How to know the dimensions of your solar roof?

30/3/2015

When installing the solar panels, the size of the roof matters a lot. Your average monthly requirement, the average daily hours of sunlight and along with the space available on the roof decides the number of solar panels to be installed.

Step 1: Calculate your energy requirements

The simple way to calculate your energy requirements is to check your monthly electricity bill, it will show you the number of units consumed or power consumed in KWHr. Your monthly energy consumption will be different in different seasons like you consume more in summers and less in winters, so take average of power consumed in summers and winters.
For example, your 12 months consumption is as follows:

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Step 2: Calculate the average daily consumption

Divide your average monthly consumption by 30, to get the average daily consumption. In our case, it is 330/30 = 11 KWHr.

Step 3: Find out the average hours of sunlight or average daily solar insolation in your area

For example Delhi/NCR receives around 5.5 hours of sunlight per day at 1 KW/m2. You can get this data from the state’s official website for renewable energy.

Step 4: Calculate the total wattage

Now, divide your average daily consumption by the average daily hours of sunlight. In our case, 11 KWHr/5.5 hours = 2 KW or 2000 Watts. This will tell you the total wattage of the panels you require to cover your energy needs in ideal conditions that is when there are no energy losses.

Step 5: Consider the energy losses

Multiply the figure you obtained in step 4 by 1.4 to cover up the losses due to inefficiencies like energy conversion losses and heat losses. We get 2000 watts x 1.4 = 2800 watts. This will be the total wattage of the panel you require to meet your energy needs.

Step 6: Calculate the shadow free area

Find out the shadow free area of your roof top by multiplying its length and breath. Let us assume that the shadow free area of the roof is, 20 feet by 11 feet, 220 square feet.

Step 7: Find out the general dimensions of the solar panels of different wattage

You can get these dimensions from the physical data sheets available in the solar company website. For roof top installation solar panels are usually come in sizes of 150 watts, 175 watts, 200 watts, 250 watts and 300 watts.
Some of the dimensions are as follows:

Step 8: Calculate the total area required by the solar panels

The total wattage of panels required, to cover your daily energy needs, is 2800 watts. Here we will calculate the total area covered by the panels of different wattage.
Condition: If the total area of the solar panels is less than the shadow free area, 220 square feet, then we will consider it for installation otherwise reject that wattage of solar panel.
Let us calculate one by one.

The number of 175 watts panels required: 2800/175 = 16 nos.
The area of one 175 watt panels is 17.7 sq. Ft (see in the table above). Therefore, the total area of sixteen panels would be 16 nos x 17.7 = 283.2 sq.ft.

(It is advisable to leave some gap between two panels, so that air can pass through and keep the panels cool in summers)

Increase the total area of the panels by 2.5%, we get, 283.2 X (1 + .025) = 290.3 sq. ft.

Result: The area of 290.3 sq. Ft > shadow free area of the roof top, the array of 175 watt panels cannot be used.
Similarly, calculate the total area of solar panels with other higher wattage, which is as follows:

It can be found out from the table above, that 240 watts of solar panel and 300 watts of solar panels are considered for the roof top installation.

Step 8: Find out the best option out of the accepted wattage of solar panels

Condition: If the Actual number of solar panels that can be fitted completely in the given area >= Required number of solar panels, then consider that power of solar panels for installation otherwise reject it.

Take the ratio of the length of the roof top to the length of the solar panel (Lroof / Lpanel)and truncate the ratio to the integer by removing the decimal part of it.
Similarly, do the same with ratio of the width of the roof top to the width of the solar (Wroof / Wpanel)
Multiplying the integer values of both the ratios gives you the number of solar panels with can be fitted completely on the given shadow free area.
Condition: If [Truncate(Lroof / Lpanel) x Truncate (Wroof / Wpanel)] >= Required number of solar panels, then consider that power of solar panels for installation otherwise reject it.

For 240 watt solar panel:

Truncate (Lroof / Lpanel) = Truncate (20 feet / 5.45 feet) = Truncate (3.7) = 3
Truncate (Wroof / Wpanel) = Truncate (11 feet / 3.25 feet) = Truncate (3.4) =3
The numbers of solar panels which can be fitted completely on the roof top are 3 x 3 = 9 panels
The required number of solar panels in this case is 12
Result: 9 < 12, reject the option.

For 300 watt panel:

Truncate (Lroof / Lpanel) = Truncate (20 feet / 6.5 feet) = Truncate (3.1) = 3
Truncate (Wroof / Wpanel) = Truncate (11 feet / 3.25 feet) = Truncate (3.4) =3
The numbers of solar panels which can be fitted completely on the roof top are 3 x 3 = 9 panels
The required number of solar panels in this case is 9
Result: 9 = 9, consider this option.

Conclusion

So, finally we find out that the 9 panels of 300 watt of total area of 195.6 square feet are the best suited for the roof top installation of area 20 feet by 11 feet = 220 square feet. The free area of 24.4 square feet (220 – 195.6) is enough for a person to do maintenance and cleaning of the solar panels, which is required from time to time.