9 Important Considerations for Your Rooftop Solar PV System

Can we really protect humanity from climatic furies

Can we really protect humanity from climatic furies

As the so-called ‘Climate Change’ is worsening month by month and abrupt/extreme weather events are becoming more frequent everywhere, more and more people are feeling the need to get away from the usual fossil fuel (coal, oil, gas) generated electric power supply. In many states there is perennial shortage of power and in many areas the problem is really acute. So people are exploring ways to install solar PV system for their home, office or industrial premises. The ministry of New and Renewable Energy (MNRE) promotes off-grid solar PV systems up to 100 kW under the Jawaharlal Nehru Solar Mission by offering 30% subsidy on capital expenditure for both residential and commercial entities. It also provides loans at 5% annual interest for 5 years. Commercial establishments have the choice to claim either capital or interest subsidies.

In India, sunshine is the most abundant natural asset throughout the country – but as with other things this has also remained largely un-harnessed so far. Depending on the location, the daily incidence ranges from 4 to 7 kWh/m2, with sunshine hours ranging from 2300 to 3200 a year. About 58% of country’s total land area (of 1.89 million km2) receives annual average Global insolation above 5 kWh/m2/day. So the future wellbeing of India lies squarely with the solar power.

Therefore, current situation demands that embracing solar power must become a national culture – it has begun to happen already as the prices of solar panels per watt keep falling. Incremental technological improvements have ensured that panels per sq foot progressively convert more sunlight into energy. Put together technological improvements, falling prices, expanding solar market and government incentives are motivating more and more people to ‘go solar’.

However, before ‘going solar’ there are some important considerations about solar PV systems that must be kept in mind.

  1. Sizing the solar PV system

Since your solar PV system will incur capital expenditure, it is important that you size it accurately. It starts by estimating your current electricity consumption. Since the daily electricity consumption will decide the size (hence, cost) of your solar system, it is prudent to first go for energy efficient appliances so that your energy requirement is minimized. Therefore, energy efficiency considerations should precede design and installation of the solar system.

The first step towards energy efficiency involves reducing power consumption by replacing all CFL/tubelights with LED lights and existing fans with energy efficient fan. (You may like to explore Advantages and drawbacks of LEDs and Super efficient and other fans and here is a 25W fan! ) It will incur some additional cost but take it as a long term investment that will pay off itself in next couple of years through savings in the electricity bill. Consider energy efficiency in all your future purchases of electrical appliances. High wattage appliances such as ACs, desert coolers, water heaters and water pump etc should not be operated through your proposed solar system in order to keep its size and cost low, though solar ACs and cooler’s are also now available in the market).

For ideas on sizing your solar PV system you may like to read:  Basics of Solar PV Design

  1. Solar insolation at your location

The insolation map shown here gives solar insolation across the country. In India, most locations receive 4-7 kWh / m2 / day. You can get an approximate idea about solar insolation at your location from this NASA website by entering your latitude and longitude numbers.

If on a particular day insolation is 6 kWh per sq meter, a 1 kW solar PV system would produce 6 kWh of energy. Obviously, more sunlight means more electricity from your solar PV system. But there are other factors that affect the system output.

  1. Panel efficiency – How much area is needed for a 1 kW solar PV plant

Efficiency (wattage per unit panel area) of the panels becomes an important issue where rooftop space is limited. Lower efficiency panels occupy greater area than higher efficiency panels, so you can place only a limited number of panels on the available area. It will limit the size of your solar PV plant. If you have limited rooftop space, choose solar panels with maximum wattage per sq foot.

Typically, a 1 kW solar PV system requires around 100 sq feet area depending upon the panel efficiency.

Efficiency of a panel is given as panel kilo-watt (kWp) per sq meter area of the panel, expressed as percentage. Consider 200 Wp and 250 Wp solar panels, both with area 1.61 sq meter. Their efficiencies will be 12.42% and 15.5%. Obviously, using 250 Wp panels would require less rooftop space.

If you want to find the rooftop area needed for a 1 kW solar PV plant, here is layman’s approach:

Using the 1.61 sq m area of the above two panels, you will require 1.61 X 5 (or 9 sq m) space if you use 200 Wp panel and 1.61 X 4 (or 6.4 sq m) with 250 Wp panels.

Approximating 1 sq m with 3.3 ft x 3.3 ft (about 11 sq feet) you need 70 – 100 sq feet rooftop space for your solar PV plant.

Note that cheaper panels come with lower efficiencies – they will require larger rooftop space.  But 100 sq feet is a good number – easy to remember as space required for a 1 kW plant.

Panel cost: Solar panel’s cost vary between Rs 30 – Rs 60 per watt. Indian make panels are available in the price range Rs 30-35. Imported modules cost around Rs 40-45.

  1. Ambient temperature

It is somewhat counter intuitive but solar panels generate less power as they heat up in hot summer days. So temperature of the panel affects its output. In fact, it is not surprising to find the best output in the winter months when panels remain cooler. Therefore, solar panels have negative temperature coefficient – for example, -0.45. It means every 1 degree rise in temperature will reduce power output by 0.45%. Likewise, a 1 degree decrease in temperature will increase panel output by 0.45%.

Thus, a 250 Wp panel (usually rated at 25 ˚C) will behave like a 256 Wp at 20 ˚C and even higher in the cold winter and as 228 Wp panel at 45 ˚C in hot summer.

