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Calculator · Updated August 2026
Roof area to production calculator
Area, orientation and shading decide production far more than panel brand does.
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The calculator
From roof area to production
Assumes about 18 sq ft per 400 W panel. Usable area is not total roof area — fire code setbacks, vents, chimneys and skylights typically remove 20–30% before you start.
This is an estimate, not a savings guarantee.
It uses published utility rates, NREL regional sun data and a standard system derate. It does not know your roof orientation, your shading, your usage pattern or what your utility will charge next year. No solar company can honestly guarantee savings, and any that does is describing a model rather than making a promise.
Usable roof area is smaller than your roof
Start from the plane you would actually use, not the building footprint. Then remove fire code setbacks — most jurisdictions require clear pathways along ridges and eaves — plus plumbing vents, chimneys, skylights, dormers and any area shaded by an adjacent roof section. On a typical house this removes 20 to 30% before a single panel is placed. Complex roofs with many planes lose more.
Why orientation is worth more than hardware
A south-facing plane in the northern hemisphere is the reference. Southeast and southwest lose roughly a tenth. East and west lose around a fifth. Northeast and northwest lose enough that they are usually not worth the hardware and the roof penetrations. That spread is larger than the difference between any two mainstream panel brands, which is why a conversation about orientation is more useful than a conversation about modules.
Roof pitch matters too, though less. Shallow pitches lose a little in winter and gain a little in summer; steep pitches do the reverse. On a normal residential pitch the effect is small enough to ignore next to orientation and shading.
Shading is the biggest single variable
Shade is not uniform and it is not annual. A tree that clears the array by 10am costs far less than one that shades it from 11am to 2pm, because midday hours carry most of the production. Modern module-level electronics reduce the penalty of partial shading compared with older string systems, but they do not eliminate it — a shaded panel still produces less light-limited output regardless of how it is wired.
Shading also changes over time. A tree that is marginal today will not be in twelve years. Any honest assessment accounts for growth, or at least tells you it did not.
What this calculator cannot see
Roof condition and remaining life — putting an array on a roof with eight years left means paying to remove and reinstall it. Structural capacity, which occasionally rules out a design. Electrical service size, since older 100-amp panels frequently need upgrading. And the actual geometry of your roof planes, which no square-footage input can capture.
That last one is why we read your address against satellite imagery before anyone visits, rather than asking you to measure anything.
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Who you would be working with
SolarAmerica
We design your system and manage the project; installation is performed by vetted licensed partners in your area. SolarAmerica is not itself the installing contractor.
Questions
The things people actually ask
How much roof do I need for solar?
Roughly 18 square feet per 400 W panel, so a 9 kW system needs about 400 square feet of usable area. Usable is the operative word — setbacks, vents and obstructions typically remove 20 to 30% of a roof plane.
How many solar panels fit on my roof?
Divide usable area by about 18 square feet per panel. A 600 square foot usable plane fits roughly 33 panels, or about 13 kW.
Does a west-facing roof work for solar?
Yes, at roughly 78% of the output of an equivalent south-facing plane. In areas with time-of-use rates and expensive late-afternoon pricing, a west-facing array can be worth more per kilowatt-hour than the raw production figure suggests.
How much does shade reduce solar production?
It depends entirely on when the shade falls. Midday shading is far more costly than early morning or late evening, because those hours carry most of the production.
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