Work out how many years solar panels take to pay back at your electricity rate.
Payback is the year in which cumulative savings overtake the installed cost, and getting it right needs two effects that a simple division misses. Panels degrade, losing roughly half a percent of output per year, so a twenty-year-old array produces about ninety percent of its first-year yield. Electricity prices rise, which pulls payback earlier — at three percent a year the savings in year ten are a third larger than in year one. The two work in opposite directions and the price rise almost always dominates. Annual yield per kilowatt installed is the input that matters most and varies enormously by location: roughly 850 kWh in northern Britain, 1,250 in central Europe or the north-eastern United States, and over 1,700 in Arizona or Andalusia.
Payback with degradation and rising prices
Annual generation = system kW x yield per kW; saving in year Y = generation x 0.995^(Y-1) x rate x (1 + rise)^(Y-1); payback is the year cumulative savings first exceed the installed cost, interpolated within that year
A projection based on the assumptions you enter, not a financial forecast or advice. Actual yield depends on orientation, pitch, shading and weather; future electricity prices, tariffs and incentives are uncertain and can change materially. Obtain a site-specific assessment before committing.
The yield per kilowatt and the electricity rate, roughly equally — they multiply together to set the saving. Getting the yield right for your latitude, roof pitch and orientation matters far more than fine-tuning the price rise.
Yes. It values every generated kilowatt-hour at the retail rate you avoid. If you export a large share at a lower feed-in tariff, blend the two rates down accordingly — self-consumption is what makes solar pay.
Budget for one inverter replacement around year twelve to fifteen, typically ten to fifteen percent of the system cost. Add it to the installed cost if you want a conservative twenty-year figure.