roof top factors

8/10/2019 Roof Top Factors

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Factors affecting roof area required by rooftop solarPV plantsThe extent of roof area required by a solar PV plant is dependent on two factors

Shade-free roof area

Panel efficiency

Shade-free roof areaUnused rooftop area will have to be assessed for incidence of shadows through the year to

determine the extent of shade-free area available for installing a rooftop solar PV plant.

We emphasise shade- free roof area because shadows affect the PV plants performance in two

ways

Output When a shadow falls on a PV panel it reduces the output from the plant

Panel damage When a shadow falls on part of a panel, that portion of the panel turns from a

conductor into a resistance and starts heating up. That portion of the panel will eventually burn out

and the entire panel will have to be replaced. This will not be covered by warranty

It is therefore critical to ensure that no shadow falls on the PV plant throughout the year.

Shadows that fall on the plant can be from

Neighbouring structures Buildings, hoardings, mobile phone towers, and even trees can cast a

shadow on a rooftop PV plant The PV plant itself One row of panels can cast a shadow on the row behind them; the further we

move away from the equator, the longer the shadow that is cast and the greater the amount of room

required between rows of panels

Panel efficiencyPanel efficiency influences rooftop space requirement because efficiency is calculated with

respect to the area occupied by the panel. We have a more detailed discussion on panel

efficiency here , but a simple way to understand the relationship between panel efficiency and

rooftop space required is to remember that a rooftop plant that uses panels with a lowerefficiency rating will require greater rooftop space than a plant that uses panels with higher

efficiency rating.

Shade-free area required at different plant capacities andpanel efficienciesIf a 1 kW plant with 15% efficiency panels requires 100 SF of rooftop space, then a 1 kW plant

with 12% efficiency panels will require 125 SF of rooftop space. We can extend this to different

combinations of rooftop plant capacity and panel efficiency for our understanding.
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Plant capacity 1 kW 2 kW 5 kW 10 kW

Panel efficiency Rooftop space required (SF)

12.0% 125 250 625 1,250

12.5% 120 240 600 1,200

13.0% 115 231 577 1,154

13.5% 111 222 556 1,111

14.0% 107 214 536 1,071

14.5% 103 207 517 1,034

15.0% 100 200 500 1,000

15.5% 97 194 484 968

16.0% 94 188 469 938

Note: These numbers are indicative only. Actual roof area required at your installation could vary

based on site- specific conditions and vendors recommendations.

Based on the above, we can see that a rooftop solar PV system typically requires 100-130 SF (about

12 m 2) of shade-free roof area per kW of capacity.

Other considerationsWEIGHT OF THE ROOFTOP PV PLANTRooftop solar PV plants are fairly heavy (about 30-60 Kgs/m 2). They do not pose a problem for

concrete roofs but cannot be installed on asbestos roofed sheds. Metal roofed facilities may or

may not be able to withstand the weight and wind load and will need to be assessed by an

expert.

MOUNTINGS THAT CAN WITHSTAND WIND PRESSURERooftop solar panel mountings would need to withstand wind pressure building up under the

panels during storms. This is an important consideration if you are located in a region prone to

cyclones. 2009s Cyclone Aila, with wind speeds up to 120 kph, took away about 60,000 solar

power systems attached to homes in the Sunderbans; the recent Cyclone Phailin brought windsof up to 200 kph. The kind of mounting required for your location and type of roof should be

discussed with the installer.

Takeaways Rooftop Solar PV plants require 100-130 SF of shade-free roof area per kW of plant capacity

Shadows falling on the panels not only reduce power output but also damage the panel

Rooftop plants weigh 30-60 Kgs/m 2 which is too heavy for asbestos roofed sheds. Installation on

metal roofed sheds should be decided on a case-to-case basis

The mounting structure should be designed to handle cyclones where wind speeds can reach 200 kph


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FACTORS AFFECTING ROOFTOP SOLAR PLANT OUTPUTThe power output of a rooftop solar system is dependent on several factors such as

Location

Orientation of the roof

Panel efficiency

Ambient temperature

LOCATION Your location determines the amount of solar insolation (sunlight falling on the panel per day).

We generally receive 4-7 KWh of solar insolation per square metre in India

The approximate solar insolation at your location can be ascertained by entering the latitude and

longitude of your location at the NASA website

To be absolutely certain of solar insolation at a particular site we would have to place sensors on-site

that measure the actual insolation received over a period of time. This is both an expensive and time

consuming process

This map shows the solar insolation across different regions in India.

Click to enlarge

OrientationIn the northern hemisphere a south-facing roof is ideal as the sun is always to the south if you

are in the temperate zone and predominantly in the south for many parts of the tropical zone.
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If a south-facing roof is not available an east-west facing roof could also be considered (as it will

cover the suns movement across the sky from east to west during the day). As the output of the

solar plant reduces in proportion to a horizontal angle greater than 15% from due south, the

output for the particular site should be calculated and assessed to understand the impact on

power generation from an east-west facing roof.

Solar PV plants are not restricted to flat roofs they can be mounted on sloped roofs as well,

with a correction in the angle of mounting for the slope of the roof.

Panel EfficiencyEfficiency of the panel is calculated as ratio of capacity of the panel (KWp) with respect to the

size (area) of the panel (m 2), expressed as a percentage. This table illustrates the calculation for

different panel capacities having the same size:Panel Capacity (Wp) Panel size (m 2) Panel efficiency [Wp/(1,000*m 2)]

200 1.61 12.42%

225 1.61 13.98%

250 1.61 15.53%

Note: Efficiency of a solar panel is calculated with respect to the size of the panel, and therefore

the efficiency percentage is relevant only to the area occupied by the panel. If two panels have

the same capacity rating (Wp), their power output is the same even if their efficiencies are

different.

To illustrate: A 1KW rooftop solar plant will produce the same power output whether it uses

lower or higher efficiency panels. The area occupied by the plant with lower efficiency panels will

be greater than the area occupied by the plant with higher efficiency panels, but the power output

is the same.

The efficiency of the panels matters where the rooftop space is limited. As the lower efficiency panels

occupy a greater area than higher efficiency panels, we will be able to install fewer panels in the same

size roof. Fewer panels mean lower plant capacity, and therefore lower power output from the plant.

