How Many Solar Panels Do I Need? A Simple Sizing Guide

To find how many solar panels you need, divide your annual electricity use (kWh) by how much one kilowatt of panels produces per year where you live. That gives the system size in kW. Then divide the system size in watts by the wattage of the panel you plan to buy, and round up. Roof space, shading and your utility's export rules decide whether you can, or should, install that full amount.

The quick formula

Here is the whole calculation in three lines:

1. System size (kW) = annual kWh you want to cover ÷ annual kWh produced per kW of panels at your location
2. Number of panels = system size in watts ÷ panel wattage (round up)
3. Roof area = number of panels × area of one panel (from its spec sheet), plus space your installer needs for access and layout

Each step is explained below with a worked example.

Step 1: Find your annual electricity use

Look at 12 months of utility bills and add up the kWh. A full year matters because usage changes with the seasons. DOE's Energy Saver planning guide points out that summer air conditioning can raise usage, so check each month rather than one bill. The University of Illinois Extension recommends averaging 12 to 24 months of bills.

For a sense of scale, the EIA reports the average US residential customer bought 10,791 kWh of electricity in 2022, about 899 kWh per month, ranging from 6,178 kWh a year in Hawaii to 14,774 kWh in Louisiana. More recently, the average home used 1,152 kWh in August 2026. Your own bills are far more useful than any average. If you don't have them handy, our kWh calculator can help you estimate usage appliance by appliance.

Decide, too, how much of that usage you want solar to cover. You don't have to aim for 100%. The Illinois Extension suggests a generation goal of 50–75% of annual use as a reasonable starting point, and where your utility pays little for exported power, a smaller system can make more financial sense. See net metering explained for why.

Step 2: Estimate production per kW where you live

The same 1 kW of panels produces different amounts of energy in different places, depending on sunshine, tilt, direction, temperature and shading. The best free tool for this is NREL's PVWatts Calculator, which estimates the energy production of grid-connected PV systems for an address you enter. Run it for a 1 kW system with your roof's direction and tilt, and note the annual kWh it reports.

If you'd rather do it by hand, the Illinois Extension uses this approach: kW × peak sun hours per day × 365 × 0.80, where 0.80 accounts for system losses. It notes that Illinois gets about 4 to 4.5 peak sun hours a day. At 4.5 peak sun hours, 1 kW produces about 4.5 × 365 × 0.80 ≈ 1,314 kWh a year.

Step 3: Divide to get system size

Using that hand estimate, a home that wants to cover 10,800 kWh a year needs about 10,800 ÷ 1,314 ≈ 8.2 kW. The table below shows how much location changes the answer for the same 10,800 kWh goal. Production values are illustrative; use PVWatts for your address.

Annual production per kW (example)	System size for 10,800 kWh/yr	Panels at 400 W (example)
1,100 kWh	9.8 kW	25
1,300 kWh	8.3 kW	21
1,500 kWh	7.2 kW	18
1,700 kWh	6.4 kW	16

How does that compare with what people actually install? Berkeley Lab's 2026 data update reports the median US residential system was 7.7 kW in 2025, with most systems between roughly 5 and 12 kW.

Step 4: Turn kW into a number of panels

Panel wattage is printed on every spec sheet. Divide your system size in watts by that rating and round up. Using the 8.2 kW example:

• With 350 W panels: 8,200 ÷ 350 = 23.4, so 24 panels
• With 400 W panels: 8,200 ÷ 400 = 20.5, so 21 panels
• With 450 W panels: 8,200 ÷ 450 = 18.2, so 19 panels

These wattages are examples, not recommendations. Higher-wattage panels mean fewer panels for the same output, which helps on small roofs. Efficiency has improved a lot: Berkeley Lab's Tracking the Sun 2024 executive summary puts the median residential module efficiency at 20.8% in 2023, up from 12.7% in 2002.

Step 5: Check your roof space

Multiply the number of panels by the dimensions on the panel's spec sheet to get the total panel area, then compare it with the usable part of your roof. "Usable" is the key word. DOE's consumer guide to rooftop solar potential notes that potential depends on the size of the roof, how much shade it gets and the direction it faces. Vents, chimneys, skylights and roof edges all eat into space, and your installer will lay panels out to meet local code. PVWatts also lets you draw your roof to see what system size fits.

If the roof can't hold the full system, you have three options: choose higher-wattage panels, accept a smaller system that covers part of your usage, or consider a ground mount if you have the land.

Things that change the right size

• Future loads. An electric vehicle, heat pump or electric water heater will raise usage. Our EV charging cost calculator can estimate how many kWh an EV adds.
• Efficiency upgrades. DOE recommends low-cost efficiency measures before going solar. Shrinking usage first means a smaller, cheaper system.
• Export credit rules. Under full retail net metering, sizing close to 100% of usage often works. Where exports are credited at lower rates, sizing to what you use on-site is usually better value.
• Shade and orientation. East- or west-facing and partly shaded roofs produce less per kW, so you need more kW for the same output.
• Batteries. A battery doesn't change how much energy you need, but it can change how much of your production you use yourself. See are solar batteries worth it.

Common sizing mistakes

• Sizing from one bill. A single summer or winter bill can overstate or understate your annual use by a wide margin. Always use 12 months.
• Using a generic production number. Production per kW differs by region, roof direction and shade. A figure that works in one state can be far off in another, so run PVWatts for your own address.
• Ignoring losses. Real systems lose energy to heat, wiring, inverters, dirt and shading. The Illinois Extension method builds in a 0.80 factor for this; PVWatts models losses for you.
• Copying the national median. Berkeley Lab's 2025 data update put the 2024 median residential system at 7.2 kW nationally but just 5.7 kW in California. Typical sizes vary because usage, sunshine and export rules vary.
• Forgetting the roof's remaining life. Panels are a long-term fixture. If the roof needs work soon, plan that first.

Let the installer confirm it

Your own estimate is for checking quotes, not replacing a site visit. DOE's planning guide says the installer makes the final sizing decision based on your usage, sunlight, system orientation and tilt, and that bids should clearly state the maximum generating capacity in watts or kW plus an estimated annual or monthly output in kWh. If one bid's production estimate is far higher than PVWatts for the same size and roof, ask why.

Next steps

Enter your annual usage and ZIP code in our solar panel calculator to get a size, panel count and payback estimate, or browse solar payback by state to see typical results where you live.