How to use it
- Choose whether your figure is per day, month or year and enter the kWh from your bill.
- Enter the average peak sun hours for your location.
- Keep 80 % system efficiency or enter your own assumption.
- Enter the panel power in watts and the share of your use you want to cover.
- Read the system size, the number of panels and the yearly output.
The formulas
- daily use = monthly kWh × 12 ÷ 365 (or yearly ÷ 365);
- system size (kW) = daily use × share ÷ (peak sun hours × efficiency);
- panels = system size × 1000 ÷ panel watts, rounded up;
- yearly output = panels × panel watts ÷ 1000 × sun hours × efficiency × 365.
Worked example
A home using 900 kWh a month, 5 peak sun hours, 80 % efficiency, 400 W panels, covering 100 %:
- daily use 900 × 12 / 365 = 29.6 kWh;
- system size 29.6 / (5 × 0.8) = 7.4 kW;
- panels 7,397 / 400 = 18.5, so 19 panels, 7.6 kW;
- yearly output 7.6 × 5 × 0.8 × 365 = 11,096 kWh, about 103 % of the 10,800 kWh used.
Assumptions and tips
- The sizing uses a yearly average: panels make more in summer and less in winter.
- The calculator does not include batteries; without them, surplus daytime power goes to the grid or is lost.
- Check the roof area too: a typical residential panel takes roughly 1.7 to 2 m².