This is illustrated in this table:

Panelefficiency

Area required for1 KW

Roof areaavailable

Plant capacity that can beinstalled(Roof area/Area required)

Lower 120 SF 1,000 SF 8.33 KW

Higher 100 SF 1,000 SF 10.00 KW

Ambient Temperature


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Solar panel temperature is an often ignored but critical parameter in a hot country like India.

Though it might seem counter-intuitive, solar PV panels generate less power in very hot

summers as the heat reduces their efficiency (the voltage reduces). In Chennai, the month of

January delivers better output than May

TEMPERATURE COEFFICIENTThe rated capacity, or power, of a solar panel (e.g. 250 Wp) is measured at 25C. The effect of

temperature on the solar panels power is measured by its thermal coefficient, expressed as %/K

or %/C. It denotes the % change in power for 1 degree change in Kelvin or Celsius (both are the

same on a unit level) above 25C. A negative (-) sign indicates the direction of the change.

A temperature coefficient of -0.447 indicates that every 1C increase in temperature over 25C

will cause a 0.447% decrease in power. Equally, every 1C decrease in temperature over 25C

will cause a 0.447% increase in power. This is illustrated in this table:

Rated panelcapacity (Wp)

Temperate( C)

TemperatureCoefficient

Effective panelcapacity (Wp) Change in Wp

250 20 -0.45% 255.59 102.24%

250 25 -0.45% 250.00 100.00%

250 35 -0.45% 238.83 95.53%

250 45 -0.45% 227.65 91.06%

Approximation of PV plant outputAs we have seen, estimating the power output from your rooftop solar plant can be a complex

exercise. Luckily we can use a simple heuristic for calculating the power output in India:

1 KWp of panel will generate about 1,400-1,600 KWh (units) per year i.e., about 4 KWh per

day. This is broadly representative of output from rooftop PV plants in India. It is an average

calculated over a year. Generation on individual days at your location will vary based on

meteorological conditions.

PV power plant performance is often denominated as Capacity Utilisation Factor or CUF. CUF isthe ratio (expressed as a percentage) of the actual output from a plant to the maximum possible

output under ideal conditions if the sun shone throughout the day and throughout the year.

The CUF for several solar-friendly Indian states and the approximate output per day for a 1 KWp

panel (calculated from the CUF) is given below.

Capacity Utilisation Factor (CUF) = Actual energy from the plant (KWh)


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Plant capacity (KWp) x 24 x 365

State CUF (%)

Output for 1 KWp

panel (KWh/day)

Andhra Pradesh 20 4.80

Gujarat 18 4.32

Karnataka 19 4.56

Madhya Pradesh 19 4.56

Maharashtra 19 4.56

Punjab 19 4.56

Rajasthan 20 4.80

Tamil Nadu 19 4.56

Uttarakhand 19 4.56

Note: The above calculation is an estimation based on average plant performance across the

state. Output at your location may vary from these estimates.

PV PLANT OUTPUTS IN DIFFERENT STATES FOR DIFFERENTROOF AREASBased on the above, we can estimate the approximate power output for PV plants on different

roof sizes in different parts of India:

Roof area (SF) 500 1,000 1,500 2,500 5,000 10,000

Plant capacity (KW)

1 KW = 100 SF 5 10 15 25 50 100

State Output (KWh/day)

Andhra Pradesh 24.00 48.00 72.00 120.00 240.00 480.00

Gujarat 21.60 43.20 64.80 108.00 216.00 432.00

Karnataka 22.80 45.60 68.40 114.00 228.00 456.00

Madhya Pradesh 22.80 45.60 68.40 114.00 228.00 456.00

Maharashtra 22.80 45.60 68.40 114.00 228.00 456.00

Punjab 22.80 45.60 68.40 114.00 228.00 456.00


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Rajasthan 24.00 48.00 72.00 120.00 240.00 480.00

Tamil Nadu 22.80 45.60 68.40 114.00 228.00 456.00

Uttarakhand 22.80 45.60 68.40 114.00 228.00 456.00

OPTIMISING ROOFTOP PV PLANT DESIGN TO MAXIMISEPOWER OUTPUTAmongst these 4 factors, location is not usually within our control when setting up a captive

rooftop solar plant. Some optimisation is possible with the other three factors.

OrientationWe can, to some extent, overcome roof orientation issues using trackers. This will, however, addboth to the initial cost and maintenance expenditure of the installation. The cost-benefit of using

trackers will have to be carefully analysed for the particular installation to determine if it is worth

the additional investment.

Panel EfficiencyIf rooftop space is a constraint we can use panels of greater efficiency to maximise the output

from the space available.

Ambient TemperatureAmbient temperature is not within our control, but we can help cool the panels by ensuring that

we provide adequate room for air to circulate around and under the PV panels. We have seen

plant performance improve significantly when panels that were mounted too close to the roof

were raised to allow greater air circulation.

Takeaways Rooftop PV plant output is dependent on

Location Roof orientation

Panel efficiency

Ambient temperature

Two panels of identical rated capacity but different efficiency will produce the same amount of

power, but occupy different amounts of space

Heat affects the panel efficiency, and peak summer months can give lower output than some winter

months

We can mitigate some of the effects of temperature by designing the plant to maximise aircooling


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Rooftop Solar PV produces about 4 KWh/day for every 1 KWp of panel capacity

What are the various components of arooftop solar system?Updated November 2013

Within this section you will find Basics of rooftop Solar PV

Components of a rooftop solar PV plant

o PV modules (panels)

o Inverters

o Mounting structureso Batteries

o Charge Controllers

Maintenance of rooftop solar PV systems

Warranties

How do I choose a good vendor for a rooftop PV system?

o Supplier Background & Credibility

o Price

How long does it take to install a rooftop PV system?

Basics of rooftop Solar PV Solar PV panels (also known as solar PV modules) work by converting sunlight into electricity.

They do not use the heat from the sun, and in fact can see a reduction in power output in hot climates

(this is discussed in greater detail here )

The electricity generated by the PV panels is Direct Current (DC). This needs to be converted into

Alternating Current (AC) using an inverter

The panels are mounted on the rooftop using special mounting structures

If solar power is required when there isnt enough sunlight for the panels to generate electricity (such

as at night), a battery backup is required

A charge controller is required to regulate the charging of batteries

These are the primary components of a rooftop solar PV plant. Other components include the

cables, switchgear, fuses, etc.

