Solar ROI calculator
Size a rooftop solar system from your electricity use and location, then see payback, NPV, IRR, the cost per kWh of solar energy, financing, and a year-by-year cash flow over its life.
Added
Works with
- Energy usage analyser Home automation and devicesopens a system here with your annual use and prices
How to use it
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Use and location — annual electricity use (add up twelve months of bills), the share to cover, and the specific yield: kWh produced per year per kW of panels. The location list gives typical figures for a well-oriented, unshaded roof; for your exact roof, tilt, and shading use NREL PVWatts (US) or PVGIS (Europe, Africa, Asia) and type the result in. Size = use × share ÷ yield, rounded up to whole panels; or type a system size to override.
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Cost and incentives — installed cost per watt (quotes typically run 2.5–4 $/W in the US before incentives), a percentage tax credit or grant, and any fixed rebate. Tick finance to pay part with a loan: the monthly payments then come out of the yearly cash flow.
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Electricity prices — your marginal price per kWh (the rate on the last kWh, including delivery charges), its yearly rise, the rate paid for exported energy, and the share of production used on site. With full net metering set the export rate equal to the price; with export tariffs, self-use is worth far more than export, which is when batteries and timing loads matter.
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Lifetime assumptions — panel degradation (≈0.5%/yr for modern panels), maintenance, an inverter replacement (string inverters often last 10–15 years), the discount rate for NPV, the analysis period, and grid carbon intensity for the CO₂ figure.
Results: payback is when cumulative cash turns positive (interpolated within the year). NPV discounts each year’s net cash at your discount rate; IRR is the rate at which NPV is zero — the equivalent interest rate the system earns. LCOE is discounted lifetime cost ÷ discounted lifetime energy: if it is below your electricity price, solar is the cheaper source. Everything is a model: weather, tariffs, and policy change, and incentives are assumed to arrive at once.
Sharing
Share link copies a URL that opens this tool exactly as configured. Every assumption, so the link reproduces the same payback and NPV. Fields you leave out keep their defaults.
For example: solar-roi/?use=8000&price=0.28¤cy=EUR.
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