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Free solar calculator

Solar System Size Calculator

Estimate how many kW and panels may be needed to cover your chosen share of annual electricity use.

Estimate your solar system size

Start from electricity use and the share you want solar to cover.

kWh
%
kWh/kW
%
W
m²
m²
✓ Planning estimate only
Your system size will appear hereEnter your annual usage and target coverage.

Your size estimate

Based on annual energy, not roof design.

Usage-based
Suggested solar capacity
4.3 kW
Approximately 10 panels
Estimated panel count
10
Estimated panel area
20 m²
Estimated annual generation
3,360 kWh
Target demand coverage
80%
Maximum panels by roof area
15
Roof fit check
Likely fits

A better starting point than guessing panel count

This calculator works backwards from annual electricity demand and local solar yield.

Target generation

Annual electricity use multiplied by desired solar coverage.

Usage × Coverage target

Required capacity

Target generation divided by effective solar yield.

Target kWh ÷ Effective kWh per kW

Panel count

Required capacity divided by panel wattage and rounded up.

Required watts ÷ Panel watts

Solar system size FAQs

Does this guarantee the number of panels I need?

No. Shading, orientation, roof pitch, inverter sizing, local rules and layout can change the final design.

Can solar cover more than 100% of annual use?

It can on an annual-energy basis, but exports, seasonal mismatch and local rules still matter.