As the amount of sunlight falling on the panels varies during the day (due to clouds, etc.), the power

output from the panels also varies. As this variation in power could damage equipment, the inverter

continuously matches the PV plants output to another source of steady power. Therefore a rooftop

solar PV that generates AC power will always needs another source of power (whether the grid or
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diesel generator or batteries) to provide a reference voltage in order to function. If such a source of

power is absent, the plant will not generate power even if there is ample sunlight.

Components of a rooftop solar PV plantFrom the above, we can see that a rooftop solar PV plant primarily requires 3, and in some cases

5, components

PV modules (panels)

Inverters

Mounting structures

If battery backup is required

Batteries

Charge controller

PV modules (panels)There are two kinds of modules: Thin-film, and Crystalline. Rooftop solar plants predominantly

use crystalline panels because they are more efficient and therefore better suited to installations

like rooftops where space is a constraint.

Panel efficiencyIt should be noted that the efficiency of a solar panel is calculated with reference to the area it

occupies. Two 250 Wp panels of different efficiency rating will generate the same amount of

power, but occupy different amounts of space on your rooftop. A more detailed discussion on

panelefficiency and its impact on space occupied by your rooftop plant can be found here .

Capacity ratingThe capacity of a solar panel is denoted in terms of watts as Wp (watt peak). E.g., 250 Wp. This

is the power output of the plant at 25C. The capacity of the plant reduces at temperatures above

25C and increases at temperatures below 25C (more details here ).

InvertersInverters are a very important component of your rooftop solar PV plant because they determine

the quality of AC power you get, and also the kind of loads that can be powered with solar

different inverters support different levels of starting current requirements which affects the kind

of machinery that can run on solar power. Inverters are also the only major component of your

solar plant that are replaced during the lifetime of the plant.

Will I get power during a power failure?Not all rooftop solar PV plants generate power during power failures. As previously mentioned , the

solar inverter uses another source of power as a reference voltage. If the inverter is designed to

use only grid power as a reference voltage, then the inverter will not be able to function in theabsence of grid power and the solar plant will not generate power.
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Therefore, if you are interested in rooftop solar to provide power during grid failures it is critical to

choose an inverter that can use other sources of power as a reference voltage and continue to

fuction even when the grid is down.

Kinds of invertersBased on the explanation above, we can classify inverters into 4 types

1. Grid-tied These inverters are primarily designed to supply the generated power to the grid and also

power the load while grid power is available. This inverter will NOT generate power during a power

failure, not only because it needs grid power as a reference voltage, but also because the inverter

shuts down the system to stop sending power into the grid and avoids the risk of electrocuting utility

personnel who are working to repair the grid (known as Anti Islanding)

2. Off-grid These inverters do not work with the grid and are designed to work only with a battery

backup or diesel generator in off-grid applications. They are suitable for applications where grid

power is not available at all, but are not the right choice if you need your solar plant to work in

conjunction with grid supply

3. Grid-interactive These inverters work both with the grid supply and with either a battery backup or

diesel generator to support the load even during a power failure.

Hybrid inverters (also known as Bidirectional or magical inverters) are a one system solution for a

complete solar PV system. They can automatically manage between 2 or more different sources of

power (grid, diesel, solar). They have inbuilt charge controllers, MPPT controller, Anti Islanding

solutions, DC and AC disconnects and other features like automatic turning on/off of the diesel

generator, automatic data logging, and various kinds of protection for the different components of

the system, making them ideally suited for applications that require management of power from

different sources

Mounting structuresSolar panels are mounted on iron fixtures so that they can withstand wind and weight of panels.

The panels are mounted to face south in the Northern Hemisphere and north in the Southern

Hemisphere for maximum power tracking. The tilt of the panels is at an angle equal to thelatitude of that location.

The proper design of mounting structures is important to power plant performance as the power

output from the PV plant will not be maximised if the mountings buckle and the panels are not

optimally oriented towards the sun. In addition, improperly mounted panels present a ragged

appearance that is not pleasing to the eye. Allowing sufficient air circulation to cool the PV panels

is also an important factor that mounting structures should be designed for because, as

mentioned above, rooftop PV plant output falls as temperatures rise above 25C.

Trackers


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Tracking is a way of mounting the panels through a mechanism that allows the panels to follow

the sun as it moves across the sky. Single-axis trackers follow the sun as it moves from East to

West during the day, while dual-axis trackers also follow the sun on its North-South journey over

the course of a year.

Trackers can increase the power output from the PV plant but add significantly to both the initial

cost of the plant and maintenance expenditure; utilisation of trackers should be decided on a

case-to-case basis after performing a cost-benefit analysis over the lifetime of the rooftop plant.

BatteriesREASONS TO USE BATTERIES

Make power available when the sun isnt shining This can be particular useful for applications

where electrical consumption is greater during the night than in the day, such as BPOs that work onnight shifts, or even residential apartments where most people are away during the day and at home

during the night

Smoothen power delivery during the day Clouds moving across the sun can suddenly reduce the

output from your rooftop plant. A battery backup can ensure that the load gets sufficient power

during such dips in plant output

Immediately cut-in during power failures If space isnt available for a large rooftop plant, solar

panels with batteries can be used to support the load until a diesel generator can be turned on

Optimise time-of-use billing If the utility charges different tariffs based on time of day, power

from the batteries can be used to reduce consumption at those times when utility power is very

expensive

DRAWBACKS TO USING BATTERIES Charge/discharge efficiency Batteries and their charging equipment are not 100% efficient. There

is a loss of energy both while charging and discharging the battery. Different models of batteries can

have different charge/discharge efficiencies. If we lose 15% of the energy while charging and another

15% while discharging, we get back only about 72% of the power that was sent to the battery

Maintenance Battery packs require careful maintenance. Maintena nce isnt limited to the physical

condition of the battery (amount of electrolyte, cleaning of terminals) but also extends to the way we

charge and discharge the battery. Repeatedly deep discharging the batteries, discharging before the

battery has reached full charge, etc., are ways in which the life of the battery can be significantly

reduced. Batteries can last as long as 10 years or give trouble within a few days, depending on how

they are used

A battery pack can add about 25-30% to the initial system cost of a rooftop PV solar system for

one day autonomy (storing an entire days output).


