Solar panel calculator
Find the system size, the number of panels and the roof area for your electricity use, with the generation to expect in each month.
Your electricity and your roof
Enter what you use and where you are. The recommended system updates as you type.
Electricity use
Your bill shows it as units consumed, or kWh. Use the average of a few months if you can.
Location
Sun figures are long-term averages for 900 cities. Pick the nearest one if yours is not listed.
Share to cover
Size for all of your use if you have the roof, or less to fit a smaller roof or budget.
Savings and payback
Optional. Add what you pay per unit to see the yearly saving, and a quoted system cost to see the payback time.
Adjust the assumptions
These are typical figures for a rooftop system. Change them if your installer has given you different ones.
Expected generation by month
Show the months as a table
| Month | Peak sun hours | Units |
|---|
Pick a city from the list to see the generation month by month.
How the answer is worked out
Four steps, with every figure open to change above.
- Start from your yearly use The units you use in a month, multiplied by twelve, and by the share you want solar to cover.
- Look up the sun where you are Peak sun hours are the solar energy reaching a flat square metre in a day, in kWh. We use the 2001 to 2020 average for your city, for the year and for each month.
- Allow for losses Panels on a roof deliver about 75% of their rated output once heat, dust, wiring and the inverter have taken their share.
- Size the system and count the panels Daily use divided by sun hours and by 0.75 gives the kW needed. That is rounded up to whole 550 W panels, and the roof area is taken at 10 square metres for each kW.
What the estimate leaves outShade, the direction the roof faces and the tilt of the panels. The sun figures are for a flat surface, and panels tilted towards the sun usually collect a little more over a year, so the estimate is slightly cautious.
Sun figures: the data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division. City names and locations are from GeoNames, used under CC BY 4.0. The roof area of 10 square metres per kW is the figure given in MNRE guidance for rooftop solar.
Common questions
How many solar panels do I need for 300 units a month?
In Delhi, 300 units a month needs about 2.75 kW. That is five 550 W panels and about 27.5 square metres of flat roof. Sunnier cities need a little less. Choose your city above for your own figure.
What are peak sun hours?
The solar energy that reaches a flat square metre in a day, in kWh. Five peak sun hours is the same energy as five hours of full-strength midday sun. It is not the number of daylight hours.
Why does generation dip in some months?
Cloud and haze. In much of India the monsoon months get less sun than April and May, even though the days are long. The chart shows the long-term average for each month, and any single year can differ.
Does it include subsidy or net metering?
No. Subsidies and net-metering rules differ by state and change over time, so the calculator leaves them out. The saving shown assumes each unit of solar replaces one you would have bought. Ask us what applies where you are.