How many solar panels fit on your roof?

"Your roof fits 14 panels" says one quote; "18 panels" says the next, for the same roof. Who's right? Panel count is pure geometry, and you can check it yourself with a tape measure — or with SolarFit. Here is what the calculation actually involves.

How big is a solar panel?

A typical residential panel today is roughly 1.76 m × 1.13 m (around 435–460 Wp). Larger formats exist, but bigger panels are heavier to handle and give you less layout flexibility — on small roof faces, two medium panels often fit where one XL panel doesn't.

You can't use the whole roof

Three things shrink the usable area:

• Edge setbacks. Installers keep a margin from ridges, eaves and side edges — commonly 30 cm or more — for wind load (suction is strongest at edges and corners) and to keep the array serviceable. Local rules and mounting-system specs vary.
• Obstacles. Chimneys, velux windows, vent pipes and dormers each knock out at least one panel, and their shadow can make neighbouring positions pointless too.
• Mounting gaps. Clamps need ±2 cm between panels; rows on flat roofs need much larger gaps to avoid self-shading.

Portrait or landscape?

On a pitched roof this is the decision that usually makes or breaks a panel. Take a roof face of 7.2 m wide × 3.8 m high (after setbacks):

Orientation	Columns	Rows	Panels
Portrait (1.13 wide × 1.76 high)	6	2	12
Landscape (1.76 wide × 1.13 high)	4	3	12
Mixed: 2 rows portrait + 1 row landscape	—	—	16

Same roof, four extra panels — a third more production — just by mixing orientations. This is exactly the kind of enumeration a human quoting ten roofs a day doesn't do, and a computer does instantly.

Does more panels always mean more value?

Not automatically. A panel squeezed into a spot that's shaded half the day produces disproportionately little (though optimizers and modern half-cell panels soften the blow). And your inverter caps total output. But in most cases, with panel prices where they are, filling the geometry is the right call — the fixed costs (scaffolding, inverter, labour) dominate.

Check your own roof

SolarFit lets you enter your roof dimensions and obstacles, then tries panel layouts for you — geometry first, assumptions explicit. Free, in your browser, no cookies. Bring your own tape measure.