Most US homes need about 15 to 25 solar panels (400 W each), or a 6 kW to 10 kW system. An average home using 865 kWh a month in an area with 5 peak sun hours needs about 18 panels, or 7.2 kW. Use the calculator below to get your own number in a few seconds.

Quick answer

Formula: (monthly kWh × 12 ÷ 365) ÷ (sun hours × 0.8) = system size in kW.
Panels: system watts ÷ panel watts, rounded up.
Average home: 865 kWh a month → about 7.2 kW, or 18 panels at 5 sun hours.
Cost: about $2.60 per watt installed in 2026, so 7.2 kW ≈ $18,720.
Roof space: about 21 sq ft per 400 W panel.

How many solar panels do I need? Use the calculator

Monthly electricity use (kWh)

Find it on your bill. US average: 865 kWh.

Peak sun hours (by state)

State averages; your site may vary.

Panel wattage (W)

System losses (%)

Heat, wiring, inverter. 15–25% is typical.

7.2 kWSystem size

18Panels needed

$18,720Est. cost at $2.60/W

378 sq ftRoof area (about 21 sq ft per panel)

Results are estimates for a grid-tied system. System size is rounded up to whole panels. For ZIP-code-level sun data, check NREL’s free PVWatts tool, linked in the sources below.

How does the solar panel calculator work?

The calculator uses the same steps an installer uses for a first estimate. It turns your monthly use into daily use, then divides by how much one kW of panels makes per day where you live.

Daily use: monthly kWh × 12 ÷ 365.
Daily output per kW: peak sun hours × (1 − losses).
System size: daily use ÷ daily output per kW.
Panels: system size in watts ÷ panel watts, rounded up.
Cost and roof area: final watts × $2.60, and panels × 21 sq ft.

Worked example: an average US home

The average US home used about 865 kWh a month in 2024, per the EIA. Here is the math with 5 peak sun hours, 400 W panels and 20% losses:

Daily use: 865 kWh × 12 = 10,380 kWh a year ÷ 365 = 28.4 kWh a day
Output per kW: 5 hours × 0.8 = 4 kWh a day
System size: 28.4 ÷ 4 = 7.1 kW
Panels: 7,100 W ÷ 400 W = 17.8, so 18 panels (7.2 kW)
Cost: 7,200 W × $2.60 = $18,720
Roof area: 18 × 21 sq ft = 378 sq ft

The same home in Arizona (6.5 sun hours) needs 28.4 ÷ (6.5 × 0.8) = 5.5 kW, or 14 panels. In Washington (3.5 sun hours) it needs 28.4 ÷ (3.5 × 0.8) = 10.2 kW, or 26 panels. Location changes the answer a lot.

Since the 30% federal tax credit (Section 25D) ended for systems placed in service after December 31, 2025, the cost shown is close to what you pay unless a state or utility rebate applies. See our solar panel cost guide for more detail.

What can make the calculator wrong?

Shade and roof direction: east- or west-facing and partly shaded roofs make less power.
Seasons: sun hours are yearly averages. Winter output can be half of summer, which matters for off-grid systems.
Future loads: an EV, heat pump or pool can add hundreds of kWh a month.
Net metering rules: if your utility pays little for exports, you may want a smaller system plus a battery.

Which solar sizing guide do you need?

The calculator gives a whole-home estimate. These guides go deeper on specific cases:

How many solar panels for a 1,500 sq ft house: use by region and a table by panel wattage.
What size solar inverter do I need?: DC-to-AC ratio, surge watts and a sizing chart.
Solar powered air conditioner: how many panels to run window, mini-split and central AC.
How many solar panels to power an off-grid house: design for the worst winter month.
How many solar panels to run a pool pump: single-speed vs variable-speed pumps.
How many solar batteries to run a house: backup vs full off-grid storage.
Do solar panels work in winter and snow?: how output changes by month.
Solar batteries guide: types, costs and how to choose.
Solar panel cost in the US: prices by system size.

Frequently asked questions

How accurate is a solar panel calculator?

It is good for a first estimate, usually within 10% to 20% for an unshaded, south-facing roof. An installer’s site survey or NREL PVWatts will account for your exact roof angle, shade and weather.

What are peak sun hours?

The number of hours a day when sunlight averages 1,000 watts per square meter. Most of the US gets 4 to 6 a day on a yearly average.

How many solar panels do I need for 1,000 kWh a month?

About 21 panels at 400 W with 5 sun hours: 1,000 × 12 ÷ 365 = 32.9 kWh a day ÷ 4 = 8.2 kW ÷ 0.4 kW = 20.5, so 21 panels.

Should I size my system to cover 100% of my bill?

It depends on your utility. With full net metering, 100% often makes sense. If exports earn a low credit, sizing to 70% to 90% of use can pay back faster.

Sources

Last updated: October 2026. Calculator results are estimates based on average sun hours and the 2026 US average installed cost; get local quotes for exact figures. This article is for information only and is not financial advice.