Solar Efficiency & Payback Calculator
Size panels by m2 across sunny, cloudy and rainy days, then compare investment vs kWh price for payback.
Size solar panels by the square meter
- Type the total panel area in m², the panel efficiency from the datasheet (usually 19–23%) and the peak sun hours of your site — not daylight hours, but equivalent hours at 1,000 W/m².
- Split a typical month into sunny, cloudy and rainy days. The tool weights them at 100%, 35% and 12% and normalizes by the days you typed, then applies system losses (14% default).
- Type the electricity price per kWh, the total installed investment and, optionally, your monthly consumption in kWh to get the bill coverage %.
- Read the generation per m² per day, the monthly kWh and savings, and the payback in months. Open Advanced to tune losses and yearly degradation.
- Check the 25-year table: every year degrades slightly, so the payback year already accounts for panels that produce a little less over time.
Solar efficiency questions
What does 20 m² at 21% efficiency generate?
At 5.2 peak sun hours with 20 sunny, 7 cloudy and 3 rainy days and 14% losses: peak 4.2 kWp, effective sun 3.95 h, 14.27 kWh/day, 428.1 kWh/month, 0.71 kWh per m² per day. At 0.95 per kWh that saves 406.70/month, so a 21,000 investment pays back in 51.64 months — about 4.3 years.
Why do cloudy days still generate something?
Panels also harvest diffuse light: the tool counts cloudy days at 35% and rainy days at 12% of a sunny day. A month of pure clouds still yields roughly a third of a sunny month per day — the day mix is what moves the result most after sun hours.
What does the payback assume?
Fixed electricity price, every generated kWh used or credited at that price, 0.5% yearly degradation and no maintenance, interest, tariff hikes or net-metering fees. Change the price or the degradation in Advanced and watch the payback year move — it is a simulation, not financial advice.
How many peak sun hours should I type?
Peak sun hours compress the whole day into equivalent hours at full sun: 3–4 in cloudy temperate zones, 4.5–6 in sunny subtropical regions, up to 7 in deserts. Your electricity bill or a solar map gives the local value; daylight length alone always overestimates it.