# Solar Rooftop Area Calculator

> Work out how many solar panels a target system size needs, the roof area they take up, and the largest system that fits on the roof you have.

Category: Construction · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/solar-rooftop-area-calculator

Divide the system size by the panel rating to get the panel count, multiply by panel area, then allow for gaps, edge setbacks and obstructions with a usable-area fraction.

Also called: Solar panel roof space calculator.

## Inputs
- Target system size (unit kWp; default 5)
- Panel rating (unit W; default 450)
- Panel length (unit m; default 1.9)
- Panel width (unit m; default 1.1)
- Usable share of roof (default 0.75)
- Available roof area (unit m²; default 50; optional)

## Results
- Roof area needed
- Panels needed
- Installed size
- Panel area
- On your roof
- Most panels that fit
- Largest system that fits

## Formula

```
Panels = ⌈Target W ÷ Panel W⌉;  Roof needed = Panels × Length × Width ÷ Usable share;  Max panels = ⌊Roof × Usable share ÷ Panel area⌋
```

Where:
- Usable share: fraction of the roof that can hold panels

## Worked example
1. Panels = ⌈5,000 ÷ 450⌉ = 12 (5.4 kWp).
2. Panel area = 12 × 1.9 × 1.1 = 25.08 m²; ÷ 0.75 = 33.44 m² of roof.
3. A 50 m² roof holds ⌊50 × 0.75 ÷ 2.09⌋ = 17 panels = 7.65 kWp.

**Result:** 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.

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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/solar-rooftop-area-calculator