The right number of solar panels for your house is not a guess. It is a three-step calculation. Add up your annual kWh use from your utility bills, calculate how much kWh a single panel produces at your address, and divide one by the other. A 1,200 kWh-per-month home with 400 W panels in a typical sunbelt climate lands at 21 panels. The same calculation in a cloudier market lands higher. The math is the same; only the inputs change.
Step one: pull your annual kWh from twelve months of bills
The first input is your real kWh usage, not an average and not your dollar bill. Every utility statement shows kWh consumed for the billing period. Pull twelve consecutive months from your online account, add them up, and you have your annual figure.
Twelve months matters. A March-only snapshot understates a household with heavy summer AC. An August-only snapshot overstates winter usage. A full year captures the actual shape of your load.
The national average residential household uses about 10,500 kWh a year. Your house might differ by a factor of two in either direction. A 1,400 square foot apartment with gas heat and gas hot water might use 5,500 kWh. A 3,500 square foot all-electric house in Phoenix with a pool and two EVs might use 25,000 kWh. Averages do not help — your bill does.
The worked example in this article uses 1,200 kWh per month, or 14,400 kWh per year — above the national average, but typical for an all-electric home with one EV. Before locking in your own number, add usage you plan to bring on within three years (a future EV at 3,000–4,500 kWh, a heat pump conversion at 1,500–3,000 kWh in moderate climates), and subtract usage you plan to remove.
Step two: how much one panel produces at your address
The production side of the equation has three inputs: panel wattage, peak sun hours, and a derate factor. The formula:
panel kW × peak sun hours per day × 365 × derate = annual kWh per panel
Panel wattage. Modern residential panels are 380 to 430 watts. The 400 W module is the volume standard — Tier 1 manufacturers, well-priced, in stock at every installer. Use 400 watts (0.4 kW) for the math unless your installer is quoting something specific.
Peak sun hours. This is the most location-sensitive variable. It measures how many hours per day the location averages 1,000 W of irradiance per square meter — effectively, the productive solar hours your roof sees. Annual averages from NREL: Phoenix 5.8, Las Vegas 5.7, Denver 5.5, Dallas 5.3, Miami 5.1, Atlanta 4.7, Chicago 4.4, New York 4.1, Boston 4.0, Seattle 3.5. Your installer can pull the exact figure for your address; for a back-of-envelope check, use your closest large city.
Derate factor. Panels do not produce their nameplate watts in the real world. Heat reduces efficiency. Dust accumulates. Wires resist. Inverters lose a few percent in conversion. The industry standard derate is 0.86 — meaning 86 percent of theoretical maximum. Lower-quality installs sometimes net out at 0.80. Aggressive partners occasionally pitch 0.88. Use 0.86 as the honest middle.
For the worked example at 5.5 peak sun hours: 0.4 × 5.5 × 365 × 0.86 = 710 kWh per panel per year.
Step three: divide and round up
Annual kWh used divided by annual kWh per panel equals panel count. Round up to the next whole panel — you cannot install 20.3 panels.
For the worked example: 14,400 ÷ 710 = 20.3. Round to 21 panels. System size: 21 × 0.4 kW = 8.4 kW DC.
This is the number an honest proposal will land near. An installer pitching 30 panels for this household has either oversized the system or is pricing in significant future load (a second EV, an addition, deeper electrification). A partner pitching 14 panels is sizing for a partial offset, which can be the right call if the roof cannot fit more — but should be flagged, not buried.
The same calculation in different climates moves the answer significantly. Phoenix at 5.8 peak sun hours produces 729 kWh per panel, so 14,400 ÷ 729 = 20 panels. Seattle at 3.5 peak sun hours produces 440 kWh per panel, so 14,400 ÷ 440 = 33 panels. Same household. Thirteen more panels in Seattle than Phoenix. Solar still works in Seattle — it just needs more roof and more capital to produce the same kWh.
When roof space and panel count disagree
"How many panels do I need" and "how many panels fit" are different questions. They usually agree. When they do not, the smaller number wins and the offset plan changes.
A typical 400 W panel measures roughly 17 square feet (about 67 by 39 inches). Twenty-one panels need around 360 square feet of clear, south-facing roof. Subtract chimneys, vent stacks, dormers, and the three-foot setback most fire codes require around the array, and the available area shrinks fast. A 1,800 square foot ranch with east-west exposure and three roof penetrations might fit only sixteen panels.
If roof capacity is the bottleneck, three options are honest. Install what fits and accept partial offset; many homes still hit a strong payback on 70 percent of usage. Add a battery so you self-consume more of what the smaller array produces during summer peaks. Or look at a ground-mount or detached structure such as a garage or carport if the yard permits.
A roof-limited system is not a failed system. It is just sized to the roof's reality.
What changes the answer in the next three years
Solar systems last 25 years. Loads change in much less than 25 years. Sizing only to today's bill almost always undershoots.
Add an EV and your home load typically rises 3,000 to 4,500 kWh per year. A Level 2 home charger on a daily commuter adds the equivalent of four to six 400 W panels.
Replace a gas furnace with a heat pump and your kWh load jumps in heating-dominant climates. Phoenix might add 1,500 kWh; Minneapolis adds 6,000-plus kWh because heat pumps work harder in cold air. A heat pump water heater removes 200 to 400 therms of gas and replaces them with 2,500 to 4,000 kWh of electricity.
Panel degradation runs about 0.5 percent per year. A 21-panel system producing 14,400 kWh in year one will produce about 12,700 kWh in year 25. If you intend to cover every year of the 25, oversize by one panel for the depreciation curve. The right system size is the size of your usage at year five, not year one.