How SolPan estimates your solar output
Last updated October 2026
SolPan is a free tool that gives a homeowner a first, rough answer to "would solar make sense on my roof?" Here is exactly what happens between typing an address and seeing a number, so you know how much to trust it.
1. Finding the roof
You enter an address, and it is converted to map coordinates. Where Google has detailed roof data for the building, SolPan starts from the roof planes Google found, each with its own size, pitch, and facing. Otherwise you trace the roof yourself on a satellite map. Either way you can adjust the outlines, and you set each plane's tilt, because tilt can't be measured from above.
2. Fitting panels
Not every square foot of a roof can hold panels, because of edges, vents, chimneys, and fire-access setbacks. SolPan assumes about 75% of each roof plane is usable. It then divides that usable area by the size of one panel to get a panel count, and multiplies by the panel tier's wattage to get the system size in kilowatts.
3. Estimating production
Each roof plane is sent to PVWatts, a free solar production model from the National Renewable Energy Laboratory (NREL), a U.S. Department of Energy laboratory. PVWatts uses many years of weather data for your location plus the plane's tilt and direction to estimate month-by-month output. The planes' results are added together. SolPan uses PVWatts' default system-loss assumption of about 14%, which covers wiring, inverter, dirt, and a general allowance for shading.
4. Estimating savings
The annual kilowatt-hours are multiplied by the average residential electricity rate for your local utility, taken from NREL's Utility Rates data. Your actual bill depends on your rate plan, how much power you use and when, and your utility's net-metering rules, so treat the dollar figure as a ballpark.
What the estimate does not include
- Shade from specific trees or neighboring buildings, beyond the general allowance above.
- Equipment cost, installation cost, financing, tax credits, or other incentives.
- Your roof's structural condition, age, or local permitting rules.
- The real-world performance of a specific panel model. SolPan uses three simplified panel tiers, and PVWatts treats panels as generic types.
An installer will measure your roof, check for shade, and look at your actual electricity use. Use SolPan to decide whether solar is worth looking into, and a professional site assessment for any decision involving money.
Related pages
How roof tilt and direction affect output · How the panel tiers compare · Try the estimator