How to use the solar panel calculator
- Pick your state.
- Enter your monthly electricity use in kWh. Your bill shows it; averaging the last 12 months smooths out summer and winter.
- Set how much of your use the panels should cover, and the panel wattage and size from the spec sheet.
- Optional: enter an installed cost per watt from a quote for a cost estimate.
The formula
- Yearly use to cover = monthly kWh × 12 × share to cover
- System size (kW) = yearly use ÷ kWh per kW for your state
- Panels = system size × 1,000 ÷ panel watts, rounded up
- Yearly production = panels × panel watts ÷ 1,000 × kWh per kW
Worked example: Texas, 899 kWh a month, 400 W panels
- Yearly use: 899 × 12 = 10,788 kWh
- System size: 10,788 ÷ 1,421 = 7.59 kW
- Panels: 7.59 × 1,000 ÷ 400 = 18.98 → 19 panels (7.6 kW)
- Production: 7.6 × 1,421 = 10,800 kWh a year; roof area about 399 sq ft
- At $2.80 per watt installed: 7,600 × 2.80 = $21,280
Peak sun hours by state
NREL PVWatts v8 for each state’s largest city: a south-facing array tilted 20° with 14% losses.
| State | City | Peak sun hours a day | kWh per kW a year |
|---|---|---|---|
| Alabama | Birmingham | 5.21 | 1,367 |
| Alaska | Anchorage | 3.17 | 922 |
| Arizona | Phoenix | 6.54 | 1,755 |
| Arkansas | Little Rock | 5.15 | 1,413 |
| California | Los Angeles | 6.08 | 1,677 |
| Colorado | Denver | 5.66 | 1,595 |
| Connecticut | Bridgeport | 4.72 | 1,340 |
| Delaware | Wilmington | 4.80 | 1,364 |
| District of Columbia | Washington | 4.82 | 1,366 |
| Florida | Jacksonville | 5.48 | 1,455 |
| Georgia | Atlanta | 5.13 | 1,402 |
| Hawaii | Honolulu | 5.82 | 1,622 |
| Idaho | Boise | 5.18 | 1,456 |
| Illinois | Chicago | 4.51 | 1,308 |
| Indiana | Indianapolis | 4.68 | 1,335 |
| Iowa | Des Moines | 4.77 | 1,370 |
| Kansas | Wichita | 5.39 | 1,526 |
| Kentucky | Louisville | 4.84 | 1,355 |
| Louisiana | New Orleans | 5.32 | 1,479 |
| Maine | Portland | 4.55 | 1,319 |
| Maryland | Baltimore | 4.88 | 1,392 |
| Massachusetts | Boston | 4.70 | 1,302 |
| Michigan | Detroit | 4.47 | 1,291 |
| Minnesota | Minneapolis | 4.61 | 1,333 |
| Mississippi | Jackson | 5.26 | 1,414 |
| Missouri | Kansas City | 5.03 | 1,433 |
| Montana | Billings | 4.76 | 1,344 |
| Nebraska | Omaha | 4.98 | 1,423 |
| Nevada | Las Vegas | 6.41 | 1,753 |
| New Hampshire | Manchester | 4.58 | 1,258 |
| New Jersey | Newark | 4.66 | 1,305 |
| New Mexico | Albuquerque | 6.42 | 1,779 |
| New York | New York | 4.50 | 1,291 |
| North Carolina | Charlotte | 5.22 | 1,446 |
| North Dakota | Fargo | 4.45 | 1,294 |
| Ohio | Columbus | 4.60 | 1,310 |
| Oklahoma | Oklahoma City | 5.45 | 1,527 |
| Oregon | Portland | 4.09 | 1,128 |
| Pennsylvania | Philadelphia | 4.81 | 1,359 |
| Rhode Island | Providence | 4.70 | 1,294 |
| South Carolina | Charleston | 5.33 | 1,454 |
| South Dakota | Sioux Falls | 4.86 | 1,409 |
| Tennessee | Nashville | 4.82 | 1,338 |
| Texas | Houston | 5.22 | 1,421 |
| Utah | Salt Lake City | 5.39 | 1,484 |
| Vermont | Burlington | 4.36 | 1,222 |
| Virginia | Virginia Beach | 5.01 | 1,427 |
| Washington | Seattle | 3.95 | 1,088 |
| West Virginia | Charleston | 4.62 | 1,232 |
| Wisconsin | Milwaukee | 4.58 | 1,334 |
| Wyoming | Cheyenne | 5.36 | 1,533 |
Frequently asked questions
How many solar panels do I need?
Divide your yearly electricity use by what 1 kW of panels produces where you live, then divide by the panel size. The average US home uses 899 kWh a month (10,788 kWh a year). In Texas, 1 kW produces about 1,421 kWh a year, so that home needs 7.59 kW, or 19 panels of 400 W.
How many solar panels do I need for 1,000 kWh a month?
With 400 W panels, about 18 in Arizona, 22 in Texas, 24 in New York and 28 in Washington. The same use needs more panels where there is less sun.
What are peak sun hours?
The day's sunlight expressed as hours of full-strength sun (1,000 watts per square meter). In this calculator's data they range from 3.17 a day in Anchorage and 3.95 in Seattle to 6.54 in Phoenix.
How much roof space do solar panels need?
A typical 400 W panel is about 21 sq ft, so 19 panels cover about 399 sq ft. You need more roof than that for gaps, edges and any clear paths your local code requires, and the area should face south, east or west without shade.
Is there still a federal tax credit for solar panels?
Not for systems installed now. The IRS Residential Clean Energy Credit paid 30% of the cost for systems installed from 2022 through December 31, 2025, and it isn't available for property placed in service after that. Check state and utility incentives instead.
How accurate is this estimate?
It assumes a south-facing roof tilted 20°, 14% system losses and the weather of each state's largest city. East- or west-facing roofs, shade and a different city can change the output noticeably. For your exact address, run NREL's free PVWatts calculator.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.