System sizing

Roof space vs output estimator

Enter your usable roof area, panel wattage, and local peak sun hours. Get an estimated panel count and annual output in seconds.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your roof
The south, east, or west-facing roof area actually clear of vents, chimneys, and shading. Not your home's total roof size.
Most US locations fall between about 3.5 and 6. NREL's PVWatts gives the figure for your exact address.
3 Panel and system choice
Most home panels quoted in late 2025 were 400 to 460 watts, according to EnergySage's marketplace data.
Use the panel's datasheet area, and add a few percent for clamps and gaps if you want a cautious count.
Accounts for inverter loss, wiring loss, dust, and temperature derate combined.

How it works

Three steps to a panel count

Step 1

Measure your usable roof area

Walk your roof plan and subtract vents, chimneys, valleys, and any shaded sections from the total. If you're not sure how much of your roof actually counts as usable, run the usable roof area calculator first to get a real number.

Step 2

Pick a panel wattage and footprint

What squeezes more output from the same square footage is power density, watts per square foot, which comes from efficiency, not wattage alone, since a higher-wattage panel is often a bigger one. If your roof space is tight, check the square footage estimator for a second view on how panel choice changes your count.

Step 3

Confirm against real-world constraints

Once you have an estimated panel count, check panel spacing and setback rules for your area, since fire codes and mounting clearances can reduce how many panels actually fit versus the raw math.

Understanding your roof's output potential

How much a typical roof can produce

A roof with 600 square feet of usable space fits about 28 panels of 21 square feet, producing roughly 16,400 kWh a year at 400 watts per panel, 5 peak sun hours a day, and a 0.80 derate. A smaller 400 square foot roof fits 19 panels and about 11,100 kWh a year. Usable area, not total roof size, is what actually limits the answer, so start with the usable roof area calculator if you haven't measured it. To see what that output is worth, the first year savings quick calculator turns a kWh figure into a first-year dollar estimate.

Why usable area is smaller than your roof looks

Usable area shrinks fast once you subtract vents, chimneys, valleys, shaded sections, faces that point the wrong way, and fire code setbacks, which the fire code setback calculator works out under the current residential and fire code provisions. NREL's lidar-based rooftop potential assessment found that only 26% of the total rooftop area of small buildings was suitable for PV, even though 83% of those buildings had at least one suitable location. A 1,000 square foot roof rarely means 1,000 square feet of installable panel space, so measure or estimate the usable portion directly.

Why panel footprint changes the count more than people expect

A compact 17.5 square foot panel and a larger 22 square foot panel can carry similar wattage, but the smaller footprint fits more panels into the same space, so the number that really sets output per roof is watts per square foot, the power density the panel efficiency vs space tradeoff tool compares. This estimator divides usable area by panel footprint, which assumes whole panels tile it cleanly, a limit the compact roof layout tool checks on a specific plane. The string sizing tool covers how panel choice interacts with inverter sizing.

When roof space is the real constraint on system size

If your usable roof area caps your system below what your usage needs, panels with more watts per square foot are the first fix, since they raise output without needing more room, and a larger wattage on its own does not help if the panel is bigger too. Recent panels make that easier: most home panels sold today fall in the 400 to 460 watt range, so higher-wattage options are easy to find. If that still isn't enough, the ground mount vs roof mount space calculator compares a ground array, and a smaller system paired with usage reduction is the other option worth checking. Berkeley Lab's Tracking the Sun report tracks how wattages and installed prices have moved over time, which matters when roof space forces the wattage decision.

Questions

Frequently asked

A roof with 600 square feet of usable space fits about 28 panels of 21 square feet, producing roughly 16,400 kWh a year at 400 watts per panel, 5 peak sun hours a day, and a 0.80 derate. A smaller 400 square foot roof fits 19 panels and about 11,100 kWh a year.

A modern 400-watt-class residential panel covers about 21 square feet, so budget at least that much usable roof area per panel, plus a little for clamps and gaps. Older 60-cell panels cover closer to 17.5 square feet.

No. Usable roof area is far smaller than total roof area once you subtract vents, chimneys, valleys, fire code setbacks, shaded sections, and faces that point the wrong way. NREL's lidar-based assessment found only about 26% of total rooftop area on small buildings suitable for PV. Measure or estimate the usable portion directly rather than starting from your home's full roof footprint.

Panels with more watts per square foot, which means higher efficiency, are the first fix for a space-constrained roof, since a higher-wattage panel is often also a bigger one. If that's still not enough, a ground-mount array or a smaller system paired with usage reduction are the next options to look at.

Related tools

Keep sizing your system

Usable roof area calculator Roof pitch output adjustment calculator Roof azimuth orientation calculator Panel spacing and setback calculator System size by square footage, quick estimator Irregular roof shape array fitter Maximum roof capacity calculator All system sizing tools