Solar Roof Calculator

How many solar panels can fit on my roof?

Fewer than most people guess. A roof face is never fully usable. Fire codes keep a strip clear below the ridge and an access path up the roof. Installers keep panels back from the edges, where wind loads are highest. Then the panels must fit as whole rectangles, so leftover strips go to waste.

This solar panel roof calculator handles each roof face separately:

1. It takes the clear setback zones off the face (ridge, access path, eave and rake edges).
2. It tries the panels in portrait and in landscape and keeps whichever fits more, with a 1-inch gap between modules.
3. It scores each face for the direction it faces, its tilt and its shade, then estimates yearly output.

A typical 2026 residential panel is rated 400 to 440 watts and measures about 67.8 × 44.6 inches (1,722 × 1,134 mm), or about 21 sq ft. As a rule of thumb, a clean, unshaded south face 40 feet wide and 16 feet from eave to ridge holds about 18 of them. That's 7.7 kW.

How to measure your roof faces

• Width along the eave: the length of the gutter edge of that face. For a hip roof face (a trapezoid), use the average of the eave and ridge lengths.
• Eave to ridge, along the slope: measured on the roof surface, not on a floor plan. From the ground, take the horizontal distance from the wall to the ridge line, add the overhang, and multiply by the pitch multiplier from our roof pitch calculator. A 6/12 roof's multiplier is 1.118.
• Area: satellite roof reports and many solar quotes list each facet in sq ft. Use the "Area of each face" tab. The calculator then assumes a face about 2.5 times as wide as it is deep, so treat the panel count as rough.
• Direction: the way water runs off the face. A phone compass held flat, pointing down the slope, gives it.
• Shade: watch the roof at 9 a.m., noon and 3 p.m. Shade in the middle of the day costs much more output than early-morning shade.

Skip faces that are cut up by dormers, skylights, vents or chimneys unless you measure the clear rectangles around them.

Fire-code setbacks for rooftop solar (approximate)

Most US jurisdictions use section R324.6 of the International Residential Code, or a local version of it, for roof access around solar arrays. The 2018 and later editions call for:

• At least two 36-inch-wide pathways from the eave to the ridge, on separate roof planes, with one on the street or driveway side. Each roof plane with panels needs a 36-inch path on that plane or next to it.
• A clear strip below the ridge of at least 18 inches when panels cover no more than 33% of the plan view roof area. Above 33%, it is 36 inches.
• With an automatic sprinkler system in the house, the 18-inch ridge setback applies up to 66% coverage.
• No pathway or ridge rules on roofs of 2/12 or flatter.

The calculator keeps this simple. It holds back one 36-inch path along one side of every face you use, plus 18 inches below the ridge. If panels would cover more than a third of the roof you entered, it switches to 36 inches. It does not model the 18-inch clearance on each side of hips and valleys. Many states and cities change these rules. Ask your installer what your fire marshal enforces.

Formulas and a worked example

Usable width = face width − edge clearance − 3 ft path
Usable depth = slope length − ridge setback − edge clearance
Panels = max(portrait fit, landscape fit)
kWh per panel per year = watts ÷ 1,000 × sun hours × 365 × direction factor × shade factor × (1 − losses) × 0.90
System cost = kW × 1,000 × cost per watt − local incentives

The 0.90 covers inverter efficiency (PVWatts uses 96%) and the output panels lose when they run hot.

Worked example (the default inputs): a south face 40 ft wide and 16 ft from eave to ridge at 6/12, US average sun of 4.98 hours, 430 W panels, 14% losses, 12-inch edges, IRC setbacks.

1. Usable area: 40 − 1 − 3 = 36 ft wide and 16 − 1.5 − 1 = 13.5 ft deep.
2. Portrait: 9 panels across (3.80 ft each with the gap) × 2 rows (5.73 ft each) = 18. Landscape: 6 × 3 = 18. So 18 panels, 7.74 kW.
3. 6/12 is a 26.6° tilt. Facing south, that scores 1.007 against the reference 20° south roof.
4. Per panel: 0.43 × 4.98 × 365 × 1.007 × 1 × 0.86 × 0.90 = 609 kWh a year. Times 18 = 10,961 kWh.
5. At 900 kWh a month (10,800 a year), that covers 101% of the home's use.
6. Cost: 7,740 W × $2.60 = $20,124. Savings at 18.5¢ are about $2,000 a year, a simple payback of about 10 years.

The north face of the same roof fits 18 panels too, but at 6/12 it would make only about 65% as much, so it is skipped by default.

How roof direction and pitch change output

The sun-hour figures in the calculator are for a south-facing roof tilted about 20°. Each face is scored against that. These factors are approximate, based on typical PVWatts results for the middle of the US. Southern states favor flatter tilts and northern states favor steeper ones.

East and west faces are often worth using. They lose only 14% to 20% and spread production across the morning and evening. That helps if your utility uses time-of-use rates. Shade costs are set at 5% for light shade, 15% for moderate and 30% for heavy.

Solar roof cost: panels vs solar shingles

Rooftop panels averaged about $2.60 per watt installed in mid-2026, or roughly $31,000 for a 12 kW system, according to EnergySage's marketplace data. State averages run from a little over $2.10 to about $3.80 per watt. The calculator shows a range from 15% below to 20% above the cost per watt you enter, because roof type, panel brand and the installer's backlog all move the price.

Solar shingles cost far more per watt, about $3.50 to $8 installed. They're roofing and solar in one, so the fair comparison is solar shingles versus panels plus the new roof you would need anyway. Shingle systems also make less power per square foot than panels, and fewer installers service them. They make the most sense when you are replacing the roof anyway and care a lot about the look.

The federal tax credit ended in 2025

The 30% federal Residential Clean Energy Credit (Section 25D) is gone for systems you own. Under the July 2025 tax law (Public Law 119-21), it does not apply to expenditures made after December 31, 2025, and the IRS treats the expenditure as made when installation is complete. Leases and power purchase agreements are different. The solar company owns that system and may claim a separate business credit under its own deadlines. Ask how that is reflected in your monthly price. Enter any state, city or utility rebates in the incentives box. The DSIRE database lists them by state.

Should you replace your roof before installing solar?

Panels last 25 to 30 years. If your roof will not, you will pay to take the panels off and put them back when you reroof. That typically runs $200 to $300 per panel, or $3,600 to $5,400 for 18 panels, before the new roof itself.

• Asphalt shingles: 15 to 30 years. Past about 12 years, replace first or at the same time.
• Metal: 40 to 70 years. Standing seam takes clamp-on mounts with no holes. See our metal roof cost calculator.
• Tile: long-lived, but tile breaks under foot and mounts cost more.

The calculator flags the roof when the middle of its typical life leaves fewer than 10 years (replace first) or fewer than 20 years (inspect first). Price a new roof with the roof replacement cost estimator. Large systems may also need a bigger service panel. See electrical panel upgrade cost.

Sources

Peak sun hours: state averages from NREL PVWatts (National Solar Radiation Database), 20° south-facing roof, as compiled by TheGreenWatt. Electricity rates: average of March and June 2026 residential prices from EIA Electric Power Monthly, Table 5.6.A. Average home use: EIA (10,791 kWh in 2022). Federal credit: IRS, Residential Clean Energy Credit. Solar cost per watt: EnergySage (June 2026). Solar shingles and panel removal costs: Fixr solar shingles and Fixr panel removal. Panel size: Qcells Q.TRON BLK M-G2+ 430 W datasheet dimensions. Setbacks: IRC R324.6 as adopted by local jurisdictions.

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