How many panels fit on my roof

Roof area divided by panel area gives a number you will never achieve. Setbacks and orientation decide the real one.

The naive calculation overstates capacity substantially, because a roof is not a clear rectangle you may fill to the edges.

Step-by-step

1. Enter the usable roof dimensions.
2. Enter your module size.
3. Compare portrait against landscape.

What you cannot use

• Edge setbacks. Panels cannot go to the roof edge — wind uplift is highest there, and regulations usually require a margin.
• Fire access. Many jurisdictions require clear pathways, often along the ridge.
• Obstacles. Vents, chimneys, aerials, skylights — plus their shadows, which extend well beyond the object.
• Shaded areas that are physically clear but not worth using.

Portrait versus landscape changes the answer

The same area fits different numbers of panels depending on orientation, because the remainder at each edge differs. On a wide shallow roof, landscape often fits an extra row; on a tall narrow one, portrait usually wins. Testing both takes a moment and occasionally gains two or three panels.

Fitting more is not always better

A panel in a corner that is shaded for three hours a day produces little and, if wired in series with others, can reduce their output too. Leaving it out sometimes increases total production.

Frequently asked questions

Why can I not fill the whole roof?

Edge setbacks for wind loading, fire access pathways required by many regulations, and clearance around vents and chimneys. Together these commonly remove a fifth or more of the apparent area.

Portrait or landscape?

Whichever fits more, which depends on the roof's proportions. It is worth testing both — the difference is occasionally two or three panels.

Should I use every available spot?

No. A panel that is shaded for hours produces little and can reduce the output of others wired with it. Sometimes fewer panels generate more.