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How many solar panels do I need?
Enter your monthly kWh and the share you want to cover: the system size and the panel count follow from your state’s sun.
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Solar to cover 100% of your use in Texas
9.3 kW
22 panels of 425 W · 13,152 kWh a year
| Use per year | 13,152 kWh |
| Output of 1 kW in Houston | 1,409 kWh a year |
| System size | 9.33 kW |
| Panels | 22 |
NASA POWER sunshine on a south-facing tilted roof, converted with the performance ratio PVWatts gives in states computed both ways; shade, orientation and roof angle change it. How this is calculated.
An average Arizona home, which used 1,075 kWh a month in 2024, needs about 7.4 kW of solar, or 18 panels of 425 W, to produce all of its electricity over a year. The rule behind that answer fits on one line: yearly use divided by what one kilowatt of panels yields where you live. PVWatts, the model run by the National Laboratory of the Rockies, puts that yield at 1,755 kWh per kW per year in Phoenix, for a south-facing roof tilted 20 degrees with 14% system losses. The same number swings by place: among the states loaded so far, Nevada reaches 1,861 kWh per kW and Alaska 922, so covering identical use takes 102% more panels in Alaska. Panel wattage only changes the count; 425 W is the median rating installed in New York in 2025. Your own bill, with twelve months of kWh, is a better input than any state average.
Three steps from your bill to a panel count
The calculator takes the monthly kWh you type, multiplies by twelve and by the share you want covered, then divides by the yearly output of one kilowatt in your state. That gives kilowatts of DC capacity. Multiply by 1,000 and divide by the panel rating, round up, and you have the number of panels. With the defaults for Arizona: 1,075 kWh times twelve is 12,900 kWh a year; divided by 1,755, that is 7.35 kW; at 425 W per panel, 18 panels.
The monthly default is the state's 2024 average from EIA form 861. The national figure was 863 kWh a month, but homes vary far more than states do: an electric water heater, a pool pump or an electric car can double the number. Add up the twelve months on your utility account, because summer air conditioning and winter heating make any single month misleading.
| State | Sun data city | kWh per kW a year | Average use a month | kW for 100% | Panels |
|---|---|---|---|---|---|
| Alabama | Birmingham | 1,367 | 1,143 kWh | 10.0 | 24 |
| Alaska | Anchorage | 922 | 578 kWh | 7.5 | 18 |
| Arizona | Phoenix | 1,755 | 1,075 kWh | 7.4 | 18 |
| Arkansas | Little Rock | 1,413 | 1,048 kWh | 8.9 | 21 |
| California | Los Angeles | 1,677 | 503 kWh | 3.6 | 9 |
| Colorado | Denver | 1,595 | 674 kWh | 5.1 | 12 |
| Connecticut | Hartford | 1,252 | 695 kWh | 6.7 | 16 |
| Delaware | Wilmington | 1,364 | 911 kWh | 8.0 | 19 |
| District of Columbia | Washington | 1,411 | 639 kWh | 5.4 | 13 |
| Florida | Orlando | 1,544 | 1,104 kWh | 8.6 | 21 |
| Georgia | Atlanta* | 1,405 | 1,074 kWh | 9.2 | 22 |
| Hawaii | Honolulu* | 1,755 | 495 kWh | 3.4 | 8 |
| Idaho | Boise* | 1,523 | 944 kWh | 7.4 | 18 |
| Illinois | Chicago* | 1,236 | 693 kWh | 6.7 | 16 |
| Indiana | Indianapolis* | 1,270 | 901 kWh | 8.5 | 21 |
| Iowa | Des Moines* | 1,346 | 832 kWh | 7.4 | 18 |
| Kansas | Wichita* | 1,504 | 876 kWh | 7.0 | 17 |
| Kentucky | Louisville* | 1,282 | 1,047 kWh | 9.8 | 24 |
| Louisiana | New Orleans* | 1,440 | 1,202 kWh | 10.0 | 24 |
| Maine | Portland* | 1,252 | 550 kWh | 5.3 | 13 |
| Maryland | Baltimore* | 1,300 | 929 kWh | 8.6 | 21 |
| Massachusetts | Boston* | 1,264 | 570 kWh | 5.4 | 13 |
| Michigan | Detroit* | 1,217 | 618 kWh | 6.1 | 15 |
| Minnesota | Minneapolis* | 1,291 | 712 kWh | 6.6 | 16 |
