Solar Panel Roof Area Calculator
Estimates how many solar panels fit on a roof section and calculates expected power output, accounting for usable area, tilt, and shading. Ideal for homeowners planning a solar installation.
About this calculator
Not all roof area is usable — vents, chimneys, setbacks, and structural constraints typically reduce it. This calculator applies a 70% usable-area factor to the raw roof dimensions. Each standard residential panel occupies roughly 17.6 sq ft. The power output formula is: Output (W) = floor((roofLength × roofWidth × 0.7) / 17.6) × panelWattage × (roofTilt / 100) × shadingFactor. The tilt factor represents how much of the panel's rated output is captured at your specific roof angle relative to ideal (a value near 1.0 for pitches around 30–35°). The shading factor (0 to 1) reduces output for trees, neighbouring buildings, or roof features that cast shadows. Together these factors give a realistic rather than theoretical peak wattage estimate.
How to use
Say your roof section is 30 ft long and 20 ft wide, with a tilt factor of 0.97, a shading factor of 0.95, and 400 W panels. Step 1 — usable area: 30 × 20 × 0.7 = 420 sq ft. Step 2 — number of panels: floor(420 / 17.6) = floor(23.86) = 23 panels. Step 3 — apply tilt and shading: 23 × 400 × (0.97) × 0.95 = 23 × 400 × 0.9215 = 8,478 W. Your roof can support approximately 23 panels with an estimated peak output of about 8.5 kW.
Frequently asked questions
How much roof area does a solar panel actually require?
A standard 60-cell residential solar panel measures approximately 65 × 39 inches, which is about 17.6 square feet. Premium 72-cell or large-format panels can be closer to 21–23 square feet. When planning a layout, you must also account for racking hardware clearance and local fire code setbacks, which typically require 3-foot clear pathways. This is why calculators use a 70% efficiency factor on gross roof area — it is a realistic planning rule of thumb.
How does roof tilt angle affect solar panel energy production?
The optimal roof tilt angle equals your latitude, pointing panels directly at the sun's average position. In the continental US, that is roughly 30–35°, which corresponds to a common 6:12 or 7:12 roof pitch. At the optimal angle, the tilt factor is effectively 1.0. Shallow roofs (less than 15°) or steep roofs (over 45°) can reduce annual output by 5–15% compared to the optimum. Mounting systems can compensate somewhat, but the roof's inherent pitch remains the primary determinant.
What shading factor should I use when calculating solar panel output?
A shading factor of 1.0 means zero shading and full output. Minimal shading from minor obstructions or distant trees typically warrants a factor of 0.95–0.98. Moderate shading — a nearby tree or chimney shadow for a few hours daily — might reduce output to a factor of 0.80–0.90. Heavy shading below 0.70 generally makes a roof section uneconomical for solar. Modern microinverters and DC optimisers can mitigate shading losses panel-by-panel, potentially improving the effective factor by 5–10%.