Backup power
Solar panel calculator
Divide your yearly kWh by what 1 kW of panels makes where you live, then by the panel size. The average US home (899 kWh a month) needs about 6.1 kW of panels (16 panels of 400 W) in Arizona, the sunniest state we model, and about 8.6 kW (22 panels) in Connecticut, the least sunny of the lower 48.
20 panels8.0 kW of 400 W panels
- Yearly production
- 11,288 kWh (941 kWh a month)
- Covers
- 105% of your use
- Roof area, panels only
- about 420 sq ft
- Value of that power
- $2,053 a year at 18.19¢/kWh
Production assumes south-facing panels at a 20° tilt with 14% system losses (the NREL PVWatts defaults). East or west roofs make roughly 10–20% less; shade can cut far more.
How the calculator sizes a solar system
yearly need (kWh) = monthly kWh × 12 × share to cover system size (kW) = yearly need ÷ kWh made per kW per year panels = system size × 1,000 ÷ panel watts (rounded up)
The key number is kWh per kW per year: how much electricity one kilowatt of panels makes in a year at your location, after real-world losses. We took it from NREL's PVWatts model (the U.S. government's standard solar estimator) for the largest city in each state, with the PVWatts defaults: a fixed roof mount facing due south, 20° tilt, standard panels and 14% system losses for wiring, inverter, soiling and mismatch. It ranges from 922 kWh in Alaska to 1,755 kWh in Arizona.
"Peak sun hours" describes the same thing from the sunlight side: the daily solar energy reaching the panel, in kWh per square meter, equal to the hours of full 1,000 W/m² sun. Production per kW works out to roughly peak sun hours × 365 × 0.75.
System size by monthly use
System size and number of 400 W panels to cover 100% of your use:
| Monthly use | Arizona (1,755 kWh/kW) | Mid-range (1,350 kWh/kW) | Alaska (922 kWh/kW) |
|---|---|---|---|
| 500 kWh | 3.4 kW · 9 | 4.4 kW · 12 | 6.5 kW · 17 |
| 750 kWh | 5.1 kW · 13 | 6.7 kW · 17 | 9.8 kW · 25 |
| 899 kWh | 6.1 kW · 16 | 8.0 kW · 20 | 11.7 kW · 30 |
| 1,200 kWh | 8.2 kW · 21 | 10.7 kW · 27 | 15.6 kW · 40 |
| 1,500 kWh | 10.3 kW · 26 | 13.3 kW · 34 | 19.5 kW · 49 |
| 2,000 kWh | 13.7 kW · 35 | 17.8 kW · 45 | 26.0 kW · 66 |
Solar production and panels needed in every state
For the average US home, 899 kWh a month, with 400 W panels. "Power worth" is the yearly production priced at the state's average residential rate (EIA, January–July 2026).
| State (city modeled) | Panels · system | kWh per kW / yr | Sun h/day | Worth / yr |
|---|---|---|---|---|
| Alabama Birmingham | 20 · 7.9 kW | 1,367 | 5.2 | $1,809 |
| Alaska Anchorage | 30 · 11.7 kW | 922 | 3.2 | $2,997 |
| Arizona Phoenix | 16 · 6.1 kW | 1,755 | 6.5 | $1,734 |
| Arkansas Little Rock | 20 · 7.6 kW | 1,413 | 5.2 | $1,538 |
| California Los Angeles | 17 · 6.4 kW | 1,677 | 6.1 | $3,792 |
| Colorado Denver | 17 · 6.8 kW | 1,595 | 5.7 | $1,813 |
| Connecticut Hartford | 22 · 8.6 kW | 1,252 | 4.6 | $3,082 |
| Delaware Wilmington | 20 · 7.9 kW | 1,364 | 4.8 | $1,949 |
| District of Columbia Washington | 20 · 7.6 kW | 1,411 | 5.0 | $2,784 |
| Florida Orlando | 18 · 7.0 kW | 1,544 | 5.6 | $1,701 |
| Georgia Atlanta | 20 · 7.7 kW | 1,402 | 5.1 | $1,727 |
| Hawaii Honolulu | 17 · 6.7 kW | 1,622 | 5.8 | $5,104 |
| Idaho Boise | 19 · 7.4 kW | 1,456 | 5.2 | $1,434 |
| Illinois Chicago | 21 · 8.2 kW | 1,308 | 4.5 | $2,112 |
| Indiana Indianapolis | 21 · 8.1 kW | 1,335 | 4.7 | $1,911 |
| Iowa Des Moines | 20 · 7.9 kW | 1,370 | 4.8 | $1,556 |
| Kansas Wichita | 18 · 7.1 kW | 1,526 | 5.4 | $1,671 |
| Kentucky Louisville | 20 · 8.0 kW | 1,355 | 4.8 | $1,547 |
| Louisiana New Orleans | 19 · 7.3 kW | 1,479 | 5.3 | $1,501 |
What the estimate leaves out
- Your roof. East- or west-facing arrays make roughly 10–20% less than south-facing ones; a flat or steep pitch and shade from trees or chimneys change it more. Run PVWatts with your address and roof for a site-specific number.
- How you're credited. Covering 100% of your kWh does not erase the bill when your utility pays less for exported power than it charges (net billing, as in California since 2023) or keeps a fixed charge. Many owners size for 70–90% of use.
- Batteries. Grid-tied panels shut off in a blackout so they don't energize the lines. For outage power you need a battery or a backup-capable inverter; size it with the home battery calculator.
- Incentives. The 30% federal Residential Clean Energy Credit does not apply to systems placed in service after December 31, 2025 (IRS). Check state and utility programs.
Safety. Rooftop solar is a permitted installation that needs utility interconnection approval. Panels produce DC voltage whenever light hits them, even with the main breaker off.
Common questions
How many solar panels do I need for a 2,000 sq ft house?
Usage matters more than square footage. A home using 1,000 kWh a month needs about 7 kW (18 panels of 400 W) in the sunniest states and 9 kW or more (23 panels) where production is lowest in the lower 48.
How much electricity does one solar panel produce?
A 400 W panel makes about 1.2–1.9 kWh a day across the lower 48 states, or roughly 450–700 kWh a year, depending on location and how it faces.
How many solar panels to power a house off-grid?
Size for your worst month rather than the yearly average, and add batteries for several cloudy days. Off-grid arrays are commonly 1.5–2× the size of a grid-tied system for the same home.
Do solar panels work during a power outage?
Not by themselves. Grid-tied inverters shut down when the grid fails. You need a battery or an inverter with backup capability.
Is there still a federal solar tax credit?
According to the IRS, the 30% Residential Clean Energy Credit (25D) is not available for property placed in service after December 31, 2025, so systems installed in 2026 do not qualify.