Step 1: find your monthly units
Look at your last 12 bills, not just one. Use the average of units consumed, because summer and winter differ.
Step 2: convert units to kW
In Bengaluru, 1 kW of panels produces roughly 4 to 4.5 units a day, or about 120 to 135 units a month. Divide your monthly units by about 125.
| Monthly units | Approx. size |
|---|---|
| 250 | 2 kW |
| 375 | 3 kW |
| 500 | 4 kW |
| 750 | 6 kW |
Step 3: check the roof
You need around 80 to 100 sq ft of shadow-free area per kW on a flat roof. Subtract the area lost to water tanks, staircase rooms, parapet shadows and trees. Shade at the wrong hour can cut output more than you expect.
Step 4: adjust for direction and tilt
South-facing is ideal in India. East or west faces still work with lower yield, and north-facing slopes lose the most. Flat roofs allow the structure to set the tilt.
Step 5: check the sanctioned load
Your sanctioned load and BESCOM's rules influence how large a system you can connect. Raise the sanctioned load first if you need a bigger plant.
Step 6: think about the subsidy
For homes, the subsidy stops growing at 3 kW. A larger system costs more for every extra kW, so size it to your consumption and roof, not to the subsidy.
Step 7: remember export is worth less
Units you use yourself save the full tariff. Exported units earn a lower tariff. Sizing near your own daytime use usually gives the best return.
Use the calculator
Our SunScore calculator follows these steps and gives a size, savings and payback estimate. A site survey then confirms the numbers.
Frequently asked questions
What if my consumption will rise?
If you plan to add an air conditioner, an EV or extra rooms, include that in the sizing, and check whether the roof and sanctioned load can take it.
Keep reading
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