What size solar system do you need?
Solar is sized to your electricity use, not to your roof or your budget alone. Get it right and the system covers the share of your bill you want without paying for panels you do not need. The math is approachable once you know your annual usage and a couple of local factors. Here is how to estimate the size you need before an installer quotes you.
Key takeaways
- Start with your annual electricity use in kWh, from your bills or your utility's online history.
- The U.S. average is about 10,500 kWh a year, but size to your own use, which can be far higher or lower.
- System size (kW) roughly equals annual kWh divided by your local production per kW, which depends on sun-hours.
- In moderate sun, each 1 kW of panels makes about 1,300 to 1,500 kWh a year; sunnier regions yield more.
- Offsetting 100 percent is not always optimal: weak export credits can make a slightly smaller system pay back faster.
Step 1: Find your annual usage
Everything starts with how much electricity you actually use. Pull a full year of bills or download the usage history from your utility's website, and note the total kilowatt-hours (kWh) for the year. A full year matters because air conditioning, heating, and daylight hours swing month to month. The U.S. average is roughly 10,500 kWh a year, about 850 to 900 kWh a month, but your own number is what counts, and an all-electric home or one with EVs can use far more.
Step 2: Turn usage into system size
A solar system's yearly production depends on its size and how much usable sun your location gets, expressed as peak sun-hours per day. A rough, widely used estimate of annual production is the system size in kW times peak sun-hours times 365, reduced by about 20 percent for real-world losses. Turned around to solve for size, you divide your annual kWh by your local production per kW. In moderate-sun regions each 1 kW of panels makes about 1,300 to 1,500 kWh a year, so a home using 9,000 kWh would need roughly a 6 to 7 kW system. Sunnier regions make more per kW and need fewer panels; cloudier ones need more. NREL's free PVWatts tool runs this calculation for your exact address.
Step 3: Reality-check against roof and budget
- Roof space: panels need room. A rough planning figure is about 55 to 70 square feet of usable roof per 1 kW, so a 7 kW system wants roughly 400 to 500 square feet of unshaded south- or west-facing roof.
- Shading and orientation: trees, dormers, and north-facing slopes cut production, so a good installer models them rather than assuming a clear roof.
- Cost: panels run about $2,300 to $3,000 per kW installed, so size is the main price driver. A 6 kW system is roughly $13,800 to $18,000 before any state or utility incentives.
Should you offset 100 percent of your usage?
The instinct is to size solar to wipe out the entire bill, but that is not always the best return. Under full-retail net metering, where exported power is worth the same as power you buy, sizing close to 100 percent of usage makes sense. Under the newer net billing, where exports earn a lower avoided-cost rate, a slightly smaller system that you mostly use on-site can pay back faster than a larger one selling cheap power to the grid. Your utility's export rules, covered in the net metering guide, shape the ideal size as much as your roof does. Your town dashboard is a good starting point for local production and payback.
Frequently asked questions
What size solar system do I need for my house?
How many kWh does a solar system produce?
Should I size solar to cover my whole bill?
See the numbers for your town
These ranges are national. Open a dashboard to see solar prices modeled for your town, with a live estimator and local factors.
Cost figures in this guide are modeled national ranges for general planning, not quotes. Local pricing varies, always get an on-site assessment from a licensed pro before you commit. Evergreen guide