TradesHow this works
Enter your annual electricity use, your site's peak sun hours, and the panel wattage you are considering. The tool sizes the array in kW, counts the panels, and estimates yearly production, then lays out three offset targets so you can see what covering 80, 100, or 120 percent of your bill takes.
Sun hours and your real usage are the two anchors, so pull a full year of bills and get a local sun-hours figure rather than a single month or a national average. And remember this sizes from usage, not from your roof, so confirm the panels fit and are unshaded with a site design.
Common questions
How many solar panels do I need?
Take your annual electricity use in kWh, divide by 365, your peak sun hours, and a loss factor of about 0.80, to get the array size in kW. Then divide the system watts by your panel wattage. For a 12,000 kWh home at 4.5 sun hours, that is roughly a 9 kW array, about 23 panels at 400 watts.
What are peak sun hours?
Peak sun hours is the daily average of full-strength sunlight your location gets, not the hours of daylight. Most of the US falls between 4 and 5. A solar irradiance map or a local installer can give you a specific number, which matters a lot because it sets the array size directly.
What offset should I aim for?
A 100 percent offset sizes the array to cover a typical year, the common target where net metering credits your summer surplus against winter. Go under 100 percent to fit a smaller or shadier roof or a tighter budget, and over 100 percent if you expect an EV, a heat pump, or rising use, subject to your utility rules.
Why the 0.80 loss factor?
Panels never deliver their nameplate rating in the real world. Inverter conversion, wiring, heat, dust and soiling, and panel mismatch all shave output, and a derate of about 0.80 accounts for it. Some tools use 0.86; a lower number is more conservative and leaves headroom.
Will this many panels fit my roof?
Maybe not. This sizes the array from your usage, but roof area, shading from trees or chimneys, and orientation decide how many actually fit and how well they produce. A south-facing, unshaded roof is ideal. Get a site-specific design and a shading study before you commit.
Sources: Array kW = annual kWh x offset / (365 x peak sun hours x 0.80 derate); panels = system watts / panel watts (NREL PVWatts method) · Peak sun hours ~4-5/day US average; derate ~0.80 for inverter/wiring/heat/soiling losses; residential panels ~400 W. Engine version 1.0.0. Risk tier 1.