Rooftop Solar Calculator (with PM Surya Ghar Subsidy)

About the Rooftop Solar Calculator (with PM Surya Ghar Subsidy)

Find the rooftop solar system that suits your home: from the units (kWh) on your electricity bill, from the roof area you have, or for a size you already have in mind. The calculator multiplies the system’s kilowatts by the sun your panels get — peak sun hours, month by month, for 237 cities from NASA POWER, or your own figure — and by a performance ratio for the losses, so you see what it makes each month and each year. It works out how many panels fit, including the gaps a flat roof needs between tilted rows so they do not shade each other in winter.

Then it puts a price on it: the savings at your tariff (with a lower rate for units sent to the grid if your utility pays less), the payback on your installer’s quote, and 25 years of savings as the panels slowly age. For homes in India it applies the central financial assistance of PM-Surya Ghar: Muft Bijli Yojana — ₹30,000 for 1 kW, ₹60,000 for 2 kW and ₹78,000 for 3 kW or more. Elsewhere, enter your own subsidy or none. An estimate to compare quotes, not financial advice.

Without a pass, the free preview shows the solar system size in kW in full, with the first months of the monthly table; the savings, the payback, the complete table, Copy result and the CSV come with a Pro pass.

Common uses

• Decide how many kW of rooftop solar your bill needs before asking for quotes.
• Check how many panels your terrace can take with proper row spacing.
• Compare installers’ quotes on payback after the PM Surya Ghar subsidy.
• See how much a system makes in the monsoon or in winter, month by month.

The calculation

• Generation: E = kW × peak sun hours × days × performance ratio. Peak sun hours are the day’s sunshine on the panels in kWh/m² — the hours of full 1,000 W/m² sun.
• Performance ratio: the share of that ideal output a real system delivers after heat, dust, wiring, mismatch and inverter losses (IEC 61724-1 defines it as final yield ÷ reference yield). The calculator starts at 0.75; your installer or a monitoring record can give a better figure.
• Size from units: kW = units a year × share to cover ÷ (peak sun hours × 365 × performance ratio), rounded up to whole panels.
• Size from roof area: a panel of efficiency η makes η kW per m² in full sun, so 1 kW of 21% panels covers 4.8 m². Panels laid flat on the roof (or flush on a sloping roof) can use the whole area you enter; tilted rows on a flat roof need gaps.
• Row spacing: rows are spaced so the front row’s shadow does not reach the next on the shortest day of the year in the hours you choose — the gap grows with the tilt and the latitude, and at 9 am and 3 pm the sun is lower and further to the side.
• Savings = units you use × tariff + units exported × export rate, the export rate equal to the tariff under full net metering. Payback = (price − subsidy) ÷ savings, year by year with the panels’ yearly output loss and any tariff rise you enter.

PM Surya Ghar (India)

PM-Surya Ghar: Muft Bijli Yojana gives households central financial assistance (CFA) for rooftop solar: 60% of the system cost for the first 2 kW and 40% of the additional cost between 2 and 3 kW, capped at 3 kW. At the benchmark prices the PIB announcement gives that as ₹30,000 for a 1 kW system, ₹60,000 for 2 kW and ₹78,000 for 3 kW or more; for sizes in between the calculator pro-rates (₹30,000 per kW up to 2 kW, ₹18,000 per kW from 2 to 3 kW). Households apply through the National Portal at pmsuryaghar.gov.in, which handles registration, the vendor and the subsidy; the amount it sanctions is the one that counts. The PIB release also says a 3 kW system will be able to generate more than 300 units a month on average.

Sun data and tilt

City values are NASA POWER’s 20-year averages of sunshine on a surface facing the equator — flat, tilted at the latitude, or at latitude + 15° — for the grid cell around each city, about 100 km across. A tilt near the latitude gives close to the most energy over a year on a fixed rack; flat panels catch more in summer and less in winter. A shaded roof, a hazy city or panels facing east or west make less than these values, so the performance ratio or your own sun figure should allow for it. These data were obtained from the NASA Langley Research Center (LaRC) POWER Project funded through the NASA Earth Science/Applied Science Program.

Sources

• PIB: Cabinet approves PM-Surya Ghar: Muft Bijli Yojana — CFA amounts
• PM Surya Ghar National Portal
• NASA POWER Data Access Viewer — solar irradiation for any location
• IEC 61724-1, Photovoltaic system performance — Part 1: Monitoring (performance ratio)

Limitations

• An estimate to compare options — not financial advice or a design. Shade, the roof’s direction, the inverter and local weather change the real output; a site survey settles it.
• Net-metering rules, export rates and sanctioned-load limits differ by utility and country; enter yours. The calculator nets a year’s generation against a year’s use.
• PM Surya Ghar amounts follow the PIB announcement; eligibility and the amount sanctioned are decided on the National Portal.
• It does not include financing, maintenance, inverter replacement or the value of backup power.

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