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Due to the above drawbacks, we do not recommend coupling solar PV plants with battery

backup unless absolutely necessary. If batteries are required, we urge you to perform a lifetime

cost-benefit analysis to understand the effect on cost of solar power from your rooftop.

Charge ControllersA charge controller regulates the DC power output from the rooftop solar panels that is used to

charge the batteries. It provides optimum charging current, and protects the batteries from

overcharging. There are two kinds of charge controllers

Pulse Width Modulated (PWM)

Maximum Power Point Tracking (MPPT)

MPPT charge controllers are more expensive than PWM but they offer much better performance

in terms of efficiency, flexibility in solar panel plant configuration, and capacity supported.

Char ge contr oll ers that are in tegrated into the in verter are preferred as the inverter di rects either

gri d power or solar power , based on avail abil ity and demand, to char ge the batter ies. Th is extends

the battery li fe compared with u sing stand-alone charge contr ollers that all ow parall el chargin g

between gr id and solar power at dif ferent power l evels, damaging th e battery

Maintenance of rooftop solar PV systemsThe basic rooftop solar PV system has no moving parts and therefore requires very little

maintenance. Additional components, such as trackers and batteries, can significantly increasethe maintenance effort and expenditure.

Solar panels These typically require little to no maintenance beyond having the dust cleaned off

them. Solar panels can be expected to last for 25 years

Inverter This can be affected by grid power quality or other issues common to power equipment

such as humidity or short-circuits caused by insects, and may require some maintenance such as

replacement of capacitors. The lifespan of an inverter is 5-10 years

Mounting structures These typically last the lifetime of the plant and do not require maintenance,

unless tracking systems are used

o Tracking mechanisms involve moving parts that can wear out and/or break. The require

lubrication, parts replacement, and sufficient room on the rooftop for maintenance access

Other parts of the system Cabling, switchgear, fuses, etc. will require minor maintenance to

ensure correct operation

Batteries As discussed above, batteries require careful maintenance to function reliably. Typical

lifespan is 3-5 years

Warranties PV Panels Industry standard warranty is


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o 5-year ma nufacturers warranty

o 0-10 years for 90% of the rated output power

o 10-25 years for 80% of the rated output power

Other systems Inverters, mounting structures, cables, junction boxes, etc. typically come with a 1

year manufacturer warranty which can be extended to 5 years

A detailed discussion on warranties and certification is provided here .

How do I choose a good vendor for a rooftop PVsystem?Choosing a good vendor is critical to getting the most out of your rooftop PV system as

carelessness in design or construction/installation can either significantly reduce the power

output from your plant or deliver a plant that isnt suited to your needs. A few things to keep in

mind when selecting a vendor are

Supplier Background & Credibility Ask for details of projects that they have already implemented

Check if they are MNRE authorised, or registered under your states energy development agency (or

equivalent body)

Check if the supplied products have been manufactured in a ISO-9001 certified plant

Verify suppliers claims about the product/component with datasheets available on the

manufacturers website (e.g., if the supplier claims that the panels are suitable for coastal areas, check

the product datasheet to see if it has cleared the salt mist corrosion test)

The cheapest vendor is not necessarily the best vendor. A vendor who has a well-established after-

sales service network may quote a higher price but will provide greater benefits in the long run

When evaluating different vendors, ensure that the plant specification, and not just the description, is

the same. E.g., 1 kW panel + 5 kW inverter may be sold as a 5 kW plant but is actually only a 1 kW

plant. Similarly, 5 kW panels + 1 kW inverter is also a 1 kW plant. Such plants can be offered at a

much lower price than a genuine 5 kW plant, but will not generate anywhere near the same amount of

power

Price The cheapest vendor is not necessarily the best vendor. A vendor who has a well-established after-

sales service network may quote a higher price but will provide greater benefits in the long run

When evaluating different vendors, ensure that the plant specification, and not just the description, is

the same. E.g., 1 kW panel + 5 kW inverter may be sold as a 5 kW plant but is actually only a 1 kW

plant. Similarly, 5 kW panels + 1 kW inverter is also a 1 kW plant. Such plants can be offered at a

much lower price than a genuine 5 kW plant, but will not generate anywhere near the same amount of

power
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How long does it take to install a rooftop PVsystem?This can vary based on plant size, site conditions, and permissions required, but typically rooftop

plants can be installed within two weeks to 3 months of the project being confirmed. It should benoted here that processing of subsidies may take much longer.

Takeaways Not all PV plants generate power during power cuts; only grid-interactive plants do

Rooftop solar PV plants primarily comprise of

o PV Modules (panels)

o Inverters

o Mounting Structures

o Optionally

Batteries

Charge Controllers

PV plants have no moving parts and require very little maintenance, unless batteries or trackers are

used

Warranties

o Panels are typically warranted against manufacturing defects for 5 years, 90% of rated power

output for 10 years and for 80% of rated power output up to 25 years

o Other system components come with 1 year warranty extendable to 5 years

Vendors should be selected based on track record and ability to perform after-sales service

A rooftop PV plant may take a couple of weeks to 3 months to be installed, excluding time to process

subsidies

What are the Warranties and Certifications I

should look for in my rooftop PV system?Updated November 2013 Within this section you will find

Warranties and Certifications

o Solar Panels

o Inverters

o Mounting structures

o Batteries

o Charge Controller/MPPT units
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1. Scoping the project

2. Calculating the amount of solar energy available

3. Surveying the site

4. Calculating the amount of energy needed

5. Sizing the solar systemo System size

o Panel size

o Inverter size

If sufficient solar energy is not available

Solar power for critical loads

Solar power for light loads

Solar-diesel hybrid

Estimating the approximate capacity of the solar PV system you require and can install for your facility

should be undertaken keeping in mind your requirements, your constraints, and the amount of sunlight

available. We list a few steps that allow a methodical approach to sizing your system.

