☀️Solar Panel Calculator

About this tool

Enter your daily electricity use, or your monthly kWh from a bill, and the peak sun hours where you live. Then enter the panel rating, system losses, the share of your use you want to cover, the area of one panel, the installed cost per watt and your electricity price. The calculator shows the system size, the number of panels, yearly production, the roof area needed, the estimated cost, yearly savings and the simple payback time.

How to size a solar system

Peak sun hours are the number of hours of full-strength sunshine, 1,000 watts per square metre, that a day's light adds up to, averaged over the year. They range from about 2.5–3 in the UK and northern Europe to 4–5 across much of the US and southern Europe, and 5–6 or more in sunny regions. System size = daily use ÷ (sun hours × (1 − losses)). A home using 30 kWh a day with 5 sun hours and 20% losses needs 7.5 kW. That is 19 panels of 400 W, or 7.6 kW, producing about 11,100 kWh a year on roughly 37 m² of roof. At an installed 2.80 per watt the system costs about 21,280. At 0.16 per kWh it saves about 1,750 a year, a simple payback of about 12 years before any incentives. Losses cover the inverter, wiring, heat, dirt and shading, and are usually 14–25%. Honest limits: savings assume every unit produced replaces one you would buy, but export tariffs, net-metering rules and batteries change that a lot, and roof direction, tilt and shade matter. Get a site survey before you buy. Not financial advice. The electricity cost calculator shows what your appliances use, the ROI calculator compares the return with other investments and the battery life calculator handles storage maths. Which appliances actually use the most electricity helps cut the load first.

Frequently asked questions