Solar Panel Calculator: How Many Panels You Need and How Long Payback Takes
- By The Eco Aesthetic Editorial Team
- Posted
Panels needed = yearly kWh you want to cover, divided by the kWh one kilowatt of panels makes in your location, divided by the watts per panel. At the EIA national average of 899 kWh a month and Kansas City, MO's PVWatts yield, that is about 19 panels of 400 W (7.6 kW). Payback uses the price you enter, because no price here is a quote.
Solar panel size and payback calculator
Answer the location and use questions first; add a quote price if you have one. Results update as you type and the numbers are saved in the page address. Nothing leaves your browser, and nothing here is a quote.
Defaults in use: location Kansas City, MO for sun (a PVWatts run, not your roof); the U.S. average rate of 18.19 cents per kWh (EIA, January to August 2026, preliminary); 899 kWh a month (EIA 2022 national average); 14% system losses (the PVWatts default); 400 W panels (our planning figure, not a sourced one, so use your datasheet); no incentive, no export credit and flat rates. Change any of them.
How the solar panel calculator works
The calculator turns your yearly use into a system size, using how much energy one kilowatt of panels makes where you live. That yield comes from NREL's PVWatts model, run for 16 cities at the PVWatts default losses. Peak sun hours are the same energy expressed as hours of full sun per day: a day with 5 kWh per square metre is 5 peak sun hours.
yield at your losses = PVWatts yield × (1 − your losses) ÷ (1 − 0.14) system size (kW) = yearly kWh to cover ÷ yield panels = round up (system size × 1,000 ÷ panel watts) yearly output (kWh) = panels × panel watts ÷ 1,000 × yield year 1 savings = min(output, use) × rate + surplus × export rate net cost = quoted price − confirmed incentives simple payback (years) = net cost ÷ year 1 savings with decline d: year n output = year 1 output × (1 − d)^(n − 1); payback year = first year cumulative savings ≥ net cost
- yield
- kWh of AC electricity per kW of panels per year, from a PVWatts run
- peak sun hours
- yearly average kWh per square metre per day on the tilted array; yield ≈ peak sun hours × 365 × performance ratio
Example: an average American home in Kansas City, MO
Worked example
Given: 899 kWh a month, 10,788 kWh a year (EIA 2022 average); Kansas City, MO: 1,433 kWh per kW per year at 14% losses; 400 W panels; 18.19 cents per kWh; a quote of $21,000, no incentive, no export credit.
- System size:
10,788 ÷ 1,433 = 7.53 kW - Panels:
7.53 × 1,000 ÷ 400 = 18.8, rounded up to 19 - Installed size and output:
19 × 400 W = 7.6 kW; 7.6 × 1,433 = 10,888 kWh a year - Year 1 savings (all output used):
10,788 × $0.18 = $1,962 - Simple payback:
$21,000 ÷ $1,962 = 10.7 years - With 0.5% a year output decline:
cumulative savings pass $21,000 in year 11
Answer: About 19 panels (7.6 kW) making about 10,888 kWh a year, and a payback near 10.7 years on a $21,000 quote at flat rates. The $21,000 is an example, not a market price.
Sun and yield by location
| Location | Sun hours a day | kWh per kW a year | Perf. ratio |
|---|---|---|---|
| Atlanta, GA | 5.13 | 1,425 | 0.76 |
| Boston, MA | 4.66 | 1,311 | 0.77 |
| Chicago, IL | 4.47 | 1,309 | 0.8 |
| Dallas, TX | 5.45 | 1,506 | 0.76 |
| Denver, CO | 5.66 | 1,595 | 0.77 |
| Honolulu, HI | 5.78 | 1,623 | 0.77 |
| Kansas City, MO | 5.03 | 1,433 | 0.78 |
| Las Vegas, NV | 6.41 | 1,753 | 0.75 |
| Los Angeles, CA | 6.08 | 1,677 | 0.76 |
| Miami, FL | 5.58 | 1,554 | 0.76 |
| Minneapolis, MN | 4.61 | 1,333 | 0.79 |
| New York, NY | 4.46 | 1,292 | 0.79 |
| Phoenix, AZ | 6.54 | 1,755 | 0.74 |
| Sacramento, CA | 5.84 | 1,629 | 0.76 |
| Seattle, WA | 3.95 | 1,088 | 0.75 |
| Washington, DC | 4.86 | 1,394 | 0.79 |
PVWatts v8 runs (API and website, same model) on 2026-11-14 using NREL's NSRDB weather data for the nearest grid cell. A different roof tilt or direction changes the numbers; run PVWatts for your address for a full estimate.
