You don't need engineering software to get a sensible solar system size — you need your electricity bill and a bit of arithmetic. This is the same back-of-envelope method I'd use, and it's exactly what the calculator on this site automates.
Step 1: find your monthly units
"Units" are kilowatt-hours (kWh) — what your bill charges you for. If your bill shows units directly, use that. If it only shows rupees, divide your bill by your per-unit tariff:
monthly units = monthly bill ÷ tariff per unit
Example: a ₹3,000 bill at ₹8 per unit is about 375 units a month, or roughly 4,500 units a year.
Step 2: know how much one kW generates
In much of India, 1 kW of rooftop solar generates roughly 4 units per day on average across the year — call it about 1,400–1,500 units annually. It's a little less in low-sunlight regions and a bit more in sunny ones. This single number is the heart of every solar estimate.
Step 3: match the system to your usage
Divide your yearly units by what one kW generates in a year:
system size (kW) = yearly units ÷ yearly units per kW
Using the example: 4,500 ÷ ~1,460 ≈ 3 kW. That's why a 3 kW system suits so many Indian homes — and conveniently, it collects the full subsidy.
Step 4: sanity-check against your roof
Panels need space. A rough guide is about 100 sq ft of unshaded roof per kW. So a 3 kW system needs roughly 300 sq ft. If your usable roof is smaller, your roof — not your bill — sets the limit, and you size down accordingly.
Should you oversize or undersize?
- Don't oversize much beyond your usage. Generating far more than you consume means exporting cheap units to the grid (often credited at lower rates), and you may not recover the extra cost. Matching usage is usually the sweet spot.
- Slightly undersizing can make sense if roof space or budget is tight — you still offset most of your bill.
- Account for future use (an EV, an AC) if you're confident it's coming, but don't size for a "maybe".
Let the calculator do the maths
See your system size, cost after subsidy, and payback period in 30 seconds — with every assumption shown.
Putting it together
That's the whole method: units in, generation per kW, divide, check against roof. The savings calculator runs exactly these steps and then adds the subsidy and cost to show your payback. Do it yourself once, though, and you'll immediately spot when a vendor is trying to sell you a system that's bigger than you need.
A note on the numbers: figures here are typical ranges as of June 2026 and change often. The PM Surya Ghar subsidy structure, your state's top-up, per-kW prices and electricity tariffs all vary — always confirm current rates on the official portal (pmsuryaghar.gov.in) and get written quotes before you buy. This is planning information, not financial advice.