Step 1: Find your yearly electricity use
Add up the kWh on your last 12 electric bills, or download the annual usage report from your utility's website. The average U.S. home uses roughly 10,500 kWh a year, but yours could easily be half or double that.
If you plan to add an EV, heat pump or pool pump soon, add that load now. An EV driven 12,000 miles a year adds about 3,500 kWh.
Step 2: Look up your peak sun hours
Peak sun hours are the average number of hours per day your roof gets full-strength sunlight. They range from about 3.5 in the Pacific Northwest and New England to 6+ in Arizona and Nevada.
Every state page in our Solar by State hub lists its average sun hours, so pick your state there and note the number.
Step 3: Calculate system size
System size (kW) = yearly kWh ÷ (sun hours × 365 × 0.8). The 0.8 accounts for real-world losses from heat, wiring, inverter efficiency and shading.
Example: 10,500 kWh ÷ (4.5 × 365 × 0.8) = about 8.0 kW.
Step 4: Convert kW into panels
Number of panels = system size in watts ÷ panel wattage. Most residential panels today are 400-440 watts.
Example: 8,000 W ÷ 400 W = 20 panels. The same home in a 6-sun-hour state would need only about 15 panels, while a 3.5-sun-hour state would need about 26.
Adjust for your utility's rules
Under full retail net metering, sizing to cover 100% of your usage usually pays best. Under net billing or avoided-cost policies, exports are worth much less, so sizing to 70-90% of usage (or adding a battery) often gives a faster payback.
Roof space, orientation, shading and your utility's system size cap can also limit how many panels fit. A vetted installer will confirm the final number with a site survey.