5 Steps to sizing your rooftop PV plant1. Scoping of the project

2. Calculating the amount of solar energy available

3. Surveying the site

4. Calculating the amount of energy needed5. Sizing the solar system

1. Scoping the projectClearly laying out what you wish to achieve with your rooftop solar PV installation is critical to designing

a plant that fits your needs. Examples of different kinds of needs we encounter in our work include

Completely supports your daytime electrical needs

Supports lighting loads

Supports critical loads during power cuts

Abates diesel consumption

Provides power for night-time use

2. Calculating the amount of solar energy availableThe amount of solar energy available to you is limited by the amount of sunlight that falls on a solar panel

per day. This is expressed in kWh/m 2/day. We expect about 4-7 kWh/m 2/day of solar insolation in India.

At crystalline panel efficiencies (which are the kind used in rooftop systems due to their higher efficiency),

we can generate 4 kWh of power per day from a 1 kWp panel. This is an average measure that can vary

across different regions in India. You can find more details on output in different Indian states here .
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The approximate solar insolation at your location can be determined from the NASA website . To be

absolutely certain of solar insolation at a particular site we would have to place sensors on-site that

measure the actual insolation received over a period of time. This is both an expensive and time consuming

process.

3. Surveying the siteThe site survey establishes the suitability of the roof for installing solar. Things to watch out for include

Space available 1 kW of panels would require 100-130 SF (about 12m 2) of shade-free roof area

Orientation A south-facing roof is ideal for those in the northern hemisphere

More information on factors affecting the rooftop solar plant output can be found here .

4. Calculating the amount of energy neededThe amount of energy needed is determined based on the load that needs to be supported. Since we have

already determined the scope of the project in step 1 we know what equipment needs to be supported. The

load represented by this equipment can be calculated as

Total energy requirement/day (Wh) = Wattage of appliance*No. of appliances*Hours of working

This should be divided by 1,000 to be converted into kWh/day. We can illustrate this formula by

calculating the load for a sample home

Appliance Number Wattage Working Hours Energy (kWh/day)

Lights 8 30 8 1.92Fans 5 50 8 2.00

TV 1 120 4 0.48

Computer 1 100 4 0.40

Refrigerator 1 300 12 3.60

Charging points 4 100 3 1.20

Total 9.60

This home would require 10 kWh of power per day to satisfy the load. At this point the plant designer

might wish to identify large/variable loads that need not be supported by solar power or that can be

operated through some other power source to reduce the investment in the solar system.

5. Sizing the solar systemLet us assume that we have limited the load to be supported by the solar PV plant to this:

Appliance Number Wattage Working Hrs Energy (Kwh/day)

Lights 5 30 4 0.6

Fans 2 50 4 0.4

Computer 1 100 2 0.2
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Charging points 2 100 3 0.6

Total 1.8

System size

This load requires 1.8 kWh/day.

Adding a 30% safety margin to this, and assuming the insolation to be 4kWh/m 2/day, we get

System size = (Energy Requirement*1.3) /insolation level

= 1.8*1.3/4 = 0.585 or 585 Wp.

Panel sizeWe calculate the panel requirement for this system size assuming we are using 130 kWp panels at 12V.

No. of panels = System size/Panel Rating

= 585/130 = 4.5

Therefore the system requires 5 panels of 130 Wp at 12V.

At this point the system designer may wish to verify if there is sufficient roof space available for installing

five 130 Wp panels. Typically, a 1 kWp system requires 100-130 SF so a 585 Wp (0.585 kWp) system

would occupy about 59-76 SF of shade-free roof area.

If sufficient roof space is not available, the system designer could revisit the loads that need to be

supported to determine which critical loads can be supported based on the amount of energy generation

that the available roof area permits.

Inverter sizeWe use a 45% safety margin when calculating the inverter size.

Required Inverter size = Total Wattage of all appliances*(1+45%)

Total wattage of appliances is calculated in this table:

Appliance Number Wattage Total Wattage

Lights 5 30 150

Fans 2 50 100

Computer 1 100 100

Charging points 2 100 200

Total 550

Therefore, required inverter size = 550 * (1+45%) = 798 W


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The inverter size is greater than the required solar panel capacity (585 Wp), eliminating the risk of the

inverter throttling the panels output.

The solar PV system required to power this load would need 5 x 130 Wp 12V panels and an inverterof at least 800 W.

If sufficient solar energy is not availableIf we find from the above steps that the rooftop system will not be able to generate sufficient energy to

support the entire load (often due to lack of sufficient rooftop space), we have several options before us:

Solar power for critical loadsIn this system the critical loads are identified and solar power with battery backup is used to ensure that the

critical loads receive power even during a power cut. More details of this system are provided here .

Solar power for light loadsIn this system the rooftop solar system is used to support non-critical loads that are not power hungry, such

as lighting. Such a system requires the light points to be wired through a separate circuit that can be

powered only through solar. The solar system can be coupled with batteries to provide lighting at night as

well.

Solar-diesel hybridThis system is favoured by consumers who consume a lot of diesel due to load shedding. Here the rooftop

solar PV system works along with the diesel generator to support the load, and helps reduce diesel

consumption. This system, including its financial returns, is discussed in detai lhere.

Due to the complexity in matching the load that can be powered with the power generating potential of the

rooftop we recommend that the final decision on sizing of your rooftop system be taken after consulting

with an experienced rooftop solar installer.

Takeaways Sizing your solar PV plant can be achieved through 5 steps

o Scoping of the project

o Calculating the amount of solar energy available

o Surveying the site

o Calculating the amount of energy needed

o Sizing the solar system

If the solar plant is unable to supply the entire load, we can consider 3 options

o Solar power for critical loads

o Solar power for light loads

o Solar-diesel hybrid

Due to the complexities involved in sizing the system relevant to your load profile we recommendworking with an experienced solar installer
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Does rooftop solar PV generate power duringa power failure?Within this section you will find

Do all rooftop solar plants generate power during power failure?

Why some rooftop solar plants shut down during power failure

o Reference Voltage

o Anti-islanding

Kinds of inverters

Do all rooftop solar plants generate power during powerfailure?

Not all rooftop solar plants generate power during a power failure. This comes as a surprise tomany of our clients, but solar plants have different designs for different purposes, and some

designs benefit from the plant not generating power during a power failure.

