Calculating Roof Area for Solar Panel Sizing
Solar installers do their first sizing pass from satellite imagery long before they put a ladder against your house. Here's how to do the same — and what corrections matter once you have a footprint number.
Footprint vs surface area
A common mistake is to assume satellite measurements give you the actual roof surface. They don't. From overhead, you measure the horizontal footprint — the shadow the roof would cast at noon if the sun were directly above. The actual sloped surface is larger.
The conversion depends on roof pitch:
- 3:12 pitch (about 14°) — surface area is ~3% larger than footprint
- 6:12 pitch (about 27°) — surface area is ~12% larger
- 9:12 pitch (about 37°) — surface area is ~25% larger
- 12:12 pitch (45°) — surface area is ~41% larger
For a typical residential roof at 6:12, multiply your satellite footprint by 1.12 to estimate the actual roof surface. If you don't know the pitch, look at the eaves in street view — shallower pitches are obvious; steeper roofs are usually 6–9:12.
Step 1: Outline the usable roof planes
Open the calculator at zoom level 20 over the property. Don't trace the entire roof — trace each plane separately. A typical hip-roofed house has 4 planes; a gabled house has 2; a more complex roof might have 6 or more.
For each plane:
- Skip planes that face north (in the Northern Hemisphere). They get less sun and aren't worth panel space.
- Skip small dormer faces — too small for full panels.
- Subtract obstructions: chimneys, vents, skylights, satellite dishes. Either trace around them or measure the obstruction separately and subtract.
Step 2: Apply the tilt correction
Take each plane's footprint area and multiply by the correction factor for that pitch (table above). If different planes have different pitches — for example, a low-pitch addition next to a steep main roof — apply the corrections separately.
Step 3: Subtract setback and shading
You can't fill the roof corner-to-corner with panels. Most jurisdictions and fire codes require a 3-foot setback from ridges and edges. As a quick rule:
- Subtract about 15–20% from each plane's surface area to account for setbacks.
- Subtract more if there's persistent shading from trees or neighboring buildings — the satellite view shows shadow patterns, but you'll want to verify with a tool like Google's Project Sunroof or a site visit.
Step 4: Convert to panel count
A standard residential solar panel today is roughly 1.7 m × 1.0 m, or about 18 sq ft. Working in metric: divide your usable roof area in m² by 1.7. Working in imperial: divide your usable area in sq ft by 18. The result is roughly the panel count.
Multiply panel count by panel wattage (typically 400–450 W for current residential panels) to get system size in watts. Divide by 1000 to get kilowatts.
Step 5: Sanity check against energy use
A 1 kW solar system produces roughly 1,200–1,600 kWh per year in the continental U.S., depending on latitude and weather. Check the system size against your annual electricity use:
- If you use 12,000 kWh/year, an 8–10 kW system covers it.
- If you use 6,000 kWh/year, a 4–5 kW system is enough.
If your roof estimate suggests a system far larger than you need, you don't have to fill the whole roof. If it's smaller than you need, your options are: efficient panels, ground mount, or accepting partial offset.
What this estimate doesn't tell you
- Structural capacity. Some roofs need reinforcement before they can carry panels.
- Electrical service. Many older homes need a panel upgrade for solar.
- Wiring path. The route from roof to inverter to service entrance has cost implications.
- Permitting and HOA. Local rules can constrain what you can install.
Use the satellite estimate to ballpark the maximum system size and decide whether solar is worth a quote. Then get installer site visits — they bring drones, irradiance models, and experience with your local rules.