A typical U.S. home needs roughly 15 to 25 solar panels to cover all of its electricity use. The exact number depends on three things: how much power you use, how much sun your location gets, and the wattage of the panels you choose. With 400-watt panels, a sunny location lands near the low end of that range and a cloudier one near the high end.
The U.S. Energy Information Administration (EIA) reports that the average U.S. residential customer bought 10,791 kWh of electricity in 2022, about 899 kWh per month. Use that as a starting point, then swap in your own bills. This guide shows you how in five steps, and the calculator below does the math for you.
Last updated: October 2026.
Key takeaways
- Panel count = daily kWh ÷ (sun hours × panel kW × performance factor), rounded up.
- Sun hours swing the answer more than almost anything else. The same home can need about 15 panels in a very sunny place and 25 or more in a dim one (illustrative figures below).
- A rough rule of thumb: every 1,000 kWh of yearly use needs about 1.7 panels at 400 W, 5 sun hours and a 0.80 performance factor.
- Roof space, shade and future needs (an EV, a heat pump) can change the final number. A qualified installer should confirm it.
What you need before you start
Set aside about ten minutes and gather these:
- Your last 12 months of electricity bills, or your utility's yearly usage summary. You need kilowatt-hours (kWh), not dollars.
- Your location, so you can look up local sun hours.
- A panel wattage. Use the number on a quote or datasheet, or the editable example in the calculator.
- A calculator, or the one further down this page.
Step 1: Find your yearly electricity use
What to do: Add up the kWh from 12 consecutive monthly bills. Many utilities also show a 12-month total in your online account.
Why: Solar output is sized against energy (kWh), and a full year captures both summer cooling and winter heating. For reference, EIA data shows how wide the spread is: in 2022 the average residential customer used 14,774 kWh in Louisiana and 6,178 kWh in Hawaii. Your home may sit far from the national average of 10,791 kWh.
You're done when: you have one number in kWh per year. Divide it by 365 to get your daily use. At the EIA average, that is about 29.6 kWh per day.
The EIA figures above describe the United States. If you live elsewhere, use your own bills. Source: U.S. Energy Information Administration, "How much electricity does an American home use?"
Step 2: Look up your peak sun hours
What to do: Find the average peak sun hours for your location. Peak sun hours are not daylight hours. One peak sun hour means one hour of sunlight at full reference strength, so a day with 10 hours of weak, hazy light might count as only 4 or 5.
Where to look: An installer's proposal usually states the figure. Free solar-resource tools such as NREL's PVWatts also estimate it for your address. Check the tool's own site for current data.
Why: Sun hours turn "panel size" into "energy per day." Populated regions commonly fall somewhere between about 3 and 7 hours as a yearly average, though your location may differ.
You're done when: you have one number of sun hours per day for your location.
Step 3: Choose panel wattage and a performance factor
Panel wattage. Most new residential panels are rated at roughly 400 to 450 watts, and older guides that assume 250 to 300 watts overstate how many panels you need. Use the wattage printed on your quote. If you have no quote yet, 400 W is a sensible placeholder.
Performance factor. A panel never delivers its full rated power. Heat, dust, wiring, inverter conversion and minor shading all take a share. Many sizing guides assume total losses of roughly 15 to 25 percent, which means a performance factor of about 0.75 to 0.85. This guide uses 0.80 as an editable example.
You're done when: you have a panel wattage (for example, 400 W) and a performance factor (for example, 0.80).
Step 4: Run the formula
- Divide yearly kWh by 365 to get daily kWh.
- Divide daily kWh by (sun hours × performance factor). This gives the system size you need in kW.
- Divide system kW by panel kW (watts ÷ 1,000).
- Round up. You can't install 0.4 of a panel.
Worked example (illustrative, not a real home): 10,791 kWh per year is about 29.6 kWh per day. With 4.5 sun hours and a 0.80 factor, 29.6 ÷ 3.6 = 8.2 kW. At 400 W per panel, that is 20.5, which rounds up to 21 panels.
How sunshine changes the answer
The table keeps everything else fixed (10,791 kWh per year, 400 W panels, 0.80 factor) and changes only the sun hours. These are illustrative calculations, not measured data for any place.
| Peak sun hours per day | System size needed | Panels at 400 W |
|---|---|---|
| 3.5 | about 10.6 kW | 27 |
| 4.5 | about 8.2 kW | 21 |
| 5.5 | about 6.7 kW | 17 |
| 6.5 | about 5.7 kW | 15 |
How your usage changes the answer
Now the sun is fixed at 5 hours, with 400 W panels and a 0.80 factor. Again, these are illustrative calculations.
| Yearly use | System size needed | Panels at 400 W |
|---|---|---|
| 6,000 kWh | about 4.1 kW | 11 |
| 10,800 kWh | about 7.4 kW | 19 |
| 15,000 kWh | about 10.3 kW | 26 |
You're done when: you have a panel count for your own numbers. The calculator below gets you there in seconds.
Try it: panel calculator
Enter your own numbers to estimate how many panels you need. The starting values are editable examples, not data about your home. Replace each one with your own figure.
Example value. Use the 12-month total from your bills.
Example value. Use the yearly average for your location.
Example value. Use the wattage on your quote.
Example value. 80 means 20 percent of output is lost to heat, wiring, inverter and similar.
Example value. 100 means cover all of it. Use a higher number to plan for an EV or heat pump.
Estimate only. Real systems depend on roof layout, shading, equipment and local rules.
Step 5: Check your roof and plan ahead
Roof space. A typical modern panel covers roughly 17 to 18 square feet, before you leave room for setbacks, vents and chimneys. Multiply your panel count by that figure and compare it with your usable roof area. Higher-wattage panels fit more power into the same space.
Orientation and shade. Shade on even part of a panel can cut output more than its size suggests. Ask any installer how they account for it.
Future use. If you plan to add an EV, a heat pump or an electric water heater, raise the "share of your use to cover" in the calculator. Sizing for tomorrow's use is cheaper than adding panels later.
Rules and money. Net metering terms, incentives and permit rules differ by utility, state and country, and they change. Check them with your utility and a licensed installer before you decide. This guide gives a general estimate, not financial advice.
You're done when: your panel count fits your roof, or you know how much you'd have to cut and what share of your use that covers.
Troubleshooting
My result is higher than an installer's quote
Check the sun hours and performance factor first. Installers often use site-specific data, and a higher-wattage panel reduces the count. Ask them to show their yearly production estimate in kWh and compare that with your yearly use.
My bills swing wildly from month to month
Use the 12-month total, not an average of the bills you remember. If your use changed recently (new appliance, new resident, working from home), estimate the new yearly figure and use that instead.
My roof can't fit the full number of panels
You have three options: choose higher-wattage panels, cut your usage with efficiency upgrades, or accept a system that covers part of your use. The calculator's "share of your use to cover" box shows what a smaller system would offset.
The answer isn't a whole number
That's normal. Always round up, then compare the system's estimated yearly output with your usage to confirm it lands close.
I already have solar and my bills look low
Your bills may understate what the home really uses, because the panels already cover part of it. EIA notes that purchased electricity can be lower than total consumption where many homes have rooftop solar. Check your inverter or monitoring app for total household consumption instead.
Your next step
Open your last 12 electricity bills, add up the kWh, and enter that number in the calculator above. Then ask two or three installers for a proposal that shows yearly production in kWh, and compare it with your total.