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Rispri LabsENGINEERING A CIRCULAR FUTURE

Free tool · Renewables

Solar Rooftop Calculator

Solar size (kW) = yearly units used ÷ (peak sun hours × 365 × performance ratio). In most of India 1 kW generates about 1,300 to 1,500 units a year and needs about 10 m² of shadow free roof. A home using 300 units a month needs about 2.75 kW.

Your inputs

Results update as you type. Example values are filled in.

kWh (units)

Average of your last 12 bills.

m²

About 10 m² is needed per kW.

hours/day

Rajasthan and Gujarat about 5 to 5.5, most of India 4.5 to 5, north east and hills about 4.

%

Losses from heat, dust, wiring and inverter. 75 to 80% is typical.

Wp
₹ per unit
₹ per kW

How to use this calculator

  1. Enter your average monthly units from electricity bills.
  2. Enter shadow free roof area.
  3. Check peak sun hours for your region and your tariff.
  4. Choose residential or commercial to apply the subsidy, then read size, savings and payback.

How do you calculate rooftop solar size?

Yield per kW (kWh/year) = peak sun hours × 365 × performance ratio

Size (kW) = yearly units ÷ yield per kW, rounded up to whole panels

Payback (years) = (cost − subsidy) ÷ yearly savings

Residential subsidy under PM Surya Ghar: ₹30,000 per kW for the first 2 kW, ₹18,000 for the third kW, capped at ₹78,000. CO₂ uses the CEA grid emission factor of about 0.71 t per MWh.

Worked example

With monthly electricity use 300 kWh (units), shadow free roof area 40 m², peak sun hours 5 hours/day, performance ratio 75 %, panel rating 550 Wp, electricity tariff 7 ₹ per unit, installed cost 55,000 ₹ per kW, subsidy: Residential: PM Surya Ghar subsidy:

Recommended system size: 2.75 kW

The calculator also gives units generated per year 3,764 kWh; units per day (average) 10.3 kWh; roof area used 28 m².

Assumptions and limits

  • South facing, shadow free panels at a suitable tilt.
  • No battery. Generation falls about 0.5% a year as panels age.
  • Subsidy rules and amounts can change; check the national portal before you buy.

Frequently asked questions

How many units does a 1 kW solar system generate?

In most of India, 1 kW of rooftop solar generates about 3.5 to 4.5 units a day, or 1,300 to 1,500 units a year.

How much subsidy is available for rooftop solar?

Under PM Surya Ghar Muft Bijli Yojana, homes get ₹30,000 per kW for the first 2 kW and ₹18,000 for the third kW, so ₹78,000 for systems of 3 kW and above. Group housing societies get ₹18,000 per kW for common facilities.

How much roof space is needed for 3 kW solar?

About 30 m² (320 sq ft) of shadow free roof, using 10 m² per kW.

References

  • MNRE, PM Surya Ghar Muft Bijli Yojana guidelines (2024)
  • Central Electricity Authority, CO₂ Baseline Database for the Indian Power Sector
  • NREL, PVWatts Calculator methodology

Built and maintained by the Rispri Labs engineering team. Last updated .