Solar Roof Analysis Calculator
Estimates the peak kilowatt capacity a rooftop can support based on available area, tilt, orientation, and shading. Use it before getting installer quotes to understand your roof's realistic solar potential.
Last updated: September 2026
Formula below · 2 sources (nrel.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The capacity a roof can host is governed by how many panels fit, and its useful output by how favorably the roof faces the sun and how much shade is present. The formula is: Effective size (kW) = (roofArea / 20) × panelWatts × tiltFactor × shadingFactor × orientationFactor / 1000. Dividing roofArea by 20 estimates the number of panels (about 20 sq ft per panel including spacing); multiplying by panelWatts gives the nameplate wattage. The three dropdown factors are multipliers between 0 and 1 that derate that nameplate for the roof: tilt (1.0 for an optimal 30–40° pitch, 0.9 flat, 0.95 steep, 0.85 very steep), shading (1.0 none down to 0.6 heavy) and orientation (1.0 south, 0.95 southeast/southwest, 0.85 east/west, 0.7 north). The result is therefore the nameplate size of an ideally sited system that would produce the same energy; the number of panels that physically fit is roofArea / 20. Dividing by 1000 converts watts to kilowatts.
How to use
Say you have 800 sq ft of usable roof, optimal 30–40° tilt (factor 1.0), south-facing (factor 1.0), no shading (factor 1.0), and 400 W panels. Panel count = 800 / 20 = 40. Each derate factor multiplies the raw capacity: 40 × 400 W × 1.0 (tilt) × 1.0 (shading) × 1.0 (orientation) / 1000 = 16.0 kW. A north-facing roof (0.7) with heavy shading (0.6) would instead give 16.0 × 0.7 × 0.6 ≈ 6.7 kW. Pick the dropdown factors that match your roof to see its realistic peak capacity.
Frequently asked questions
How does roof orientation affect solar panel output and system size?
True south orientation (azimuth 180° in the Northern Hemisphere) captures the most sunlight throughout the day, giving a roof orientation factor of 1.0 in this calculator. Roofs facing southeast or southwest lose roughly 10–15% of potential output, while east- or west-facing roofs (90° or 270°) lose around 20–30%. North-facing roofs in the Northern Hemisphere are generally unsuitable for solar. If your roof is not ideally oriented, you can partially compensate by adding more panels to the available area, though at higher upfront cost.
What shading factor should I use for trees or nearby buildings near my solar roof?
The shading factor is a multiplier: 1.0 means no shading, 0.9 minimal, 0.8 moderate and 0.6 heavy shading (for example a large tree or neighbouring building shading the roof for several hours a day). Shading is especially damaging in traditional string-inverter systems because one shaded panel can reduce output of the entire string; microinverters or DC optimisers significantly reduce this penalty. A professional shade analysis tool such as SolarEdge's Designer or Google's Project Sunroof can give you a site-specific estimate.
How much roof area do I need per solar panel and how many panels fit on an average roof?
A standard residential solar panel measures roughly 17–20 square feet (about 1.6 m²), so this calculator uses 20 sq ft as a per-panel allocation including spacing. An average U.S. home has 1,500–2,000 sq ft of total roof area, but usable south-facing sections after excluding vents, skylights, and setbacks are typically 300–600 sq ft, accommodating 15–30 panels. At 400 W per panel that translates to 6–12 kW of installed capacity — enough to cover most or all of a typical household's annual electricity consumption.