In fact, the ‘cold desert’ Laddakh in J&K is the best place for solar PV plants because the temperatures are very low and sky generally clear. Of course, if you are located in say, Chennai or Delhi this information has no meaning for you. But it helps to know that ideal locations would have low temperatures and higher insolations.

Although you have no control on ambient temperature at your location, but can plan to position the solar panels to allow higher air circulation rather than at enclosed locations.

  1. Shade free area

Even if you have a large roof your solar PV power plant needs shade-free area. Shadows may come from neighboring structures like other buildings, mobile phone towers, hoardings, trees. It may even come from the PV plant itself – a row of panels may cast shadow on the rows behind it.

Note that at higher latitudes shadows are longer and large roof space is required for the plant. However, in India it is a minor issue.

Shadows not only reduce the panel output, they may also cause panel damage. Shadowed areas turn from being conductor to insulator and start heating up – it can eventually burn up those areas. Such damages are not covered by warranty.

  1. Panel orientation

India lies in the northern hemisphere with its southern tip quite close to equator. In northern hemisphere you would ideally like to mount the panels on a south facing roof because if you are in the temperate zone the sun is always towards south and if in the tropical zone the sun is predominantly in the south.

If there is problem with southern side, you may also mount panel facing 45 degree east or west of south.

Tilt or angle with respect to horizontal is another important parameter. For fixed panel system it is typically taken as the same as the latitude of your location.

In case of confusion, the best option is to consult a solar PV system installer.

  1. Power output of a 1 kW solar PV plant

Actual power output of your solar PV plant is a complex thing that depends upon so many variables – insolation, panel orientation, tilt angle, temperature and so on. But in India, we have some indicative data for guidance.

A 1 kW PV plant generates about 1600 – 1700 kWh (units) per year which is about 4-5 kWh (4000 – 5000 Wh) per day. This is also about the daily power consumption of a small family. Therefore, a 1 kW solar PV plant can serve most households.

Conventionally, PV plant’s performance is given as capacity utilization factor (CUF). CUF is the ratio (expressed as percentage) of the actual output of a plant to the maximum possible output under ideal condition (assuming that the sun shone every day of the year).

The CUF for many solar friendly Indian states is in the range 18-20 ­[for example, Gujarat (18), Andhra Pradesh (20), Uttarakhand (19)] and the average output of 1 kW plant in these states lies in the range 4.3 – 4.8 kWh.

Annual units expected from a plant (kWh)  =  Plant size in kW x CUF x 365 x 24

Thus, a 1kW plant in Andhra (CUF 20) is expected to generate 4.8 kWh (or 4800 Wh) per day.

(Please note that this is a state average and can vary significantly at different locationseven within a state.)

TIP: Just look at the daily insolation in kWh on any day – it tells you electricity units produced on that day. So a 1kW system in Delhi may produce on a winter cold day (3kWh x 1) 3kWh of power on that day. On summer days the same system could give you (6kWh x1) 6kWh. [This is just an illustrative example]

  1. Weight of the PV plant

This is an important factor in windy areas because solar panels along with their mountings can be fairly heavy. It is not a problem for concrete roofs but asbestos and other roofs may be too fragile and pose safety problems. So the roof should be sturdy enough to withstand the weight and wind load of the plant.

In recent years, cyclones such as Aila and Phailin with wind speeds of 120 – 200 kph have knocked down thousands of solar systems. In coastal areas prone to such calamities, this issue should be particularly discussed with the installer.

Consider this data sheet for 125Wp and 150Wp off grid solar panels from EMMVEE. Module weights are 12Kg and 16Kg. If you use 7 150Wp modules for a 1 KW solar PV rooftop system, weight of the panels alone would be 112Kg – you may add roughly an equal weight of the mountings. So 200 – 250 Kg for the whole system on your roof.

  1. Batteries and inverter

You will need to store the DC current from the panels into batteries and then convert it into AC through an inverter for end use. Remember that the type of batteries and inverter will also determine the overall efficiency of your solar PV system.

‘Deep cycle’ lead acid batteries are best suited for your solar application. They generally last for 5-10 years and are about 80% efficient. They are designed to repeatedly discharge 80% of their rated capacity. Note that automotive batteries are shallow cycle and are not suitable. Tubular positive plate or sealed maintenance free (SMF) batteries are best suited for solar PV systems. Size the batteries so that they can store enough power for your needs during cloudy or rainy 2-3 days.

A pure sine wave inverter is the ideal choice for all kinds of appliances but is always costlier. Modified sine wave inverters are relatively cheaper, less efficient (they waste some power as heat) and create a buzzing sound with appliances. So make a choice between the two types based on your budget and other requirements.

Inverter/UPS Models and Their Efficiencies

In summary, energy efficiency considerations must always precede solar PV design; it further helps to know various factors that dictate the actual power output from your solar system.

Disclaimer: This blog has no business interest in products of any company whose website is mentioned here.


About Goodpal

I am a firm believer in healthy people (mind and body both), healthy societies and healthy environment. Please feel free to comment, share and broadcast your views -- I like rational and intellectual discussions. Thanks for stopping by. Have a Good Day!
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3 Responses to 9 Important Considerations for Your Rooftop Solar PV System

  1. padmasri says:

    very useful information and good guidelines.

  2. amjad says:

    can i have some more information about the rooftop solar panels its a wonderful post btw.

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