Solar Roof Area Calculator
Find how much solar capacity fits on your roof, or the area a system needs.
Please note: Estimates for planning only. Actual solar output depends on your latitude, roof orientation, shading, weather and equipment. Get a site-specific assessment and quotes before committing.
What the Solar Roof Area Calculator does
Roof capacity depends on panel dimensions rather than wattage alone. Real roofs lose a quarter or more of their area to edge setbacks, vents, chimneys and maintenance access, so the fitted area is always well below the measured area.
Formula
Panel area = Panel length × Panel widthUsable area = Roof area × Usable sharePanels that fit = ⌊Usable area ÷ Panel area⌋Capacity = Panels × Panel watts ÷ 1,000
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| What do you want to find? | one of 2 options | Yes | — |
| Roof length | number | In some modes | Accepts more than 0. Shown What do you want to find? is Capacity that fits my roof. |
| Roof length unit | one of 6 options | In some modes | Shown What do you want to find? is Capacity that fits my roof. |
| Roof width | number | In some modes | Accepts more than 0. Shown What do you want to find? is Capacity that fits my roof. |
| Roof width unit | one of 6 options | In some modes | Shown What do you want to find? is Capacity that fits my roof. |
| Target system size | kW | In some modes | Accepts more than 0. Shown What do you want to find? is Area needed for a target system. |
| Panel rating | W | Yes | Accepts 1 or more. |
| Panel length | mm | Yes | Accepts 1 or more. |
| Panel width | mm | Yes | Accepts 1 or more. |
| Usable share of roof | % | Yes | Setbacks, vents, chimneys and access paths reduce the fitted area. |
How to use it
- Choose What do you want to find?.
- Enter Panel rating, Panel length, Panel width and Usable share of roof.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
A 10 m × 6 m roof, 400 W panels measuring 1,750 × 1,100 mm, 75% usable.
- Roof
- 10 × 6 m
- Panel
- 400 W
- Size
- 1750 × 1100 mm
- Usable
- 75
Roof 60 m², usable 45 m². Each panel is 1.925 m² → 23 panels fit, giving 9.2 kW.
Frequently asked questions
Why can I not use my whole roof?
Fire codes require access pathways and ridge setbacks, and vents, chimneys and dormers break up the usable plane. 70–80% is realistic on a clean roof.
Does roof orientation matter?
Considerably. A south-facing plane in the northern hemisphere is the benchmark; east or west typically loses 15–20%, and north-facing is rarely worthwhile.