Solar Roof Calculator
Enter your roof dimensions — find out how many solar panels fit and the maximum system your roof can hold.
How to Use This Calculator
Enter your roof dimensions
Measure the south-facing roof face where panels will be installed — not the entire roof or the building footprint. If you have a simple gable roof, measure the length of the ridge line and the rake (slope distance from ridge to eave). Use length × width or enter total square footage directly.
Set usable percentage
Not all roof area can hold panels. Subtract for:
- Fire code setbacks — Most jurisdictions require a 3-foot setback from all roof edges, ridge, and valley lines. This typically removes 20–30% of gross area.
- Irregular roof shapes — Hip roofs, dormers, and valleys create unusable triangles.
- Shading zones — Areas that would receive significant shading from trees, chimneys, or adjacent structures.
The 75% default is appropriate for a typical rectangular roof face with standard setbacks. Drop to 65% if your roof has multiple peaks or obstructions.
Enter obstructions area
For each obstruction (chimney, skylight, vent pipe, dormer), estimate its area plus the required 3-foot clearance zone around it. A 2×3 ft chimney with 3 ft clearance on all sides blocks approximately 48 sq ft of roof space.
Select panel wattage
Higher-wattage panels (450W, 500W) are physically larger but pack more power per panel. If your usable area is limited, higher-wattage panels let you fit more system capacity in the same space. The calculator automatically adjusts panel footprint based on your wattage selection.
The Formula
The floor function means we round down — partial panels don't exist, so we always take the lower whole number. This makes the result conservative, which is appropriate for planning: you'll always fit at least this many panels, potentially more with careful layout optimization by an experienced installer.
Note that this calculator gives the maximum physical capacity of your roof — not the recommended system size. If your roof can fit 30 panels (12 kW) but you only need 20 panels (8 kW), you'd install 20 panels. Use our Solar System Size Calculator to find the right size for your consumption.
Example
A typical 3-bedroom ranch house roof
A 42 × 26 ft south-facing roof face with a chimney (estimated 40 sq ft including clearance) and one skylight (30 sq ft including clearance). Using 400W panels, 75% usable area.
Result
This roof can physically accommodate up to 41 panels, far more than the 20–22 panels this typical home needs for 100% offset. The homeowner should install only the number of panels needed for their consumption — installing extra just to fill the roof is wasteful and typically not economically justified under most net metering policies.