Solar Panel Calculator

How to size a solar panel system

1. Enter the energy you use in a day, in kWh, from your electricity bill.
2. Enter the peak sun hours for your location and the system losses.
3. Enter the panel rating, and the days of autonomy and battery details for an off-grid system.

Frequently asked questions

What is the formula?

Array size (kWp) = daily kWh ÷ (peak sun hours × (1 − losses)). 10 kWh a day with 4 peak sun hours and 20% losses needs 3.13 kWp, which is seven 450 W panels.

What are peak sun hours?

The number of hours a day the sun would need to shine at full strength (1,000 W/m²) to deliver the same energy. It is typically 2 to 3 in northern Europe and 5 to 6 in sunny regions, and it changes a lot between summer and winter.

What losses should I use?

About 20% covers the inverter, wiring, dust, heat and panels not facing the sun exactly. Shading adds much more.

How big should the battery be?

Days of autonomy × daily use ÷ the share of the battery you can use. Lithium batteries usually allow 80 to 90%, lead-acid about 50%.

Are my numbers sent anywhere?

No. Everything is worked out in your browser and nothing is stored or uploaded.