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How Many Solar Panels Do I Need?

Most U.S. homes need somewhere between 15 and 25 solar panels to cover the bulk of their electricity use. The exact number comes down to three things: how much electricity you use, how much sun your location gets, and how powerful each panel is.

A simple way to estimate it

You can get close with a two-step calculation:

• Step 1 — find the system size you need. Every 1 kW of panels produces roughly 1,200 to 1,700 kWh of electricity per year in the U.S., depending on sunshine. Divide your yearly usage by that figure to get the kilowatts you need.
• Step 2 — convert kilowatts to panels. Modern residential panels are around 400 watts each, so a 1 kW is about 2.5 panels. Multiply your system size by 2.5.

Worked example: the average U.S. home uses about 10,800 kWh a year. At roughly 1,400 kWh per kW, that is about 7.7 kW of panels — which, at 400 watts each, works out to around 19 panels.

Rough guide by household usage

• Lower usage (about 6,000 kWh/year) — roughly a 4–5 kW system, or 11 to 13 panels
• Average usage (about 10,800 kWh/year) — roughly a 7–8 kW system, or 18 to 20 panels
• Higher usage (about 15,000 kWh/year) — roughly a 10–11 kW system, or 25 to 28 panels

Other things that change the number

• Sunlight. A sunny state like Arizona needs fewer panels than a cloudier one to produce the same energy.
• Panel wattage. Higher-wattage panels mean fewer panels for the same output — useful if roof space is tight.
• Roof space. Each panel takes up roughly 18 to 20 square feet, so a 20-panel system needs about 350–400 square feet of usable roof.
• How much you want to offset. Some homeowners size a system to cover 100% of their bill; others cover less to keep the upfront cost down.

Get a tailored figure

Rather than estimate by hand, the free SunWise solar calculator sizes a system for your state and usage and shows the costs and savings that go with it. It is the quickest way to turn these rules of thumb into numbers for your own home.

Next, it is worth reading how much solar panels cost and how long they take to pay for themselves.