How to work out how many solar panels you need
Solar is sized by energy, not by house size. You need to know how many kWh you use in a year and how many kWh one kilowatt of panels makes on your roof. Here is the math the calculator runs with its starting values: a Texas home using 900 kWh a month, 400-watt panels, a south-facing roof with no shade.
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1
Find your yearly use
Add up 12 months of kWh from your bills. With 900 kWh a month, that is 900 × 12 = 10,800 kWh a year.
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2
Find what 1 kW of panels makes
Multiply peak sun hours by 365 days and by the system efficiency. Texas averages 5.4 sun hours, and 77% is what reaches your meter after heat, wiring, inverter and dirt losses: 5.4 × 365 × 0.77 = 1,518 kWh per kW per year.
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3
Divide to get the system size
10,800 ÷ 1,518 = 7.12 kW of panels to cover 100% of your use.
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4
Divide by the panel size and round up
7,116 watts ÷ 400 watts = 17.8, so 18 panels. That is a 7.2 kW system making about 10,927 kWh a year, 101% of your use.
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Check the roof and the price
Each 400 W panel is about 21 sq ft, so 18 cover 378 sq ft. Allow for fire-code walkways and you need about 504 sq ft of good roof. At $3.00 per watt, 7,200 watts cost $21,600 installed.
If you only know your bill, pick "$ bill". The calculator divides the bill by your rate to estimate kWh. A $150 bill at Texas's 16.03¢ average is about 936 kWh a month, which needs 19 panels. Fixed monthly fees inflate that estimate a little, so kWh from the bill is better.
Panels needed by monthly use and sunshine
400-watt panels, south roof, no shade, 77% system efficiency, 100% of use. Four sun hours is typical of the Pacific Northwest and the Great Lakes, five of much of the Southeast and Midwest, and six of the desert Southwest. Tap a usage to load it into the calculator with your current state.
| Monthly use | 4 sun hours | 5 sun hours | 6 sun hours |
|---|---|---|---|
| 500 kWh | 14 (5.6 kW) | 11 (4.4 kW) | 9 (3.6 kW) |
| 750 kWh | 21 (8.4 kW) | 17 (6.8 kW) | 14 (5.6 kW) |
| 900 kWh | 25 (10 kW) | 20 (8 kW) | 17 (6.8 kW) |
| 1,200 kWh | 33 (13.2 kW) | 26 (10.4 kW) | 22 (8.8 kW) |
| 1,500 kWh | 41 (16.4 kW) | 33 (13.2 kW) | 27 (10.8 kW) |
| 2,000 kWh | 54 (21.6 kW) | 43 (17.2 kW) | 36 (14.4 kW) |
One kW of panels makes about 1,124 kWh a year at 4 sun hours, 1,405 at 5 and 1,686 at 6. The same house can need half again as many panels in Seattle as in Phoenix. The calculator's state list gives the sun hours for each state's largest metro; if you live far from it, edit the number.
Panel size and roof space
Most rooftop panels sold today are about the same size. Higher-wattage panels get there with more efficient cells, not a bigger frame, so fewer panels do the same job on the same roof.
| Panel | Typical size | Area | Panels per kW |
|---|---|---|---|
| 400 W, 108-cell | 1,722 × 1,134 mm (67.8 × 44.6 in) | 21.0 sq ft, 1.95 m² | 2.5 |
| 430 W, 108-cell | 1,722 × 1,134 mm | 21.0 sq ft, 1.95 m² | 2.33 |
| 450 W, 120-cell | about 1,903 × 1,134 mm (74.9 × 44.6 in) | 23.2 sq ft, 2.16 m² | 2.22 |
Panels can't run edge to edge. Building codes based on section R324.6 of the International Residential Code ask for 36-inch walkways from the eave to the ridge and a clear strip along the ridge, so firefighters can reach and vent the roof. Many towns allow smaller setbacks for small arrays, and some add their own rules. Vents, chimneys and skylights take more room. The calculator assumes you can fill about 75% of a clear roof face, which is an estimate. NREL's national rooftop study covered 98% of each suitable sloped roof plane with modules, but it did not subtract fire-code walkways, so real layouts fill less. Your installer's layout is the real answer.
If the panels don't fit, you have three levers: higher-wattage panels, a second roof face, or a smaller share of your use. A 7.2 kW system with 430 W panels needs 17 panels instead of 18.
Direction, tilt and shade
The sun hours in the calculator assume a south-facing roof tilted about 20°. Other roofs make less, and the calculator adds panels to make up for it:
| Roof | Output vs. south | Default home needs |
|---|---|---|
| South, no shade | 100% | 18 panels |
| Southeast or southwest | about 96% | 19 panels |
| East or west | about 85% | 21 panels |
| South, light shade | about 90% | 20 panels |
| South, heavy shade | about 75% or less | 24 panels |
These factors are our rounded estimates from PVWatts runs and vary with latitude and roof pitch. North-facing roofs are usually not worth covering in the U.S. Shade hurts more than its size suggests, because one shaded panel can drag down a whole string unless the system uses microinverters or optimizers. For a precise figure, run your address in NREL's free PVWatts tool.
