About the Solar System Size Calculator
Solar array size in kWp needed to cover a given share of your electricity use.
Also known as: what size solar system, kwp calculator, solar array sizing, solar system for my home.
How to use this calculator
- Enter annual electricity use, share to cover with solar, peak sun hours per day, system efficiency and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
kWp = target kWh ÷ (sun hours × 365 × derate)
Worked example
Using the values pre-filled in the calculator above:
- Annual electricity use = 3,800 kWh
- Share to cover with solar = 80 %
- Peak sun hours per day = 3.5
- System efficiency = 80 %
- Panel rating = 450 W
- Available roof area = 40 m²
the calculator returns:
- System size: 2.975 kWp
- Panels required: 7
- Roof area needed: 14.7 m²
- Expected annual generation: 3,219 kWh