How to use this calculator
- Enter or choose target system size.
- Enter or choose panel rating.
- Enter or choose panel length.
- Enter or choose panel width.
- Enter or choose usable share of roof.
- The roof area needed updates as you type. Use Copy result or Share to keep it.
Method
Panels = ⌈Target W ÷ Panel W⌉; Roof needed = Panels × Length × Width ÷ Usable share; Max panels = ⌊Roof × Usable share ÷ Panel area⌋
- Usable share
- fraction of the roof that can hold panels
Worked example
- Panels = ⌈5,000 ÷ 450⌉ = 12 (5.4 kWp).
- Panel area = 12 × 1.9 × 1.1 = 25.08 m²; ÷ 0.75 = 33.44 m² of roof.
- A 50 m² roof holds ⌊50 × 0.75 ÷ 2.09⌋ = 17 panels = 7.65 kWp.
A 5 kW target needs 12 panels of 450 W and about 33 m² of roof; a 50 m² roof could take up to 7.65 kWp.
What the result means
Use this for an early feasibility check. Orientation, tilt, shading and roof structure decide whether the space is worth using.
Assumptions and limits
- All panels are the same size and rating.
- The usable share covers row gaps and setbacks.
- Does not estimate energy yield.
- Roof loading, wind uplift and local setback rules need checking by an installer.
Questions
Why round the panel count up?
You cannot install part of a panel, so meeting the target means the next whole panel, which slightly exceeds it.
Does a sloped roof change the area?
Enter the sloped roof area you can mount on; panels lie flat on the roof plane.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.