Type in your monthly electricity use and this calculator tells you what size rooftop solar system you need, how many panels, how much roof space, the likely cost after the PM Surya Ghar subsidy, and how many years it takes to pay for itself.
The formula behind the sizing
A solar panel's rated wattage is measured in lab conditions. On your roof, heat, dust, wiring losses and inverter losses mean you get around 75% of that. So the system size is:
kW needed = (monthly units ÷ 30) ÷ (peak sun hours × 0.75)
Peak sun hours is the number of hours of full-strength sunlight your location receives on an average day. Most of India falls between 4 and 5.5. Rajasthan and Gujarat are at the top end, while the North-East and coastal Kerala are lower.
Worked example
A household using 300 units a month in Maharashtra (4.5 peak sun hours) needs about 2.96 kW. Rounding up to six 540 W panels gives a 3.24 kW system, which should generate around 330 units a month. At ₹55,000 per kW the system costs about ₹1.78 lakh. The subsidy at 2024 rates is ₹78,000, so the net cost is about ₹1 lakh. At ₹8 per unit, yearly savings are about ₹28,800, giving a payback of roughly three and a half years.
Before you sign a contract
- Check the roof is shadow-free from 9 am to 4 pm. One shaded panel can drag down the whole string.
- Ask for net metering. Surplus units go to the grid and are adjusted in your bill.
- Insist on the approved vendor list on the national portal if you want the subsidy.
- Budget for cleaning panels every two to three weeks in dusty months.
Frequently asked questions
How much roof area does 1 kW of solar need?
Plan for about 100 sq ft (roughly 10 m²) of shadow-free roof per kW, including space for walking and cleaning.
How many units does a 1 kW system generate?
Around 3.5 to 4.5 units a day in most of India, or 100 to 130 units a month depending on season and location.
Is the subsidy still available?
The PM Surya Ghar scheme was launched in 2024 with the rates used in this calculator. Subsidy rules can change, so confirm the current amount on the official portal before you plan your budget.