SolarToolsHow This Tool Works
The Roof Solar Potential Calculator evaluates whether your roof is suitable for solar panels. Not every roof is a good candidate — orientation, tilt, shading, and usable area all affect how much electricity solar panels will produce. This calculator combines all those factors to give you a single "solar potential" score and estimate how many panels can fit.
A south-facing roof with no shading at 30° tilt scores 100% — optimal. East and west-facing roofs score 85–88% — still good. North-facing roofs (in the Northern Hemisphere) score 55% — usually not worth it. Heavy shading can drop any roof's potential below 50%, making solar impractical without tree removal.
The calculator also estimates how many panels fit on your usable roof area. Most roofs lose 20% of gross area to setbacks (fire code requires 3ft paths), vents, skylights, chimneys, and obstructions. The calculator accounts for this automatically.
- Measure roof area — length × width of the section facing the optimal direction. Use Google Earth or a measuring tape.
- Pick orientation — use a compass or Google Maps. South is best (Northern Hemisphere), north is worst.
- Roof tilt — estimate from the ground. Flat = 0°, typical pitched roof = 18–30°, steep = 35–45°.
- Shading — observe your roof through the day. Trees, chimneys, neighbor's buildings all matter.
- Peak sun hours — from NREL PVWatts for US, or local solar atlas.
- Panel wattage — 400W is standard. Higher wattage = fewer panels but same area.
The "solar potential %" combines all factors. Above 85% is excellent; below 55% consider ground-mount or a community solar program.
When to Use This Calculator
Orientation: south vs east vs west vs north
In the Northern Hemisphere, true south is optimal — panels face the sun's path directly. East-facing roofs produce 88% of south (more morning production). West-facing produces 85% (more afternoon production — better for TOU rate plans). North-facing produces only 55% — usually not worth installing. In the Southern Hemisphere, reverse everything: north is optimal.
Tilt: flatter vs steeper
Optimal tilt equals your latitude (30–40° for most of the US). Deviations lose 0.5% per degree. Flat roofs (0°) lose 15% but allow easier installation and maintenance. Steep roofs (45°+) lose 8–10% but shed snow better. For most homes, the roof's existing pitch is close enough — flush-mounting is standard.
Shading: the solar killer
Even partial shading dramatically reduces solar production. A single tree branch shading 10% of one panel can reduce that panel's output by 50% (string inverter) or 10% (microinverter/optimizer). Shading assessment requires a solar pathfinder or Shade Report — eyeballing from the ground is unreliable. Free tools: Sun surveyor app, Google Project Sunroof (US only).
Usable area: the 80% rule
Not all roof area can hold panels. Fire codes require 3-foot setback paths on all sides. Vents, skylights, chimneys, and valleys consume space. Complex roof shapes (hips, dormers) reduce usable area. As a rule of thumb, 80% of south-facing roof area is usable. North-facing sections are typically not used (poor production).
Panel density
A 400W panel measures about 6.5' × 3.5' = 17.5 sq ft. At 80% usable area, a 1,200 sq ft roof holds about 54 panels — a 21.6 kW system. That's larger than most homes need (typical: 7–10 kW). Roof area is rarely the constraint — shading and orientation are.
When a roof isn't suitable
If your roof scores below 55% potential (heavy shade, north-facing, or very steep), consider: (1) ground-mount solar in your yard, (2) community solar program (subscribe to a shared array), (3) tree removal if shading is the only issue (costs $500–$3,000 but can unlock 20%+ production), or (4) wait for solar carports or solar roof products to improve.
Frequently Asked Questions
Good signs: south-facing, minimal shading, 15–35° tilt, 400+ sq ft of usable area. Bad signs: north-facing, heavy tree shade, very steep or flat roof, complex shape with many dormers. Use this calculator to get a solar potential score — above 85% is excellent, below 55% is marginal.
Divide usable roof area by 17.5 sq ft (one 400W panel). A 1,200 sq ft south-facing roof with 80% usability holds about 54 panels (21.6 kW). Most homes only need 15–25 panels, so roof area is rarely the constraint — shading and orientation matter more.
Technically yes, but it's usually not worth it. North-facing panels (Northern Hemisphere) produce only 55% of south-facing. The reduced production extends payback beyond the system's warranty. Consider ground-mount, community solar, or a solar carport instead.
Dramatically. Even 10% shading of one panel can reduce that panel's output 50% (string inverter) or 10% (microinverter). Heavy shading (most of the day) can make solar impractical. Tree removal costs $500–$3,000 but can unlock 20%+ production. Get a shade report before deciding.
Yes. Flat roofs use ballasted racking (weighted blocks, no penetrations) tilted at 10–15° for drainage and optimal angle. Flat roof installations lose about 10% production vs optimal tilt but are easier to install and maintain. Common on commercial buildings; increasingly popular on modern homes.
About 200 sq ft for a minimal 5-panel (2 kW) system — enough to cover a small home or offset part of a larger bill. For a typical 7 kW system (18 panels), you need about 350 sq ft of usable south-facing roof. Most single-family homes have enough roof area; the constraint is usually shading or orientation.
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Energy & Solar Glossary
Plain-English definitions of every term used in this calculator and across the site.