An average US home needs about 14 to 23 solar panels of 430 watts to cover a year of electricity. That range comes from EIA’s 2024 average of 863 kWh a month per home and from our runs of NLR’s PVWatts for nine US cities on 3 November 2026. Where your own home falls in that range comes down to three numbers: how many kWh you use in a year, how many kWh one kW of panels makes on your roof, and the wattage of the panel on your quote. This hub explains each of them, what your roof’s angle and direction do to the answer, and how to check the size and production figure on an installer’s quote.
Leaf & Watt sells and installs nothing and takes no referral fees. The figures in these guides come from public data and model runs we record, with our assumptions labelled, as set out on our how we estimate page.
The three numbers that set the size
The whole calculation fits in one line: panels = yearly kWh × share to cover ÷ (kWh per kW × panel watts ÷ 1,000), rounded up. Worked through for a Houston home that uses the Texas average of 1,096 kWh a month:
- Yearly use: 1,096 × 12 = 13,152 kWh.
- Output per kW: PVWatts gives 1,420 kWh a year per kW in Houston (south-facing, 20° tilt, 14% losses).
- System size: 13,152 ÷ 1,420 = 9.26 kW of panels, rated in DC watts.
- Panels: 9,262 W ÷ 430 W = 21.5, so 22 panels. That is 9.46 kW, about 102% of the home’s use, on about 462 square feet of roof.
Our how many solar panels do I need calculator runs the same math on your own use, city or PVWatts figure, roof and panel, and shows the roof area and the lowest and highest month. The page beside it has the full table of panel counts by city and yearly use.
Your use: twelve bills, not one
Add up the kWh on your last 12 bills. One summer or winter bill times 12 can be far off, because air conditioning and electric heat move use a lot by season. State averages are a rough guide only: EIA’s 2024 figures run from 495 kWh a month in Hawaii to 1,202 in Louisiana. Add any load you plan to add. An electric car driven 12,000 miles a year at 30 kWh per 100 miles uses 3,600 kWh, which in Houston is about 6 more 430 W panels.
Output per kW: where you live matters most
In our PVWatts runs, one kW of south-facing panels at 20° makes about 1,754 kWh a year in Phoenix, 1,420 in Houston, 1,307 in Chicago and 1,087 in Seattle. The same home in Seattle needs about 60% more panels than in Phoenix (our arithmetic).
Be wary of the “peak sun hours” shortcut. For Houston it turns 5.22 kWh of sunlight per square meter per day into 1,905 kWh per kW a year, about a quarter more than PVWatts’ 1,420, because it leaves out losses from soiling, shading, wiring, heat and the inverter. Sized with the shortcut, the Houston home would get 17 panels and cover about 79% of its use.
Panel wattage and roof space
Panel wattage changes the count, not the system size: 400 W panels need about 7.5% more panels than 430 W ones. Roof space is often the real limit. Berkeley Lab’s 2026 data update found panels on 2025 installs typically cover 16% to 39% of a home’s roof, and put the median US home system at 7.7 kW, with most between roughly 5 and 12 kW. Vents, chimneys, shade and local fire-code setbacks all take space, and the installer’s layout and your building department decide what fits.
Roof angle and direction
The city figures above assume a south-facing roof at 20°. Most roofs aren’t exactly that, and the good news is that pitch matters less than people expect. In 788 runs of the European Commission’s PVGIS model on 3 November 2026:
- The best year-round angle for a south-facing array runs from about 19° in Honolulu and 25° in Miami to 40° in Minneapolis, usually a few degrees below the latitude.
- Most pitched roofs are close enough. Any south-facing roof from 5/12 to 12/12 (23° to 45°) got within about 5% of the best angle in all nine calculator cities. Setting the angle equal to the latitude, the classic rule, cost at most 1.2%.
- Direction matters more. Turning a 6/12 roof from south to southeast or southwest cost about 3% to 8%; east or west cost about 11% to 23%, generally more in the North.
- Flat roofs lose the most. Panels laid flat gave up 4% to 19%. For the 9.46 kW Houston example that is about 1,170 kWh a year, and a 10° tilt frame cuts the loss to about 480 kWh.
Our solar panel tilt angle calculator gives the best angle for 25 US cities, converts your roof pitch to degrees, and shows what your own pitch and direction give up in kWh and dollars a year. It also covers seasonal adjustment, which only suits ground mounts or racks built to move: changing the angle twice a year added about 3.4% to 5.4% in our runs, roughly $100 a year for the Houston example at EIA’s Texas average rate. A west-facing roof makes less in total, but its later-day output can be worth more under time-of-use or net billing rates.
Winter months and how much to cover
A system sized for the year makes more than you use in summer and less in winter. The 9.46 kW Houston system makes about 814 kWh in December and 1,315 kWh in July, against use of 1,096 a month. In Seattle the same system makes about 312 kWh in December.
Whether that summer surplus pays for the winter gap depends on how your utility credits exports, and that decides whether covering 100% of your use is the right target. Covering 100% makes sense when an exported kWh earns about what a bought kWh costs. California’s investor-owned utilities credit new systems’ exports at hourly avoided-cost prices, not the retail rate. Florida’s investor-owned utilities carry credits forward up to 12 months, then pay out what is still unused at year end at a rate based on the utility’s as-available energy tariff, not your retail rate. In Texas the credit depends on your retail plan or local utility. Where exports earn less, try 70% or 80% in the panel calculator, then compare sizes in the solar payback calculator. Our guide to net metering and net billing explains the rules, and California after NEM 3.0 goes deep on that state. Some utilities also cap the size: Southern California Edison’s net billing tariff limits a system’s estimated output to 150% of your past year’s use, unless you attest to new load.
If a battery is part of the plan, size the panels first, then the battery with the home battery size calculator. More on backup is in our batteries and backup hub.
Checking the size on a quote
Both calculators are built to check an installer’s numbers, not replace them. Before you sign, ask for:
- The use the size is based on, in kWh, and any loads the installer added.
- The production estimate and the tool behind it. Divide the yearly kWh by the system’s kW DC. If that is well above your own PVWatts run for the same roof, ask why in writing.
- Tilt, direction and shade for each roof face, with east and west arrays shown separately.
- Panel model, wattage and count. Count × watts should equal the system size.
- kW DC and kW AC, so you compare like with like. PVWatts’ default DC-to-AC ratio is 1.2.
- The export assumption: which tariff or plan the savings use, at what credit per kWh.
Any tilt frame needs to be covered by the design, the structural calculations and the permit. Panel, inverter and battery wiring is work for a licensed electrician or installer, and the system needs your area’s permits and inspections and the utility’s interconnection approval before it’s switched on.
Where to start
- Getting a first idea of size: start with the panel calculator above, using 12 months of bills.
- Your roof isn’t a south-facing 20°: run the tilt angle calculator next, then put its kWh per kW back into the panel calculator.
- Deciding whether the size pays: go on to the payback calculator and our worth it and costs hub.
Production figures are model estimates for city centres on typical-year weather; real years and real roofs vary. Your utility decides export credits and size limits, and your installer and building department decide what fits. This page is general information, not professional, tax, legal or financial advice.
Sources checked on 4 November 2026.
