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.
About this calculator
The system capacity (kW) a roof can host is governed by how many panels fit, how favorably the roof faces the sun, and how much shade is present. The formula used here is: Capacity (kW) = (roofArea / 20) × panelWatts × (roofTilt / 90) × (1 − shadingFactor) × (1 − |roofOrientation − 180| / 180) / 1000. Dividing roofArea by 20 estimates the number of panels (assuming ~20 sq ft per panel). Multiplying by panelWatts gives raw wattage. The tilt factor (roofTilt/90) scales output relative to a perfectly vertical surface; a 30° tilt yields 0.33. The orientation factor penalises roofs that deviate from true south (180°). The shading factor (0 = no shade, 1 = full shade) reduces effective output. Dividing by 1000 converts watts to kilowatts.
How to use
Say you have 400 sq ft of usable roof, a 30° tilt, south-facing orientation (180°), 10% shading factor (0.10), and 400 W panels. Panels = 400 / 20 = 20. Raw watts = 20 × 400 = 8,000 W. Tilt factor = 30 / 90 = 0.333. Shading factor = 1 − 0.10 = 0.90. Orientation factor = 1 − |180 − 180| / 180 = 1.0. Capacity = 8,000 × 0.333 × 0.90 × 1.0 / 1000 ≈ 2.40 kW. A due-south roof with minimal shading maximises the result.
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 decimal from 0 (no shading) to 1 (completely shaded). Light partial shading from a distant tree might warrant a factor of 0.05–0.10, while heavy shading from a large neighbouring building could reach 0.30–0.50. 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 precise site-specific shading factor.
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.