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How to calculate your solar payback period

Payback period is the year your cumulative electricity savings first catch up to what you spent upfront. A rough "cost ÷ annual savings" divide gets you in the right neighborhood, but it quietly ignores two things that pull the real answer in opposite directions. Here's the full calculation.

The formula

Why degradation and inflation both matter

Solar panels lose a small amount of output every year as they age — typically around 0.5% annually for quality panels — so a system generating 8,000 kWh in year one might generate closer to 7,600 kWh by year ten. On its own, that stretches out payback. Working against it, grid electricity tends to get more expensive over time. If your tariff rises 5% a year, each kWh your panels generate is worth more in year ten than it was in year one, even though there are slightly fewer of them. These two effects partially cancel out, which is exactly why a flat "divide the cost by year-one savings" estimate is usually a bit pessimistic — real payback tends to arrive a little sooner once inflation is factored in.

A full worked example

Take a 5 kW system costing Rs 800,000 upfront, generating 22 kWh a day (5.5 sun hours × 5 kW × 0.8 efficiency) against a tariff of Rs 55/kWh, with 5% annual inflation and 0.5% annual degradation.

A flat estimate (Rs 800,000 ÷ Rs 441,650) would have suggested roughly 1 year and 10 months — close, but the year-by-year version accounting for inflation is the more defensible number, and the gap widens the further out you project. The calculator above runs this exact year-by-year projection for 25 years and plots the full curve, so you can see the crossover point rather than just the final answer.

What a "good" payback period looks like

There's no universal number — it depends entirely on local electricity prices and installation costs, which vary enormously by country. In markets with high grid tariffs and relatively low installation costs, payback in the 2–4 year range isn't unusual. In markets with cheaper grid electricity or higher installation costs, 6–10 years is more typical. What matters more than comparing your number to a rule of thumb is comparing your payback period to the panels' expected working life — most quality panels are warrantied for 25 years, so even an 8-year payback still leaves well over a decade and a half of largely free electricity afterward.

Inputs that move your payback period the most

• Installation cost per kW — the single biggest lever, and the one most worth getting multiple quotes on.
• Electricity tariff — higher grid prices mean each kWh you generate is worth more, shortening payback.
• Grid offset percentage — a larger system costs more upfront but saves proportionally more each year; the payback period itself often changes less than people expect.
• Regional sun hours — more sun hours means more generation from the same array, improving payback without touching cost.