Assumptions and limits
- Location: the nearest-city yields are for a south-facing array tilted 20° on an open rack with a standard module. Your roof, shade and sun can differ, and the city you pick may be far from you. Run PVWatts for your address and enter the yield for a more exact estimate.
- Losses: the 14% PVWatts default is a bundle of typical losses. Changing it rescales the yield in proportion, which is how PVWatts applies system losses to DC output; it does not model inverter clipping.
- Panel wattage (400 W) is our planning figure, not a sourced one. It changes the panel count, not the system size in kW.
- Prices and incentives: no price on this page is a market price. Enter your own quote. The federal credit ended for systems placed in service after December 31, 2025, and incentives change; check with your installer or a tax professional.
- Rates: a flat cents-per-kWh figure from EIA's January to August 2026 state averages (preliminary). Time-of-use rates and net metering rules can change the savings.
- Left out: financing, maintenance, inverter replacement, roof work, batteries, and any change in your own use.
Frequently asked questions
How many solar panels do I need for a 2,000 square foot house?
Square footage does not set the answer; electricity use and sun do. EIA's average American home bought 10,788 kWh in 2022 (899 a month). In Kansas City, MO that needs about 7.5 kW of panels, or 19 panels of 400 W. A larger house with electric heating needs more, and a home in Phoenix needs fewer. Enter your own kWh and location.
How do I find my kWh?
Look at your electric bill. It shows kWh used for the month, and many utilities show a twelve month history or let you download usage data from your online account. Add twelve months and divide by twelve for an average. Use at least a year, since heating and cooling change use. If you are about to add an electric car or heat pump, add that expected use.
What is a payback period?
It is the time for savings to cover the net cost of the system. Simple payback divides the net cost by first-year savings. This calculator also shows the year cumulative savings pass the cost when output declines 0.5 percent a year (the median NREL found for panel degradation), which is usually a year or so later. Neither counts financing, maintenance, inverter replacement or roof work.
Do incentives change the result?
Yes, and they change often. The IRS says the 30 percent Residential Clean Energy Credit is not available for property placed in service after December 31, 2025, so the incentive field starts at zero. State, utility or local programs may exist; enter only what you have confirmed and subtract it from the price. A lower net cost shortens payback proportionally.
How do roof orientation and shade affect the estimate?
The yields here come from PVWatts runs for a south-facing array tilted 20 degrees with the PVWatts default 14 percent losses, which already include 3 percent for shading. A west or east roof, a steeper tilt, or heavy shade will produce less. Raise the system loss percentage to reflect shade, or run PVWatts with your roof tilt and direction and enter its result as your own yield.
Is net metering assumed?
Not by default. The calculator values each kWh you make at your retail rate up to the kWh you use in a year, and values any surplus at the export rate you enter, which starts at zero. If your utility credits exports at retail, set the export rate equal to your rate. Real net metering rules vary by state and utility, so check your tariff.
Sources
- NREL PVWatts Calculator v8: API documentation and the runs used for each city (developer.nlr.gov) Run on 2026-11-14; see the city table below.
- NREL PVWatts help: System Losses (default 14 percent and its categories) (pvwatts.nlr.gov) Accessed 2026-11-14.
- U.S. EIA: How much electricity does an American home use? (FAQ, 2022 data) (eia.gov) Last updated 2024-01-08. Accessed 2026-11-14.
- U.S. EIA, Electric Power Monthly, Table 5.6.B (residential price by state, year to date through August 2026) (eia.gov) 2026 values are preliminary. Accessed 2026-11-14.
- IRS: Residential Clean Energy Credit (irs.gov) Accessed 2026-11-14. Tax law changes; confirm with a tax professional.
- Jordan and Kurtz (NREL, 2012): Photovoltaic Degradation Rates, an Analytical Review (docs.nlr.gov) Median 0.5 percent a year, mean 0.8. Accessed 2026-11-14.
- U.S. EPA: Greenhouse Gas Equivalencies Calculator, calculations and references (epa.gov) eGRID 2022 national marginal rate. Accessed 2026-11-14.