Solar Panel Square Footage Calculator
Calculate available roof square footage for solar panels. Standard residential panels are about 17.5 sq ft each (65 × 39 inches) and produce 350-400 watts.
Solar Roof Area Calculator
How much roof space per panel
Standard residential solar panels are 65 × 39 inches = 17.6 sq ft per panel. Including the framing and rail spacing between panels, plan on ~20 sq ft per installed panel.
100 sq ft of usable roof space holds about 5 panels = ~1,750 watts (1.75 kW). 500 sq ft holds about 25 panels = ~9 kW. A typical 6 kW residential system needs around 350 sq ft of clear south-facing roof.
Roof area limitations
Code typically requires a 3-foot setback from the ridge and from gable ends for fire access. Some jurisdictions require setbacks from valleys and dormers as well. These setbacks can reduce usable area by 30-50% on small or complex roofs.
Shading from chimneys, vents, and adjacent buildings further reduces effective area. South-facing roof planes are most productive; east and west still produce well; north-facing roofs in the northern hemisphere are usually not viable.
How many panels fit on my roof
Standard residential solar panels are about 17.5 sq ft each (65 × 39 inches). So 1 panel ≈ 17.5 sq ft of roof space, including some spacing.
Most home installations use 20-30 panels, requiring 350-525 sq ft of usable roof area.
Not all roof area is usable: south-facing slopes are best (north-facing in southern hemisphere). East/west facing work but produce 15-20% less. North-facing slopes are usually skipped.
Shaded areas can't be used. A tree shading a roof for 2 hours/day kills the productivity of those panels.
Setbacks: most jurisdictions require 18 inches from roof edges and ridges for fire safety. This reduces effective panel area by 5-15% on small roofs.
Panel watts and house energy needs
Modern residential panels produce 350-450 watts each. So 20 panels at 400W = 8 kW total system.
Average US household uses 10,500 kWh per year = 875 kWh per month = 29 kWh per day.
An 8 kW system in moderate climate produces about 11,000-13,000 kWh per year — enough to cover most household usage.
By region: Southern California or Arizona produces 1,500-1,700 kWh per kW per year. Pacific Northwest: 1,000-1,200. Northeast: 1,100-1,300. Florida: 1,400-1,500.
Worked example: 2,000 sq ft Texas home with $200/month electric bill. Annual usage ~12,000 kWh. Need 8-9 kW system. 20-23 panels at 400W each. Required roof: 350-400 sq ft south-facing.
Solar cost per sq ft and per watt
Solar panel installed cost: $2.50-4.00 per watt of capacity (2024). 8 kW system = $20,000-32,000 before tax credits and incentives.
Federal tax credit (Inflation Reduction Act): 30% of total system cost (extended through 2032). State and local incentives can add 10-30% more.
Effective cost after incentives: $14,000-22,000 for 8 kW system in typical state.
Payback period: 6-10 years in most US states. Faster (4-6 years) in states with high electric rates and good solar incentives.
Cost per sq ft of roof used: roughly $50-100 per sq ft of solar panel area. Doesn't include the rest of your roof, just the area occupied by panels.
Pro tips
Use roof slope, not floor area
A pitched roof has more surface area than its floor footprint. A 7/12 pitch roof has about 1.16× the area of the floor below it.
Account for setbacks early
Don't calculate "roof area × panel count" - first subtract code-required setbacks. The usable area is often 60-70% of the gross roof plane.
Solar production varies by orientation
A south-facing 6 kW system in Phoenix produces about 9,500 kWh/year. The same system facing east drops to ~7,500 kWh. Use a tool like PVWatts for precise estimates.
Get a site assessment
Solar installers offer free assessments that include shading analysis, roof condition, and electrical panel capacity. The square footage calc is a starting point only.