Why some rooftop solar plants shut down during powerfailureThere are two important reasons why some solar plants shut down during a power failure

REFERENCE VOLTAGE As the amount of sunlight falling on the panels varies during the day (due to clouds, etc.), the

power output from the panels also varies. As this variation could damage equipment that is

powered by solar, the inverter continuously matches the PV plants output to another sourc e of

steady power. Therefore a rooftop solar PV that generates AC power will always needs another

source of power (whether the grid or diesel generator or batteries) to provide a reference voltage

in order to function. If the inverter is designed to use only grid power as a reference voltage, the

plant will not generate power even if there is ample sunlight.

ANTI-ISLANDING When a power failure occurs, a portion of the grid isnt energised. This non -energised portion of

the grid is known as an island. If the solar plant is pumping electricity into this non-energised

portion of the grid during a power failure, it might cause utility personnel who are working on the

grid to be electrocuted. To eliminate this risk, the inverter in the solar power system turns off the

power from the plant.

Kinds of inverters
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As it is the inverter that determines whether the plant continues to function or not during a power

cut, we need to only understand the different kinds of inverters to ensure we have a rooftop solar

plant that generates electricity even during power cuts. There are 4 kinds of inverters

1. Grid-tied These inverters are primarily designed to supply the generated power to the grid and also power the load while grid power is available. This inverter will NOT generate power during a power

failure because it uses only grid power as a reference voltage and cannot function in the absence of

grid power

2. Off-grid These inverters do not work with grid power and are designed to work only with a battery

backup or diesel generator in off-grid applications. They are suitable for applications where grid

power is not available at all, but are not the right choice if you need your solar plant to work in

conjunction with grid supply

3. Grid-interactive These inverters work both with the grid supply and with either a battery backup ordiesel generator to support the load even during a power failure.

Hybrid inverters (also known as bidirectional or magical inverters) are a one system solution for

a complete solar PV system. They can automatically manage between 2 or more different

sources of power (grid, diesel, solar). They have inbuilt charge controllers, MPPT controller,

Anti Islanding solutions, DC and AC disconnects and other features like automatic turning

on/off of the diesel generator, automatic data logging, and various kinds of protection for the

different components of the system, making them ideally suited for applications that require

management of power from different sourcesIt is therefore critical to understand the purpose the rooftop solar PV plant is to fulfil before

selecting the inverter. As vendors use various terms to refer to different components we urge you

to verify if the inverter will supply power during power failure by specifically discussing this issue

with the vendor, rather than going by any label assigned to the product.

Takeaways Not all rooftop solar PV plants generate power during power failure; only some do

Whether the plant generates electricity during power failure or not lies with the inverter

o The inverter matches the power from the solar plant with another source of stable power to

ensure quality of electricity supplied. If another source is not available the inverter will not

deliver power

o The inverter can also shut down the solar plant in the event of grid failure for the safety of those

repairing the grid

Only grid-interactive or hybrid inverters (which are a kind of grid-interactive inverter) will provide

electricity even during power failure because they can utilise several sources of power, not just grid

power, to provide the reference voltage that solar PV should match


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All solar PV power plants that deliver AC power require a reference voltage, whether from grid

power or battery or diesel genset, to function

Do I have to build my own rooftop plant orcan I just buy solar power?Within this section you will find

Methods of procuring solar powero 3rd Party Sale using Open Access

Advantages Drawbacks

o Group Captive Advantages Drawbacks

o BOO(T) Build Own Operate (Transfer) Advantages Drawbacks

Summary of procurement options by consumer category Illustration of charges applicable to each option

While there are many advantages to building your own rooftop solar power plant, there is no denying thatit does come with a few issues, primarily the initial investment required and (to some extent) the extraeffort involved in making sure the plant is working correctly. It would be much more convenient if wecould just buy solar power on a per-unit-of-consumption basis the way we do with grid power.

Do such options exist for solar power? Luckily, they do. Subject to government regulations and vendorconditions, a large energy consumer can procure solar power from a solar Independent Power Producer

(IPP).

Methods of procuring solar powerThere are 3 ways in which an intensive energy consumer can procure solar power

3 rd Party Sale using Open Access Group Captive BOO(T) Build Own Operate (Transfer)

3 rd Party Sale using Open AccessOpen Access is the freedom given to consumers with connected load greater than 1 MW to choose theirown supplier of power i.e., they are not restricted to buying power from the utility and can instead buy

power from any 3 rd party supplier of power. Therefore, the consumer can contract with a solar IPP to buy power generated from their solar PV plant. A consumer with connected load less than 1 MW may apply foropen access; the utility is not obliged to grant open access in such cases, but may do so at its discretion.

Advantages The consumer doesnt have to invest in t he power plant. The consumer only pays for the electricity

supplied by the IPP The consumer doesnt need to maintain the power plant, or be concerned with warranties, quality of

components, etc. The consumer is no longer restricted by available rooftop spa ce. The IPPs solar plant may be much

larger than what could have been installed on the consumers rooftop Inter-state open access is not allowed. Power has to be procured only from power producers within

the state Open access charges in many states are very high Cross subsidy charges are imposed on 3 rd party sale
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There are many delays in getting permissions Congestion and transmission constraints may limit the amount of power that can be procured

DrawbacksDue to these reasons, 3 rd party sale of electricity is witnessed only in a few states, primarily AndhraPradesh, Karnataka, Maharashtra, and Tamil Nadu.

Due to the various charges and regulations involved in the procurement of power from 3 rd partydevelopers, we recommend a careful estimation of the landed cost (total cost to obtain the power atyour distribution board) before deciding on procuring solar power from a 3 rd party solar IPP.

Group CaptiveUnder the Group Captive scheme a group of persons/entities holding 26% shares in a RE generatingcompany can each treat the power consumed as captive power provided they jointly consume more than51% of the RE power generated.

