Solar Panels Per Square Calculator
Roofing contractors and homeowners ask the same question: how many solar panels will actually fit? Enter your sloped roof area, subtract the fire and edge setbacks, and this calculator returns the panel count, system size in kW, annual production in kWh, and the share of a typical utility bill it offsets — free, no signup.
How to calculate solar panels per square
A roofing square is 100 sq ft of roof surface, so a 20-square roof gives you 2,000 sq ft to work with. Solar modules are measured in square inches and convert cleanly: the standard 39 x 72 inch residential panel is 2,808 sq in, or 19.5 sq ft. Divide the usable roof area by that 19.5 and round down, because you cannot order a fraction of a module and you cannot buy panels that overhang the eaves.
The other half of the answer is the setback. Code and fire access rules keep panels clear of ridges, valleys, and roof edges, and leave walkways between array sections. Ten percent is a reasonable starting point on a simple gable; complex cut-ups with dormers and multiple planes need more. That is why the calculator subtracts a percentage before dividing rather than dividing the raw roof area.
Sloped roof area, not footprint. Apply your pitch multiplier first.
Fire pathways and roof-edge clearance. Start at 10%.
From a utility bill. Used for the bill-offset estimate.
Panel and site assumptions
Defaults match a common 400 W residential module. Blank fields fall back to those defaults.
Panels that fit
92
System size
36.80 kW
Annual output
42982 kWh
Bill offset
100%
Array size
92 panels x 400 W
36.80 kW
Panel footprint
3.25 ft x 6 ft
19.50 sq ft each
Roof area used
17.94 roofing squares
89.7% of roof
Usable roof area
10% set aside
1800 sq ft
Annual production
4 peak sun hours/day
42982 kWh
Electricity offset
12000 kWh annual use
100.0%
Estimated savings
$0.16/kWh
$6877/yr
| Sizing step | Assumption | Result |
|---|---|---|
| Array size | 92 panels x 400 W | 36.80 kW |
| Panel footprint | 3.25 ft x 6 ft | 19.50 sq ft each |
| Roof area used | 17.94 roofing squares | 89.7% of roof |
| Usable roof area | 10% set aside | 1800 sq ft |
| Annual production | 4 peak sun hours/day | 42982 kWh |
| Electricity offset | 12000 kWh annual use | 100.0% |
| Estimated savings | $0.16/kWh | $6877/yr |
This is area-based sizing. A stamped layout also has to satisfy fire setbacks from ridges and eaves, roof-edge clearances, vents and skylights, row-to-row shading on tilted racking, inverter and string limits, and the module count your electrical service can carry. Most installs use fewer panels than the roof can physically hold, because the array is sized to load and to the utility interconnect cap rather than to full coverage.
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Solar panel calculator formulas
- Usable roof area = roof surface area × (1 − setback %).
- Module area = panel width (ft) × panel length (ft). A 39 × 72 in panel = 3.25 × 6 ft = 19.5 sq ft.
- Panels that fit = round down (usable roof area ÷ module area).
- System size = panels that fit × module wattage ÷ 1,000.
- Annual production = system size (kW) × peak sun hours × 365 × 0.8 system efficiency.
- Bill offset = annual production ÷ annual household consumption, capped at 100%.
- Estimated savings = annual production (kWh) × local electricity rate.
The 0.8 factor is the industry rule of thumb for end-to-end system efficiency: inverter conversion, soiling, temperature coefficient, and wiring losses on a properly installed array. Peak sun hours is the one number you should localize — it runs about 4 hours per day across much of the continental United States, higher in the Southwest, lower in the Northeast. Production figures here are planning estimates, not a guarantee; a production model from the installer is authoritative.
How to measure your roof for a solar array
Two numbers work: the footprint from a satellite image, or your own measurements. If you only have the building footprint, convert it to true sloped surface area first with the Calculate your pitch multiplier tool, then enter the surface area here. Measuring from the ground works too: measure one rafter run from eave to ridge in inches, measure the rise over that same run, and use the rise-in-12 form. Multiply the footprint by the resulting factor and you have the sloped area the modules actually need.
If you are estimating materials for the roof at the same time, the shingle waste calculator turns the same square footage into bundles, underlayment rolls, and ridge cap. Many replacements are sold as roof and solar together, so both numbers come off the same takeoff.
What area-based sizing leaves out
Dividing area is a first pass, not a layout. It ignores the shape of each roof plane, so a 30-square roof split into two awkward halves may not accept 90% of the modules that pure area suggests. It also assumes one consistent module footprint, while real arrays interleave portrait and landscape modules to close out rows. Row-to-row shading is not modelled either, and on tilted racking the spacing between rows to prevent one row shading the next can consume more roof than the panels themselves.
Electrical limits bind before area does more often than contractors expect. The array still has to fit the main panel rating, the inverter capacity, and the longest string the model allows, and the utility will cap an interconnect at a share of your peak demand. That is why a full roof often holds more panels than anyone should install — the system is sized to load, not to coverage. Use this number for scoping and conversation, then confirm against a stamped layout.
Selling solar alongside a roof replacement
Solar changes the re-roof decision more than most homeowners expect. Panels have to be removed before a tear-off and reset after, which adds labor, and the roof underneath has to be sound enough to rack against. If the roof is already at the end of its life, replacing it first and racking on new decking is cheaper than doing both twice. Use the roof replacement ROI estimator to put the payback on the roof side of that conversation, then size the array with the calculator above.
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