Roof Area to Solar Panels Calculator

How much roof do you need for solar?

Most calculators start from your bill and tell you the kW you need; this one starts from the roof and tells you the kW you can host — which is the real constraint for many town houses and apartments. The arithmetic is simple: a modern 540 W module occupies about 2.55 m² (roughly 27.5 sq ft), so after deducting unusable strips and row spacing, a typical flat roof yields about a kilowatt for every 80–110 sq ft of gross area.

Why the usable share matters most

Hand a designer a ‘600 sq ft roof’ and the first thing they subtract is everything that is not really available: the water tank and its shadow, the staircase headroom, parapet set-backs, and the walkways you need for cleaning access. On flat roofs, tilted rows must also be spaced so the front row does not shade the one behind it in winter — that alone costs 20–30% of the area. Being honest about the usable share up front (the default here is 70%) avoids the classic disappointment of a quote for half the panels the gross area suggested.

Pitched vs flat roofs

A sun-facing pitched roof is the efficient case: panels mount flush, pack at ~90%, and the tilt is built in. A flat roof gives you freedom to choose the optimal tilt angle but pays for it in row spacing. East–west ‘butterfly’ layouts on flat roofs pack far more kW per square metre at a small per-panel yield penalty — worth asking your installer about if area is tight.

From fit to bill savings

Once you know the kW your roof supports, check whether it covers your usage with the system size calculator, estimate output precisely with the output calculator, and turn it into money with the savings calculator. If the roof supports less than you need, that is worth knowing before anyone quotes you.

Frequently asked questions

With modern 540–575 W panels, about 5–6 m² of panel area — which usually means 80–110 sq ft of gross roof once usable share and row spacing are counted. Older 330 W panels needed ~100 sq ft of panels alone.

Typically 12–14 modern panels (about 6.5–7.5 kW) if 70% is usable at an 85% packing factor — but tanks, shading and set-backs vary hugely, which is why the usable-share input matters. Run your own figures above.

Yes — tilted rows must be spaced so each row clears the winter shadow of the one in front, typically costing 20–30% of the area. Flush east–west layouts trade a little per-panel yield for much denser packing.

Both: the bill tells you what you need, the roof tells you what is possible. If the roof supports less than you need, you can prioritise high-value loads or consider higher-wattage panels; if it supports more, net metering economics decide whether to fill it.