The size of a solar system comes down to how much electricity you use and how much sun your roof gets. Your electric bill gives the first; your location, roof direction and shading give the second.
A home paying an example $150 a month at 18 cents per kWh uses about 833 kWh a month. With 5 peak sun hours a day and about 15% system losses, covering that takes about a 6.8 kW system: 17 panels of 400 watts, about 366 sq ft of roof, producing about 10,549 kWh a year.
Solar Panel Calculator
System size, production and payback from your bill.
How the calculator sizes a system
- Usage: monthly bill ÷ your rate = monthly kWh, ÷ 30 for daily kWh.
- Sun: peak sun hours, the hours of full-strength sunlight a day, × (1 − system losses) × shading.
- Size: daily kWh ÷ effective sun hours = kW; ÷ 400 W, rounded up, gives the number of panels.
The sun hours by state are approximate. For your address, roof angle and direction, NREL's free PVWatts calculator estimates production from local weather data.
Panels by how much you use
Most people know their monthly kWh, not an abstract kilowatt figure. The counts below use the calculator's U.S.-average default of 5 peak sun hours and the calculator's default 85% system efficiency, so effective sun is 5 × 0.85 = 4.25 hours a day.
| Monthly usage | Daily usage | Panels (400 W) | Installed size | Roof area (20% panels) |
|---|---|---|---|---|
| 500 kWh | 16.7 kWh | 10 | 4.0 kW | 215 sq ft |
| 800 kWh | 26.7 kWh | 16 | 6.4 kW | 344 sq ft |
| 1,000 kWh | 33.3 kWh | 20 | 8.0 kW | 431 sq ft |
| 1,500 kWh | 50.0 kWh | 30 | 12.0 kW | 646 sq ft |
| 2,000 kWh | 66.7 kWh | 40 | 16.0 kW | 861 sq ft |
Take 1,000 kWh a month, which is 33.3 kWh a day. Divided by 4.25 effective sun hours that is a 7.8 kW raw need; at 400 watts a panel it rounds up to 20 panels, an 8.0 kW installed system.
How location changes the size
The same $150 bill needs about a 4.7 kW system (12 panels) at Arizona's roughly 7 peak sun hours, about 6.8 kW (17 panels) at the U.S. average of 5, and about 7.8 kW (20 panels) at New York's roughly 4.2. Less sun means more panels for the same production.
The table below holds the example home fixed at 833 kWh a month and changes only the sun hours. Each raw requirement rounds up to whole 400-watt panels, so the installed size sits a little above the raw need.
| State (example) | Peak sun hours | Panels (400 W) | Installed size |
|---|---|---|---|
| Arizona | 7.0 | 12 | 4.8 kW |
| Nevada | 6.8 | 13 | 5.2 kW |
| Colorado | 6.2 | 14 | 5.6 kW |
| California | 6.0 | 14 | 5.6 kW |
| Texas | 5.8 | 15 | 6.0 kW |
| Florida | 5.5 | 15 | 6.0 kW |
| North Carolina | 5.2 | 16 | 6.4 kW |
| U.S. average | 5.0 | 17 | 6.8 kW |
| New York | 4.2 | 20 | 8.0 kW |
| Washington | 3.8 | 22 | 8.8 kW |
Roof area
A 400-watt panel takes about 21.5 sq ft at the calculator's assumed 20% efficiency (monocrystalline), 23.9 at 18% and 35.9 at 12% (thin-film). The default system's 17 panels need about 366 sq ft, before setbacks and spacing your installer and local code require.
How panel wattage changes the count
The panel count is the system watts divided by each module's wattage, so bigger modules mean fewer panels for the same system size. The example home needs about 6,500 watts before rounding, which splits three ways.
| Module wattage | Panels needed | Installed size |
|---|---|---|
| 350 W | 19 | 6.65 kW |
| 400 W | 17 | 6.8 kW |
| 450 W | 15 | 6.75 kW |
Total roof area barely moves, because it is the same system size and a higher-wattage panel of the same 20% efficiency is simply larger. At 20% efficiency a 350-watt module covers about 18.8 sq ft and a 450-watt module about 24.2 sq ft, so all three options land between about 358 and 366 sq ft.