| Mississippi | Jackson* | 1,442 | 1,156 kWh | 9.6 | 23 |
| Missouri | Kansas City* | 1,379 | 1,001 kWh | 8.7 | 21 |
| Montana | Billings* | 1,403 | 852 kWh | 7.3 | 18 |
| Nebraska | Omaha* | 1,377 | 956 kWh | 8.3 | 20 |
| Nevada | Las Vegas* | 1,861 | 930 kWh | 6.0 | 15 |
| New Hampshire | Manchester* | 1,264 | 619 kWh | 5.9 | 14 |
| New Jersey | Newark* | 1,271 | 662 kWh | 6.3 | 15 |
| New Mexico | Albuquerque* | 1,831 | 654 kWh | 4.3 | 11 |
| New York | New York* | 1,271 | 571 kWh | 5.4 | 13 |
| North Carolina | Charlotte* | 1,420 | 1,015 kWh | 8.6 | 21 |
| North Dakota | Fargo* | 1,341 | 1,029 kWh | 9.2 | 22 |
| Ohio | Columbus* | 1,214 | 846 kWh | 8.4 | 20 |
| Oklahoma | Oklahoma City* | 1,520 | 1,079 kWh | 8.5 | 21 |
| Oregon | Portland* | 1,214 | 882 kWh | 8.7 | 21 |
| Pennsylvania | Philadelphia* | 1,311 | 817 kWh | 7.5 | 18 |
| Rhode Island | Providence* | 1,301 | 567 kWh | 5.2 | 13 |
| South Carolina | Charleston* | 1,467 | 1,050 kWh | 8.6 | 21 |
| South Dakota | Sioux Falls* | 1,380 | 994 kWh | 8.6 | 21 |
| Tennessee | Nashville* | 1,327 | 1,154 kWh | 10.4 | 25 |
| Texas | Houston* | 1,409 | 1,096 kWh | 9.3 | 22 |
| Utah | Salt Lake City* | 1,592 | 774 kWh | 5.8 | 14 |
| Vermont | Burlington* | 1,155 | 574 kWh | 6.0 | 15 |
| Virginia | Virginia Beach* | 1,373 | 1,032 kWh | 9.0 | 22 |
| Washington | Seattle* | 1,138 | 955 kWh | 10.1 | 24 |
| West Virginia | Charleston* | 1,245 | 1,027 kWh | 9.9 | 24 |
| Wisconsin | Milwaukee* | 1,208 | 645 kWh | 6.4 | 16 |
| Wyoming | Cheyenne* | 1,545 | 863 kWh | 6.7 | 16 |
What a kWh-per-kW figure already includes
PVWatts, the model of the National Laboratory of the Rockies, simulates a typical weather year from satellite data for one city per state, hour by hour, and reports what 1 kW of standard panels on a fixed roof mount delivers after 14% of losses for wiring, inverter conversion, soiling, mismatch and similar effects. Its free access allows only a few runs a day, so 10 states use it directly; the others use the yearly sunshine on a tilted roof from NASA POWER, multiplied by the performance ratio that PVWatts gives in the states computed both ways (0.801, within 10% of PVWatts in each of them). It does not know your roof. An array facing east or west, a lower or steeper pitch than 20 degrees, or a tree that shades the panels after 3 p.m. all cut output, and an installer's shade report replaces the state figure with one for your address.
Climate inside a state matters too. The sun data for Arizona come from Phoenix; a mountain town or a foggy coast in the same state can differ noticeably.
One panel, one year
A single 425 W panel in Phoenix makes about 746 kWh a year, roughly 62 kWh a month. That is the quickest way to translate a gap on your bill into panels: a monthly shortfall of 100 kWh needs about 2 more.
Covering part of your use
Many owners size below 100%. Covering half of Arizona's average use takes 3.7 kW, 9 panels. Roof space, budget and the way your utility credits power sent back to the grid all push in that direction: where exported kWh earn less than the retail price, the last panels of an oversized system pay back slowest. The payback calculator lets you set that export share and credit. A figure above 100% makes sense if you plan to add an electric car or a heat pump; the EV charging calculator estimates what a car adds in kWh.
Bigger panels, fewer of them
Panel ratings change the count, not the system size. The median home system installed in 2023 was 7.4 kW (Berkeley Lab): 22 panels at 350 W, 18 at 425 W, 15 at 500 W. Higher-wattage panels are physically larger, so they help on a cramped roof only when their efficiency is higher too. Once you know the size, the cost page prices it for your state.