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Solar Panel Calculator: What Size Rooftop Solar Do I Need?

Enter your monthly units from the electricity bill and the panel wattage. You get the system size in kW, panels needed, roof area and yearly generation.

Solar system size

2.5kW

Panels
5 × 540 W
System with whole panels
2.7 kW
Roof area needed
27 m² · 290.6 sq ft
Yearly generation
3,942 units (kWh)

Estimate only. Real results depend on your equipment, its age and conditions.

PM Surya Ghar subsidy as of 27 Oct 2026: ₹30,000/kW up to 2 kW, ₹18,000 for the 3rd kW, max ₹78,000. Check the portal for current rules.

Show working
  1. 300 ÷ 30 = 10 kWh/day
  2. 5 × 0.8 = 4 kWh/kW/day
  3. 10 ÷ 4 = 2.5 kW
  4. 2,500 ÷ 540 = 4.63 → 5

How to use

  1. Enter average monthly units from your bills.
  2. Check peak sun hours for your city (4.5–5.5 in most of India) and panel wattage.
  3. See system size, panels and roof area.

Good to know

System kW = daily units ÷ (peak sun hours × (1 − losses)). Daily units = monthly ÷ 30. With 300 units a month, 5 sun hours and 20% losses (heat, dust, wiring, inverter), you need about 2.5 kW; with 540 W panels that is 5 panels (2.7 kW).

MNRE’s consumer FAQ (checked 27 Oct 2026) says 1 kWp generates about 4 to 5.5 units a day on a clear day and needs about 10 m² of shadow-free roof. Peak sun hours vary by region and season; Rajasthan and Gujarat are higher, the north-east and monsoon months lower.

Under PM Surya Ghar Muft Bijli Yojana (as checked on 27 Oct 2026), central subsidy for homes is ₹30,000 per kW up to 2 kW, ₹18,000 for the third kW, capped at ₹78,000 for 3 kW and above. Subsidy, net-metering rules and approvals come from your DISCOM and the national portal; check current terms there.

Questions

How many kW solar do I need for 300 units a month?

About 2.5 kW at 5 sun hours and 20% losses, which is 5 panels of 540 W.

How much roof area does 1 kW need?

About 10 m² (around 108 sq ft) of shadow-free roof, as per MNRE’s guidance.

How many units does a 1 kW system make?

About 4–5.5 units a day in clear weather, roughly 1,300–1,600 units a year in most of India.

What is the subsidy for a 3 kW system?

As of 27 Oct 2026, ₹78,000 central subsidy under PM Surya Ghar (₹30,000/kW for 2 kW + ₹18,000 for the 3rd). Check pmsuryaghar.gov.in for current rules.

Should I size for my highest bill?

Size for your yearly average. Net metering credits extra summer units against other months, subject to your DISCOM’s rules.

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