Solar Panel Calculator

How many solar panels do you need?

Sizing a solar system starts with one number from your electricity bill: how many kilowatt-hours (kWh) you use in a month. From there it's a matter of how much sun your region gets and how much of the panels' rated output survives real-world losses. This calculator turns those into an estimated system size in kilowatts and a panel count.

The math is straightforward: divide your daily usage by your region's average peak sun-hours and a derate factor. NREL's solar-resource data gives the sun-hours; the 0.80 derate is the standard allowance for inverter, wiring, temperature, and soiling losses.

Peak sun-hours by region

A "peak sun-hour" is an hour of full-strength sunlight (1,000 watts per square metre). The daily average varies a lot across the country, which is why the same-size system produces far more power in Arizona than in Seattle.

• Desert Southwest (AZ, NV, NM, southern CA): about 6 hours/day.
• Mountain West & South-Central (CO, UT, TX): about 5.5 hours/day.
• Southeast & Florida: about 5 hours/day.
• Midwest & Mid-Atlantic: about 4.5 hours/day.
• Northeast & New England: about 4 hours/day.
• Pacific Northwest: about 3.8 hours/day.

Why this is an estimate

Two identical houses can need very different systems. Roof tilt and direction, shade from trees or a neighbour's chimney, winter snow cover, panel temperature on a hot roof, and your utility's net-metering rules all change how much a panel actually delivers. Treat this figure as a starting point for conversations with installers, not a final design — a site assessment is the only way to get the real number.

Frequently asked questions

Should I size for 100% of my bill? Not necessarily. Because of net-metering caps and diminishing returns on a partly shaded roof, many homeowners get the best payback offsetting 80–90% of their use rather than chasing a full offset. This calculator estimates a full offset so you can scale down from there.

How much roof space does a system need? A modern residential panel is roughly 2 m². A 20-panel system therefore needs about 39 m² of unshaded, well-oriented roof — more if the roof faces east/west rather than south.

Do bigger panels mean fewer panels? Yes — a 450 W panel produces more than a 350 W one, so a given system size needs fewer of them. But total roof area and system kW matter more than panel count; higher-wattage panels mainly help when roof space is tight.

Does the derate factor really cut output by 20%? Roughly. Inverters, wiring, heat, dust, and DC-to-AC conversion each take a small slice; together they typically total about 20%, which is why installers size to nameplate figures well above your actual usage.

These are planning estimates based on NREL regional averages and standard loss assumptions. Your actual production depends on your specific roof and equipment — get a site-specific quote before buying. See our electrical guides for more.