Rooftop Solar Calculator (system size in kW)
Enter your monthly electricity units to estimate the solar system size, number of panels and roof space you need.
Quick answer
300 units a month: about 2.67 kW system, 5 panels.
How it is calculated
- Daily units = monthly units ÷ 30 = 10 kWh.
- System kW = daily units ÷ (sun hours × efficiency) = 10 ÷ (5 × 0.75) = 2.67 kW.
- Panels = system watts ÷ panel watts, rounded up. Roof: about 100 sq ft per kW.
Worked example
A home uses 300 units a month. With 5 sun hours a day and 75% efficiency it needs about 2.67 kW: 5 panels of 540 W, about 267 sq ft of roof.
| Monthly units | 300 kWh |
| Sun hours | 5 h/day |
| Panel wattage | 540 W |
| System efficiency | 75 % |
| System size | 2.67 kW |
| Panels | 5 |
| Roof area (approx) | 267 sq ft |
Formula
kw = Um/30/(Sh×Pr/100) System size = kw Panels = ceil(kw×1000/Pw) Roof area (approx) = kw×100
Good to know
- Check government subsidy (PM Surya Ghar) and net-metering rules with your electricity board.
- Shade on the roof cuts output a lot.
Questions people ask
How many units does 1 kW of solar make a day?
Usually 4–5 units a day in India, depending on season and place.
How many kW for 300 units?
About 2.7–3 kW.
Roof space for 1 kW?
About 80–100 sq ft of shade-free roof.
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Updated 13 Nov 2026 · Desktop N Calculator v1.2.0