Advantages The consumer can gain economies of scale by investing jointly with other consumers in a very large

plant The consumers need to hold only 26% of the equity in the project. If the project is funded with 70%

debt and only 30% equity, then the consumers need to jointly hold only 26% of 30% i.e., they investonly 7.8% in the cost of the project

Usually, a power plant developer builds the plant under a SPV company and offers 26% equityto the consumer(s). An agreement to buy back the shares on the termination of the procurementcontract is also entered into

Cross-subsidy charges are not levied as the supplied power is treated as captive consumption As part owners of the plant, consumers are eligible for Renewable Energy Certificates (RECs) and

can further monetise Group captive arrangements through sale of RECs Procuring power through group captive arrangements requires open access. Therefore group captive

suffers from similar problems to 3 rd party sale, such as high open access charges and obtaining

permissions, though it does not attract cross-subsidy charges Some organisations may not wish to hold equity in the SPV

DrawbacksGroup captive schemes have been seen primarily in the states of Karnataka, Maharashtra, and Tamil Nadu.

Both 3 rd Party Sale and Group Captive mechanisms of procuring power utilise the grid infrastructure todeliver electricity from the power plant to the consumer. The supply of power is therefore affected byany event that affects the grid. Unless the consumer has a dedicated feeder, 3 rd party and group captive

power will not be supplied during load shedding or grid failure.

BOO(T) Build Own Operate (Transfer)In the BOO(T) model, the rooftop system provider installs the plant on the consumers rooftop but onlysells the power from the plant to the consumer. In this arrangement, the system provider would bear thecapital expenditure for the solar unit provided the customer fulfils certain criteria and enters into a power

purchase agreement (anywhere between 5-15 years) with the developer. If both parties agree, theownership of the plant may be transferred to the consumer after a period of time.

Advantages As the system is installed on the consumers rooftop, it does not use grid infrastructure to deliver

power and is therefore not affected by grid outages or grid congestion Open access is not required and open access charges do not apply as the plant is independent of the

grid The amount of power that can be procured is limited by the extent of roof space available for

installing the plant


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The rooftop system provider may require the consumer to have a good credit rating and/or provide payment security

The system provider is the owner of the plant and enjoys all incentives provided by the government,such as accelerated depreciation or RECs

Drawbacks The amount of power that can be procured is limited by the extent of roof space available

for installing the plant

The rooftop system provider may require the consumer to have a good credit rating

and/or provide payment security

The system provider is the owner of the plant and enjoys all incentives provided by the

government, such as accelerated depreciation or RECs

Summary of procurement options by consumer categoryThe following table gives a quick summary of available options for different categories of

consumers

OpenAccess

Available Options

Customersconnectedload

Customerhasdedicatedfeeder?

BOO System atthe Premises

Procure REPower from

3 rd Party/GroupCaptive

During powercuttime

Duringnon

powercuttime

During powercuttime

Duringnon

powercuttime

> 1 MW

Yes Yes

Yes No

No Yes

No No

< 1 MW Yes No

No No

As can be seen, power failures dramatically limit the options available to consumers without

dedicated feeders if they require solar power to be supplied even during a power failure.

Illustration of charges applicable to each optionThis table gives a comparison of the different charges that should be taken into account

when calculating the landed cost of the procured power.

Charges 3 rd Party Sale Group Captive BOO(T)

Price of Power (at generationpoint) 3.500 3.500 3.500


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Tax savings from accelerated depreciation

Item Rs.

Cost of a 100 kW rooftop solar plant (A) 1,00,000.00

Accelerated depreciation @80% 80,000.00

Corporate tax rate* 35%

Tax saved through depreciation (B) 28,000.00

Net cost of rooftop solar plant (A)-(B) 72,000.00

*Tax rate can vary for different assesses

MNRE SubsidyThe Ministry of New and Renewable Energy (MNRE) provides Central Financial Assistance

through capital and/or interest subsidy (depending on the nature of the applicant). The summary

of the subsidy scheme is provided in the table:

S.

No. Category

Maximum

capacity

GOI Support

System with

battery

backup

System

without

battery

backup

Interest

Subsidy

1

Individuals for all

applications 1 kWp

Rs.51/watt or30% of

project cost

whichever is

less

Rs.30/watt or30% of

projectcost

whichever is

less

Soft loans

@5% p.a.

2

Individuals for

Irrigation, & community

drinking water

applications 5 kWp

Rs.51/watt or

30% of

project cost

whichever is

less

Rs.30/watt or

30% of

project cost

whichever is

less

Soft loans

@5% p.a.

3

Non-commercial/

commercial/industrial

applications 100 kWp

Rs.51/watt or

30% of

project cost

whichever is

less

Rs.30/watt or

30% of

project cost

whichever is

less

Soft loans

@5%

p.a.*

4 Non-commercial/commercial/industrial 250 kWp

Rs.90/watt or 30% of projectcost whichever is less

Soft loans@5%


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mini-grids p.a.*

*for commercial/ industrial entities either of capital or interest subsidy will be available

Note: 1 The benchmark cost for setting up a solar PV plant is Rs. 170/Wp (With battery providing

6 hours of autonomy) and Rs. 100 per Wp (without battery) i.e. if the actual project cost exceeds

this amount then project cost will be deemed to be the benchmark cost for calculating the

subsidy.

Note 2: Benchmark costs are for systems with 5-year warranty for all components (inverters,

batteries, switchgear, etc.) other than PV modules which are warranted for 90% of output at end

of year 10 and 80% at end of year 25. PV modules have to be made in India to avail subsidy.

Note 3: Capital subsidy is increased to 90% of benchmark cost for special category states (North

Eastern states, Sikkim, Jammu & Kashmir, Himachal Pradesh, and Uttarakhand).

The subsidy calculation is illustrated in this table:

Savings from capital subsidy

Item Rs.

Cost of a 1 kW rooftop solar plant with battery backup 1,60,000.00

Benchmark cost 1,70,000.00

Subsidy @30% of actual cost 48,000.00Net cost after subsidy benefit 1,12,000.00

Please see here for more details on how both the subsidy and accelerated depreciation work

together to reduce the cost of your rooftop solar system even further.

Renewable Energy Certificates (RECs)Renewable Energy Certificates are an avenue to further monetise your rooftop solar PV plant.

RECs are available for rooftop plants of 250 kW or greater capacity. Every 1 MWh (1,000 units)

of energy generated is eligible for 1 REC. These RECs are traded on power exchanges, where

they are sold to organisations that need to satisfy a Renewable Purchase Obligation (typicallyutilities).