Roof direction and tilt
In the northern hemisphere the sun tracks across the southern sky, so a south-facing roof plane collects the most energy over the year. East-facing and west-facing planes produce less because they catch strong sun only for part of the day, and a north-facing plane is the weakest.
Tilt matters too: a steeper pitch favors the low winter sun, a shallower pitch favors the high summer sun, and most fixed rooftop arrays settle somewhere in between. Shading from trees, chimneys or neighboring roofs cuts output during the shaded hours, which is what the calculator's shading factor approximates.
Because the loss from a given direction, tilt and shade pattern depends on your address, run each roof plane through NREL's PVWatts, which models them from local weather data.
Cost and payback
Installed prices vary widely by region and installer, so the calculator uses your quote: enter the price per watt. At an example $3.00 a watt, the default 6.8 kW system costs $20,400, saves about $1,873 a year at 18 cents per kWh, and pays back in about 10.9 years.
The federal Residential Clean Energy Credit (25D) ended for expenditures made after December 31, 2025. State and utility incentives and net metering rules vary; check the DSIRE database for your state.
CO2 offset
At the U.S. grid average of 0.823 lb of carbon dioxide per kWh (EPA eGRID2022), the default system's 10,549 kWh a year offsets about 8,681 lb a year. Where the grid is cleaner, the offset is smaller.
Grid-tied vs off-grid sizing
A grid-tied system sizes to your annual net usage and leans on the grid as a buffer. It banks surplus in sunny months through net metering, then draws from the grid at night and in winter, and the figures above assume this setup.
An off-grid system has no grid to fall back on, so it must be oversized and paired with enough battery storage to carry the home through nights and stretches of low-sun weather. That raises both the panel count and the cost well beyond a grid-tied system of the same annual production.
Batteries
Solar panels alone shut down during a grid outage for safety. To keep power on, you need a battery and the right inverter setup; see the home battery backup guide.
Frequently asked questions
How many solar panels do I need?
For a $150 monthly bill at 18 cents per kWh and 5 peak sun hours, about 17 panels of 400 watts (6.8 kW). Sunnier locations need fewer.
How much roof space do solar panels need?
About 21.5 sq ft per 400-watt panel at 20% efficiency, so roughly 366 sq ft for 17 panels, plus required setbacks.
Is there still a federal solar tax credit?
Not for homeowners: the Residential Clean Energy Credit ended for expenditures after December 31, 2025. State and utility incentives may still apply.
How long does solar take to pay back?
It depends on your price per watt, sun and electricity rate. At an example $3.00 a watt, the default system pays back in about 10.9 years at 18 cents per kWh.
How many solar panels does it take to power a whole house?
For an average U.S. household using about 863 kWh a month (EIA), roughly 17 panels of 400 watts at the U.S.-average 5 peak sun hours. Sunnier regions need fewer and cloudier regions more, so your own bill and location set the real number.
How many solar panels for 1,000 kWh a month?
About 20 panels of 400 watts, an 8.0 kW system, at the U.S.-average 5 sun hours. That is 33.3 kWh a day divided by 4.25 effective sun hours, a 7.8 kW raw need that rounds up to 20 panels.
How many solar panels for a 2,000 sq ft house?
Floor area does not set the number; your electricity usage does. An average U.S. household uses about 863 kWh a month (EIA), roughly 17 panels of 400 watts at 5 sun hours, but two homes of the same size can differ widely by appliances, climate and habits.
Do solar panels still produce in winter or on cloudy days?
Yes, panels produce whenever daylight reaches them, just less of it. Output drops on overcast days and through shorter winter daylight, which is why sizing uses a yearly average of peak sun hours rather than a single sunny day.
Sources & References
- NREL: PVWatts calculator (location-specific production estimates)
- IRS: Residential Clean Energy Credit (25D)
- EPA: Greenhouse Gas Equivalencies, calculations and references (eGRID2022)
- EIA: Electric Sales, Revenue, and Average Price (average residential consumption)
- DSIRE: State and utility incentives and net metering