Solar System Size Calculator

By in
Updated October 27, 2026
kWh
hours
W
kWp
calculated
calculated
calculated
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How to use this calculator

Enter your monthly electricity use, local peak sun hours, performance ratio and panel rating. The system size, panel count and approximate roof area are calculated.

Example

400 kWh a month, 5 sun hours, 75% PR, 450 W panels:

  • System = 13.3 ÷ 3.75 = 3.56 kWp
  • Panels = 8
  • Roof area ≈ 17.6 m²

Frequently asked questions

How many solar panels do I need?

Divide daily use by (peak sun hours × performance ratio) to get kWp, then divide by the panel rating.

How much roof space does a panel need?

A typical 400–450 W panel is about 2 m² (around 2.0 × 1.1 m).

Should I cover 100% of my use?

It depends on local net-metering rules and budget. Many systems are sized to cover 70–100% of yearly use.

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