What the system costs
Installers price residential solar per watt. Marketplace quotes averaged about $2.60 per watt in 2026, while Berkeley Lab's median for homeowner-owned systems installed in 2025 was about $3.60, so the calculator starts at $3.00. A 7.2 kW system comes to $18,700 to $25,900 across that range. Compare quotes on price per watt, not on the total, since each installer sizes a little differently.
The 30% federal tax credit for home solar does not apply to systems placed in service after December 31, 2025. Some states, cities and utilities still offer rebates. Whether the system pays for itself depends on your rate and how your utility credits exports; the solar payback calculator runs that math year by year. If you want backup power, size a battery with the home battery calculator.
Tips and common mistakes
- Use a full year of bills. A single July or January bill can be off by half. Many utilities show 12 months of kWh in the online account.
- Plan for new loads. An electric car driven 12,000 miles a year adds roughly 3,000 to 4,000 kWh, about 5 to 7 more panels in Texas. A heat pump replacing gas heat adds more in cold states.
- Don't oversize for profit. Most utilities pay less for power beyond what you use, and some cap the system at your past use.
- Fix the roof first. If the shingles have less than about 10 years left, replace them before the panels go up. Removing and reinstalling an array costs thousands. Our roofing calculator prices the job.
- Check the main panel. A 100-amp service panel may need an upgrade to accept a larger system, which installers often price separately.
- kW is not kWh. System size (kW) is the panels' rated power. Production (kWh) is energy over time. Quotes should list both.
Rooftop solar means work at height, roof penetrations and a grid-tied electrical connection. It needs a permit, an inspection and your utility's approval before you switch it on, and most homeowners hire a licensed installer. Panels make voltage whenever light hits them, so never open a junction box or touch wiring on an installed system.
Common questions
How many solar panels do I need for my house?
Divide your yearly electricity use by what 1 kW of panels makes where you live, then divide by the panel wattage. A home using 900 kWh a month in Texas needs about 7.1 kW, which rounds up to 18 panels of 400 watts. The same home needs about 25 panels at 4 sun hours and about 17 at 6 sun hours.
How many solar panels does it take to power a 2,000 square foot house?
Floor area is a poor guide, because use depends on climate, appliances and whether the home heats with gas or electricity. Many 2,000 square foot homes use 900 to 1,200 kWh a month, which works out to roughly 17 to 33 panels of 400 watts depending on your sun. Use 12 months of bills for a real number.
How much roof space do solar panels need?
A typical 400-watt residential panel measures about 67.8 by 44.6 inches, or 21 square feet. Eighteen of them cover about 378 square feet. Fire-code walkways and edge setbacks mean you should plan on roughly a third more roof than the panels themselves, about 500 square feet in that example.
How much electricity does one solar panel produce?
A 400-watt panel on a south-facing roof makes about 450 to 675 kWh a year in most of the U.S., or roughly 37 to 56 kWh a month. It makes more in the Southwest and less in the Pacific Northwest and the Great Lakes.
What size solar system do I need for 1,000 kWh a month?
1,000 kWh a month is 12,000 kWh a year. At 5 peak sun hours, 1 kW of panels makes about 1,405 kWh a year, so you need about 8.5 kW, or 22 panels of 400 watts. At 4 sun hours it takes about 10.7 kW, and at 6 sun hours about 7.1 kW.
Should I size solar panels for more than 100% of my use?
Usually not. Power beyond what you use in a year often earns only a low export credit. Size up only if you expect to add an electric car, a heat pump or other new loads soon, and check that your utility allows the larger system.
Sources and assumptions
Sun hours, rates and prices match our solar payback and electric bill calculators, so the numbers agree across the site. Checked October 2026; every input can be edited.
- Peak sun hours: solar radiation on a 20° south-facing surface from the National Solar Radiation Database (NREL, renamed the National Laboratory of the Rockies in December 2025), read through PVWatts for each state's largest metro.
- System efficiency: 77% by default, calibrated against PVWatts results (its 14% default system losses plus inverter and heat losses). Direction and shade factors are our rounded estimates from PVWatts runs, not code values.
- Electric rates (used only to turn a bill into kWh): EIA Electric Power Monthly, Table 5.6.B, residential average, January to August 2026. Typical use of about 900 kWh a month: EIA FAQ.
- Panel sizes: 1,722 × 1,134 mm for 108-cell 400 to 430 W modules, per distributor listings such as Greentech Renewables; about 1,903 × 1,134 mm for 120-cell 450 W modules (sizes vary by a few millimeters between makers). Check your panel's datasheet.
- Roof access: pathway and ridge setback rules follow IRC section R324.6 as adopted locally; see for example Larimer County's residential layout guide. The 75% usable-roof figure is our estimate.
- Usable roof share (estimate): 75% of a clear roof face. For an upper bound, NREL's Rooftop Solar Photovoltaic Technical Potential in the United States (Gagnon et al., January 2016, Table 1) assumes modules cover 98% of a suitable tilted roof plane and 70% of a flat roof, with no fire-code setbacks. The same study found only 26% of all small-building roof area suitable once shade and direction are counted.
- Installed price: EnergySage marketplace (about $2.60/W, June 2026) and Berkeley Lab's Tracking the Sun (median about $3.60/W for 2025).
- Federal credit: IRS, Residential Clean Energy Credit, not available for property placed in service after December 31, 2025.
- Not included: panel aging (about 0.5% a year), snow cover, batteries, roof repairs and electrical upgrades.