One of the first questions homeowners ask when exploring solar is how big the system needs to be. Installers will give you a number in kilowatts, but understanding where that number comes from — and how to sanity-check it — puts you in a much stronger position before any sales conversation.
Start with your electricity usage, not your roof size
System size should be driven by how much electricity your home uses, not by how much roof space you have. The goal is to generate enough power to offset close to 100% of your annual consumption. Oversizing means paying for production you'll never use. Undersizing means you're still buying electricity you could have been generating yourself.
Pull your last 12 months of electricity bills and add up the kilowatt-hours (kWh) used. This number is on every bill. The annual total is the figure your system needs to match. If you can't find 12 months of bills, your utility's online portal usually has usage history going back two years.
The US average is around 10,500 kWh per year, but individual homes vary widely. A smaller home with gas heating and no electric vehicle might use 6,000 kWh. A larger home with electric HVAC, a pool, and an EV charger might use 20,000 kWh or more.
The formula installers use
Once you have your annual usage, the calculation is straightforward. Divide your annual kWh by your location's peak sun hours per day, then divide again by 365 days. The result is the system size in kilowatts you need.
For example: a home using 12,000 kWh per year in Phoenix (6.0 peak sun hours per day) needs roughly a 5.5 kW system. The same home in Boston (4.2 peak sun hours) would need a 7.8 kW system to produce the same annual output — about 40% larger, just because of the difference in sunlight.
This is why system size comparisons between states are misleading. A 6 kW system in Arizona produces significantly more electricity than a 6 kW system in Ohio. What matters is whether your specific system covers your specific usage.
What affects peak sun hours in your area
Peak sun hours aren't the same as daylight hours. They represent the equivalent hours of full-strength sunlight your panels receive on an average day. A cloudy region might have 10 hours of daylight but only 3.5 peak sun hours. A desert location might have 6.5.
Our Best States for Solar guide includes peak sun hour data for every state. As a rough guide, the Southwest averages 5.5 to 7 peak sun hours, the Southeast 4.5 to 5.5, the Northeast 3.5 to 4.5, and the Pacific Northwest 3.5 to 4.5.
Account for efficiency losses
Real-world systems don't operate at 100% efficiency. Heat reduces panel output. Inverter conversion losses, wiring resistance, and occasional shading all take a small cut. A standard efficiency factor of around 80% is commonly applied, which means you size your system about 20% larger than the raw calculation suggests to hit your actual usage target.
Most installers build this into their estimates automatically. When you're comparing quotes, ask each installer what efficiency factor they've used. If one quote assumes a much higher efficiency than another for the same roof and location, push for an explanation.
Sizing for future usage changes
If you're planning to add an electric vehicle, replace a gas appliance with an electric one, add a pool, or add on a room in the next few years, factor that additional load into your sizing now. Adding panels later is possible but more expensive than getting the right size upfront — you'd pay for a second permitting process, additional installation labor, and potentially a panel upgrade.
A good rule of thumb: if you expect to add an EV in the next two to three years, size your system to cover your current usage plus an estimated 3,000 kWh per year for the vehicle. Most EVs average 3,000 to 4,000 kWh annually depending on how much you drive.
What a typical system looks like
For reference, most residential installations fall in the 6 to 10 kW range. At around 400 watts per panel, a 6 kW system uses 15 panels and a 10 kW system uses 25 panels. Roof space required runs roughly 300 to 500 square feet of clear, unobstructed south-facing surface.
If your roof can't accommodate the size your usage requires, you have a few options: prioritize the highest-usage circuits, consider a ground-mount system if you have yard space, or accept partial offset and pair the system with lifestyle changes to reduce consumption.
Sources
- U.S. Energy Information Administration — Use of Electricity (average residential consumption data)
- NREL PVWatts Calculator — Peak sun hour and system output estimates by location
- U.S. Department of Energy — Homeowner's Guide to Going Solar
- NREL — Residential Solar-Adopter Income and Demographics (2022)