Criteria The project should have a minimum generating capacity of 250 kW

The power generated should not be sold to any distribution licensee at a preferential tariff

Captive solar power generators should not be

Availing promotional wheeling charges

Availing promotional banking charges

Not receiving any exemption/waiver of electricity taxes or duties
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Only grid-connected projects can avail RECs. Off-grid projects are not eligible

The solar project should be accredited with the State Nodal Agency 6 months prior to the date of

commissioning of the project

The solar project should be registered with the Central Agency 3 months prior to the date of


The solar generator has to apply to the Central Agency for the RECs based on electricity

generated that is certified by the State Load Despatch Centre (SLDC) through a separate meter

Issued RECs can be traded only through power exchanges through a closed double-sided

auction

Procedure The solar project should be accredited with the State Nodal Agency 6 months prior to the date of


The solar project should be registered with the Central Agency 3 months prior to the date of


The solar generator has to apply to the Central Agency for the RECs based on electricity generated

that is certified by the State Load Despatch Centre (SLDC) through a separate meter

Issued RECs can be traded only through power exchanges through a closed double-sided auction

Price of Solar RECsThe price of solar RECs has been fixed within a band of Rs. 9,300 (minimum) and Rs. 13,400

(maximum) per solar REC until FY 2016-17.

Risks associated with RECsThere are two risks associated with RECs in India

Current market The market for RECs exist only if RPOs are enforced. The track record of

enforcement by most state governments is rather poor. As there is a minimum price at which RECs

can be sold, the effect of poor demand is felt in the number of RECs sold: only about 15% of the solar

RECs offered for sale in November 2013 found buyers

Future price The floor price has been set only till 2017. There is uncertainty on pricing beyond this

period. Unless enforcement of RPOs improves we expect the price for solar RECs to be in the Rs.

1,500-3,900 range between 2017 and 2022

Further information on RECs can be found at the REC registration website .

State schemesSeveral states in India have released solar policies that further incentivise rooftop solar. We

provide a brief snapshot of a few state solar policies for rooftops.

GujaratCapacity addition 25 MW
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targeted

Consumer segment 80% Government; 20% Residential

Project Type Rent-a-Roof

Incentives

1. 5 MW rooftop programme on the PPP model in the capital

which is now extended to about 5 more cities and towns

2. Monthly incentive of Rs.3/kWh for the roof owner

Offtaker/Power purchaser State Distribution Agency

Base requirement

Various sizes of SPV systems ranging from 500 KW, 100 KW, 50

KW, 20 KW, 10 KW, 5 KW, 1 KW and more

KarnatakaCapacity addition

targeted 250 MW

Consumer segment All buildings with rooftop space

Project Type Rent-a-Roof

Incentives

1. Rs 3.40/KWh

2. Net Metering

3. Any other incentives available to rooftop systems


Base requirement

Developers should guarantee a minimum of 450 kWh a year for half

kW systems and 900 kWh for 1 kW

KeralaCapacity addition

targeted 10 MW

Consumer segment Residential only

Project Type Owner owned

Incentives

1. 30% Subsidy from MNRE +

2. Rs.39000/system from the Government of Kerala

Offtaker/Power purchaser Captive (home use)

Base requirement Minimum system size of 1 kW

Rajasthan


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Capacity addition

targeted 50 MW

Consumer segment All buildings with rooftop space

Project Type Owner owned/Rent-a-Roof

Incentives Tariff-based competitive bidding


Base requirement Small solar power plants connected at 11 kV of a minimum of 1 MW

Tamil NaduCapacity addition

targeted 350 MW

Consumer segment Residential and Commercial

Project Type Owner owned/Rent-a-Roof

Incentives

1. Rs. 2/kWh for first two years; Rs. 1/kWh for next two years; Rs.

0.5/kWh for subsequent two years

2. Net metering

3. 10,000 1 kW domestic systems eligible for Rs. 20,000 subsidy

in addition to 30% MNRE subsidy

Offtaker/Power purchaser Captive and State Distribution Agency

What is net metering?Several state policies mention net metering. It refers to an incentivising model where excess

power generated by the rooftop plant (such as power generated on weekends or national

holidays) can be pumped into the grid, and the generator receives a credit for the number of units

supplied to the grid against the number of units received from the grid i.e., it is as if the meter ran

in reverse when power flowed from the rooftop plant into the grid. In its purest form it would becalculated like this:

Particulars Solar power supplied togrid

Grid powerconsumed

No. of units 100 2,000

Net units 1,900

Grid tariff for power consumed:0-100 kWh Rs. 3.00100-500 kWh Rs. 3.75500-1,000 kWh Rs. 4.50>1,000 kWh Rs. 5.00 Rs. 8,550


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Numbers are for illustration only

In some state policies the energy supplied to the grid is not directly credited against the number

of units consumed from the grid. Instead, another tariff is used to calculate the credit for the

energy supplied to the grid. This is illustrated in this calculation:

Particulars Solar power supplied togrid

Grid powerconsumed

No. of units 100 2,000

Solar tariff for power supplied to grid:Rs. 3/kWh Rs. 300

Grid tariff for power consumed:0-100 kWh Rs. 3.00100-500 kWh Rs. 3.75

500-1,000 kWh Rs. 4.50>1,000 kWh Rs. 5.00 Rs. 9,050

Total 300 9,050

Net bill amount Rs. 8,750

Numbers are for illustration only

Solar power supplied to the grid under net-metering may not qualify for RECs. For e.g., in Tamil

Nadu such power is not eligible for RECs as it is deemed to qualify for the DISCOMs RPO.

Net metering requires a net meter that can record both power consumed from, and supplied to,

the grid.

It should be noted that without net metering, the excess power generated is still supplied to the grid.

The generator doesnt receive any benefit from doing so in the absence of a net metring policy.

PermissionsTypically, permissions are not required to set up rooftop installations with capacity


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o Accelerated depreciation

o MNRE subsidy

o Renewable Energy Certificates

Several states provide additional incentives based on their solar policies

Net metering, or reward for excess power supplied to the grid, is slowly gaining ground in India

Permissions required for installing grid connected rooftop solar systems primarily involve receiving

approvals from the local power distribution authorities, who may need to ensure that the grid

infrastructure does not become congested

roof top factors

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