Solar Panel Calculator
Start from your electric bill and this sizes the system: the kilowatts, the number of panels, and the roof they will need. The sun does different amounts of work in Phoenix and in Seattle, so the region setting matters more than any brochure admits, and the arithmetic behind every step is shown so you can argue with any line of it. What the system costs and when it pays back is its own page, one link away.
The arithmetic, from bill to roof
Three honest numbers drive it. Your yearly usage, which your bill already knows. Your region's peak sun hours, the day's solar energy expressed as hours of full-strength sun, which NREL's solar resource maps put near 6 a day in the desert Southwest and closer to 3.5 in the Pacific Northwest; our five bands sit inside those published ranges and the override box takes your local figure. And the derate: real systems deliver about 80 percent of their laboratory rating after inverter losses, wiring, soiling and heat, a stated planning figure in line with NREL's PVWatts defaults. Everything else is multiplication, shown in the result so you can check any step.
Worked example
A $150 monthly bill at the average 18 cent rate, mid-country sun, covering all of it with 425 W panels:
$150 ÷ $0.18 = 833 kWh a month, 10,000 kWh a year, almost exactly the average American home. One kilowatt of panels here produces about 1,314 kWh a year (4.5 sun hours × 365 days × 0.80), so the system needs 7.6 kW, which is 18 panels and roughly 400 square feet of usable roof.
Same house, three very different roofs
Move that identical home around the country and the answer moves with the sun: about 14 panels in Phoenix, 18 in Kansas City, 24 in Seattle. This is the number one reason online solar estimates disagree with each other and with the quote you eventually get. A calculator that does not ask where you live is answering for a city you may not live in, and an installer's site-specific production model, which also sees your roof's direction, pitch and shading, will always beat a regional band. Treat this page as the sanity check that keeps that quote honest.
Usable roof is smaller than roof
Eighteen panels want about 400 square feet, and the operative word is usable: south-facing (east and west work at roughly a 15 to 20 percent production discount, north rarely makes sense), unshaded through the middle of the day, and clear of vents, skylights and the fire setbacks most codes require at ridges and edges. Plenty of big roofs hold surprisingly few panels once those subtractions are made, which is why the site visit exists. Our square footage calculator helps measure what you actually have.
What it costs is the other half
Sizing is the easy half of the decision. The solar panel cost calculator prices this system at current installed rates, runs the savings year by year against your utility rate, and covers the thing that changed hardest in 2026: the federal residential tax credit ended on December 31, 2025, and calculators still applying it are overstating everyone's case by 30 percent. If part of your plan is charging a car on this roof, the EV charging cost calculator prices that side.
Data reviewed: September 2026. Figures here come from published sources and change over time. How we verify
Copy one snippet and give your visitors a working Solar Panel Calculator.
Frequently asked questions
How many solar panels do I need?
For a typical American home using about 10,000 kWh a year in a mid-country sun region, about 18 modern 425-watt panels, a 7.6 kW system taking roughly 400 square feet of roof. The honest range is wide: the same usage needs about 14 panels in the desert Southwest and 24 in the Pacific Northwest, because the panels are identical and the sun is not.
How do I size a solar system from my electric bill?
Divide the monthly bill by your electricity rate to get monthly kWh, multiply by 12 for the year, then divide by what one kilowatt of panels produces annually in your region: roughly 1,750 kWh in the Southwest down to about 1,000 in the Northwest, once real-world losses are counted. A $150 bill at the average 18 cent rate is about 10,000 kWh a year, which is almost exactly the national average home.
What are peak sun hours?
The day's total solar energy expressed as hours of full-strength sun. Phoenix averages around 6 a day across the year, the middle of the country about 4.5, Seattle closer to 3.5. Multiply by 365 and by a system's losses and you get the yearly production per kilowatt of panels, which is the single number this whole calculation stands on. Our bands are set within the ranges NREL's solar resource maps publish, and the field is editable if you know your local figure.
Why the 80 percent derate?
Panels are rated in laboratory conditions. Real systems lose production to inverter conversion, wiring, soiling, heat (panels produce less when hot, which is why the desert does not win by as much as its sun suggests), and occasional downtime. Our 20 percent total is a stated planning figure in line with NREL's PVWatts defaults; installers with premium equipment will quote a little better, and the field is yours to change.
How much roof space do solar panels take?
A modern residential panel is roughly 22 square feet, so the 18-panel example needs about 400 square feet of usable roof, and usable is the working word: south-facing, unshaded, and clear of vents, chimneys and code-required setbacks. A roof can be big and still have little usable area, which is one of the first things a site visit checks.
Should I size for 100 percent of my usage?
Not automatically. If your utility credits exported power at less than the retail rate, which is increasingly common as net metering gives way to net billing, overproduction earns less than avoided consumption, and a system sized at 80 to 90 percent of usage often pays back faster per dollar. Planning an EV or a heat pump pushes the other way. The offset field lets you try both stories.
Do more efficient panels mean fewer panels?
Fewer panels for the same watts, not more energy per watt of rating. A 450-watt panel and two 225-watt panels produce the same power; efficiency decides how much roof a watt occupies, not what the watt does. Efficiency is worth paying for on a small roof and rarely decisive on a big one.
What will this system cost and when does it pay back?
That is the other half of the decision and it changed sharply in 2026, because the 30 percent federal residential tax credit ended on December 31, 2025. Our solar panel cost calculator prices the system this page sizes, runs the payback year by year, and shows exactly what the credit's end did to the arithmetic.