Most solar calculators start from your electricity bill and work out how many panels you need. This one works the other way round: you have a roof of a certain size, and you want to know how big a system it can physically hold. That's the right question when roof space — not budget or demand — is your limiting factor.
The basic method
It comes down to simple division. Take your available roof area, keep only the usable fraction, then divide by the size of one panel to get the number of panels that fit. Multiply that by the panel wattage and you have the system size in kilowatts. For example, 40 m² of roof at 75% usable is 30 m² of usable space; at 2 m² per panel that's 15 panels, and at 450 W each that's about a 6.75 kWp system.
Why you can't use the whole roof
The usable percentage matters more than people expect. Fire-code setbacks around roof edges, chimneys, vents, skylights, and maintenance walkways all eat into the space, and any section that's shaded or facing the wrong way isn't worth using. On a clean, simple roof you might reach 85%; on a cut-up roof with lots of obstructions it can drop below 70%. When in doubt, be conservative.
Panel efficiency changes everything
Two panels of the same physical size can differ a lot in wattage. A higher-efficiency panel packs more watts into the same square metre, so it fits more kilowatts onto a fixed roof. That's why the chart above shows two lines — on a tight roof, higher-efficiency panels can be worth the extra cost because they turn the same area into a bigger system.
From roof capacity to real output
Knowing the system size is the first step. To see how much energy that system will actually generate where you live, feed the kilowatt figure into the energy production calculator — output depends heavily on your peak sun hours. To size a system to your electricity demand instead of your roof, use the panel calculator, and to check the payback, the ROI calculator.
Related tools
The panel calculator sizes a system from your energy use, the energy production calculator estimates output, and our guide on how many solar panels to run a house ties roof space and demand together.
Frequently asked questions
How many solar panels fit on my roof?
Divide the usable roof area by the area of one panel. A 40 m² roof at 75% usable (30 m²) fits about 14–15 panels of ~2 m² each — roughly a 6.5 kWp system at 450 W per panel.
How much roof area do I need per kW of solar?
Roughly 5–7 m² of usable roof per kilowatt with today's panels. Higher-efficiency panels need less area per kilowatt.
Why can't I use my whole roof area?
Setbacks, chimneys, vents, skylights, walkways, and shaded or wrong-facing sections all take space. A usable fraction of 70–85% is typical.