The number of solar panels you need comes down to three things: how many units of electricity you use, how much sunshine your state gets, and the wattage of the panels you buy. Here is the exact method, with tables for typical Indian homes.
Your electricity bill shows the units (kWh) you used in the billing period. Use the average of the last 12 months if you can, because summer bills with air-conditioning can be twice the winter ones. If you only know the rupee amount, divide it by your tariff: a ₹3,000 bill at ₹7 per unit is about 430 units. Your state's electricity tariff is listed on our tariff pages.
Divide your daily use by your state's peak sun hours:
Peak sun hours are the number of hours a day of full-strength sunlight your location gets on average. They range from about 4.5 in Kerala and West Bengal to 6.5 in Rajasthan. For 300 units a month at 5 peak sun hours: 300 ÷ 30 = 10 units a day, and 10 ÷ 5 = 2 kW. This is exactly how the rooftop solar calculator sizes an on-grid system, rounded up to the next 0.1 kW.
Real panels never deliver their full rating. Heat, dust, wiring and the inverter cost about 22%, so each kW produces roughly 4 units a day (about 1,400 a year) at 5 peak sun hours. Our solar generation per kW guide lists the figure for every state. Sized this way, the system covers around 80% of your annual use, which keeps most of the output for your own consumption rather than export at the lower buyback rate. If you want to offset close to 100% of your units, divide by 0.78 as well: 300 units a month then needs about 2.6 kW.
Rooftop panels sold in India today are mostly in the 400–450 W and 500–550 W ranges, with larger 560–600 W+ modules used on bigger roofs. A 2 kW system is 2,000 W, so it needs 2,000 ÷ 550 = 3.6, rounded up to 4 panels of 550 W, or 2,000 ÷ 400 = 5 panels of 400 W.
Because you can only buy whole panels, the installed capacity usually ends up slightly above the target. Four 550 W panels are 2.2 kWp, not 2 kWp. That is normal, and the PM Surya Ghar subsidy is worked out on the rated capacity of the panels actually installed. To claim the subsidy, the panels must also be domestically manufactured (DCR) modules installed by a registered vendor. See the subsidy guide for the slabs.
Figures below use 5 peak sun hours and the same sizing rule as the calculator. Panel area is for Mono PERC panels (about 19.5% efficient). The roof figure adds space for walkways and shade-free gaps.
| Monthly units | System size | 400 W panels | 550 W panels | Panel area | Roof to plan | Units per year |
|---|---|---|---|---|---|---|
| 100 | 0.7 kW | 2 | 2 | 39 sq ft | 55–70 sq ft | ~990 |
| 150 | 1.0 kW | 3 | 2 | 55 sq ft | 80–100 sq ft | ~1,420 |
| 200 | 1.4 kW | 4 | 3 | 77 sq ft | 110–140 sq ft | ~2,010 |
| 300 | 2.0 kW | 5 | 4 | 110 sq ft | 160–200 sq ft | ~2,850 |
| 400 | 2.7 kW | 7 | 5 | 149 sq ft | 215–270 sq ft | ~3,830 |
| 500 | 3.4 kW | 9 | 7 | 188 sq ft | 270–340 sq ft | ~4,860 |
| 600 | 4.0 kW | 10 | 8 | 221 sq ft | 320–400 sq ft | ~5,690 |
The same 300 units a month needs a bigger system where there is less sun. These sizes use each state's peak sun hours from the calculator:
| State | Peak sun hours | System size | 400 W panels | 550 W panels |
|---|---|---|---|---|
| Kerala, West Bengal | 4.5 | 2.3 kW | 6 | 5 |
| Tamil Nadu | 5.0 | 2.0 kW | 5 | 4 |
| Karnataka | 5.3 | 1.9 kW | 5 | 4 |
| Delhi, Maharashtra | 5.5 | 1.9 kW | 5 | 4 |
| Gujarat | 5.8 | 1.8 kW | 5 | 4 |
| Rajasthan | 6.5 | 1.6 kW | 4 | 3 |
Each of these systems produces roughly the same 2,850–3,000 units a year, because the extra panels in cloudier states make up for the weaker sun.
Mono PERC panels take about 55 sq ft per kW (about 50 sq ft with TOPCon and 67 sq ft with polycrystalline). Including walkways, edge clearance and gaps that keep rows from shading each other, plan for 80–100 sq ft of shadow-free roof per kW. A 3 kW home system therefore needs about 240–300 sq ft. If space is tight, higher-efficiency panels fit more capacity on the same roof. Our panel types guide compares them.
Enter your bill or appliances and your state. The calculator gives system size, number of panels, roof area, cost after subsidy and payback.
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