Find out how many solar panels will fit on your roof and what size system you can install.
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You don't need to get on the roof. Here's how to estimate your available area from the ground or from planning documents.
The simplest method. Measure the horizontal width of your roof from inside the loft space (rafter to rafter). Height: measure along a rafter from the eaves to the ridge. Don't include chimneys or valleys.
Google Maps or Apple Maps satellite view lets you estimate roof dimensions. Right-click → Measure distance to get approximate width and depth. Pitched roofs: multiply depth by 1.12–1.25 to account for the slope.
If you have original planning or building regulation documents for your home, the elevations and roof plan will show exact dimensions. These are usually the most accurate method.
Modern 430–440 Wp monocrystalline panels are the UK standard. Most installers fit the same panel format; higher-wattage options (460–480 Wp) use the same physical size but slightly different cell technology.
| Spec | Value |
|---|---|
| Typical wattage | 430 Wp |
| Width | ~1.13 m |
| Height (portrait) | ~1.72 m |
| Panel area | ~1.94 m² |
| Weight per panel | ~22 kg |
Installers may quote slightly different dimensions — always confirm with your installer before finalising the roof layout.
The calculator uses an 80% usability factor for pitched roofs and 38% for flat. Here's why real installations always use less than the gross area:
Edge setbacks
Installers leave ~300 mm from the ridge, eaves, and verges for structural and weather sealing reasons.
Panel row alignment
Panels must align in full columns and rows — partial panels can't be cut to fit. Irregular roof shapes waste more space.
Flat roof row spacing
Angled frames on flat roofs need ~3 m between rows to avoid self-shading at low winter sun angles. This dramatically reduces usable capacity vs. the gross area.
Chimneys, skylights, valleys
Obstructions require panels to be split into sub-arrays. In shaded areas, individual panels or strings must be avoided entirely.
Your installer notifies the Distribution Network Operator (DNO) after installation. No approval needed in advance. This is automatic for most single-phase homes.
Your installer must apply for G99 approval before installation. This typically takes 4–12 weeks and may require a technical assessment. Most applications are approved — it's a process, not a barrier.
Read our full guides on costs, battery storage, and the Smart Export Guarantee before you get quotes.