Solar Panel Calculator: How Many Solar Panels Do I Need?

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At the 2024 U.S. average of 863 kWh a month, a home with 4.5 peak sun hours a day needs about 20 solar panels of 400 W, an 8 kW system that makes about 10,848 kWh a year.

How many solar panels do you need?

Enter your average monthly electricity use from your bills, the peak sun hours for your address, and your panel wattage. System losses start at 14%, the NREL PVWatts default. Results: system size, panel count, yearly output, and roof area.

kWh
hours
W
%
%

Estimates only: a licensed installer sizes the real system, and your utility must approve the connection. Peak sun hours are the daily solar radiation (kWh per sq m) that NREL PVWatts shows for your address. Includes a 96% efficient inverter; roof area assumes 19.9 sq ft per panel (a 71.7 x 40 in, 400 W panel).

To size solar panels by hand, divide your yearly use by what 1 kW of panels makes in a year where you live, then divide by your panel's wattage in kW and round up. Penn State's solar design course sizes a grid-connected system the same way, from yearly use and peak sun hours.

Our solar panel calculator also estimates the yearly output and the roof area the panels cover. It doesn't design the system: a licensed installer sizes it for your roof, shade, and electrical service, and your utility has to approve the connection.

What to enter in the solar panel calculator

  1. Monthly electricity use. Add up the kWh on 12 months of bills and divide by 12. The U.S. Department of Energy (DOE) says to "Review electricity bills to determine annual electricity needs." The default, 863 kWh, is the 2024 U.S. residential average from the Energy Information Administration (EIA); California's was 503 kWh and Texas's 1,096. To see what one appliance adds to that, the electricity cost calculator turns its watts and hours of use into kWh a month.
  2. Peak sun hours per day. Get them for your address from NREL's free PVWatts calculator (see below). PVWatts asks for the panels' tilt in degrees; for panels mounted flush with the roof, the roof pitch calculator turns a pitch such as 6/12 into 26.6°. The default 4.5 is an example, not a typical value.
  3. Panel wattage from your quote or the panel's datasheet. The default 400 W is one of the power classes on REC's Alpha Pure datasheet.
  4. System losses. Leave 14%, the PVWatts default, unless you expect more shade or heat (see below).
  5. Share of your use to cover. 100% sizes the panels for all of it; 50% for half.

The solar panel formula

  • Yearly use (kWh) = monthly kWh × 12 × share to cover
  • Yearly kWh from 1 kW of panels = peak sun hours × 365 × (1 − losses) × 0.96
  • System size (kW) = yearly use ÷ yearly kWh from 1 kW
  • Panels = kW × 1,000 ÷ panel watts, rounded up
  • Yearly output (kWh) = panels × watts ÷ 1,000 × yearly kWh from 1 kW
  • Roof area (sq ft) = panels × 19.9

Penn State's course defines a peak sun hour as "the equivalent number of hours per day when solar irradiance averages 1,000 W/m2." NREL's PVWatts manual uses the same 1,000 W per square meter as its reference irradiance for panel ratings. So 1 kW of panels in 4.5 peak sun hours makes 4.5 kWh a day before losses. The 0.96 is PVWatts' default inverter efficiency, kept out of the 14%: "The inverter efficiency is not included in the system loss."

Worked example (the calculator's default values)

A home using 863 kWh a month, 4.5 peak sun hours (an example), 400 W panels, 14% losses, covering 100%:

  1. Yearly use: 863 × 12 = 10,356 kWh.
  2. 1 kW of panels: 4.5 × 365 × 0.86 × 0.96 = 1,356 kWh a year.
  3. Size: 10,356 ÷ 1,356 = 7.64 kW, which is 19.09 panels of 400 W, rounded up to 20 (8 kW).
  4. Output: 8 × 1,356 = 10,848 kWh a year, 105% of the use.
  5. Roof: 20 × 19.9 = about 398 sq ft of panels.

The calculator shows: "You need about 20 solar panels of 400 W: an 8 kW system." and "Use: 863 kWh a month x 12 = 10,356 kWh a year. At 4.5 peak sun hours a day, 14% losses, and a 96% efficient inverter, 1 kW of panels makes about 1,356 kWh a year (4.5 x 365 x 0.86 x 0.96). 10,356 / 1,356 = 7.64 kW, or 19.09 panels of 400 W, rounded up to 20. Estimated output: 10,848 kWh a year (904 a month), 105% of your use. Panel area: about 398 sq ft of roof at 19.9 sq ft per panel, plus the clear paths fire codes require."

Other homes and setups

  • Covering half the use: 10 panels (4 kW), about 5,424 kWh a year, 52% of the use.
  • 4.22 peak sun hours, the figure Penn State's course uses for State College, Pennsylvania: 21 panels (8.4 kW).
  • 6.5 peak sun hours, the figure in a University of Arizona Cooperative Extension example for a south-facing array: 14 panels (5.6 kW).
  • 430 W panels at the default use and sun: 18 panels (7.74 kW).
Solar panels for 863 kWh a month: about 20 panels of 400 watts, an 8 kW systemLeft, a chart of sunlight on the panels over an illustrative day from 6 am to 6 pm. The area under the curve, 4.5 kilowatt-hours per square meter, equals a rectangle 1,000 watts per square meter high and 4.5 hours wide, so the day has 4.5 peak sun hours. Right, 20 solar panels in 4 rows of 5 on a roof plane, each 71.7 by 40 inches, about 19.9 square feet. The work: 863 kWh a month times 12 is 10,356 kWh a year. One kilowatt of panels makes 4.5 times 365 times 0.86 for 14 percent losses times 0.96 for the inverter, about 1,356 kWh a year. 10,356 divided by 1,356 is 7.64 kW, or 19.09 panels of 400 watts, rounded up to 20 panels, 8 kW. They make about 10,848 kWh a year, 105 percent of the use, and cover about 398 square feet.SOLAR PANELS FOR 863 KWH A MONTHPeak sun hours1,0000W per sq m6 amnoon6 pm4.5 ha day's sunlight: 4.5 kWh per sq msame energy: 4.5 h at 1,000 W per sq m20 panels of 400 W = 8 kWclear strip at the ridgeeach 71.7 × 40 in, 19.9 sq ft20 × 19.9 = 398 sq ft of panelsUse: 863 kWh a month × 12 = 10,356 kWh a year1 kW of panels: 4.5 × 365 × 0.86 × 0.96 = 1,356 kWh a year10,356 ÷ 1,356 = 7.64 kW = 19.09 panels of 400 W → 20 (8 kW)Output: 8 × 1,356 = 10,848 kWh a year, 105% of the useRoof: 20 panels × 19.9 sq ft = 398 sq ft, plus fire-code clearances
A peak sun hour is one hour at 1,000 W per sq m, so a day's 4.5 kWh per sq m of sunlight is 4.5 peak sun hours. At that, with 14% losses and a 96% efficient inverter, 1 kW of panels makes about 1,356 kWh a year: 863 kWh × 12 = 10,356 kWh needs 7.64 kW, or 20 panels of 400 W (8 kW), which make about 10,848 kWh a year and cover about 398 sq ft before fire-code clearances.

Solar panels needed by monthly use and sun hours

Panels of 400 W to cover 100% of the use, with 14% losses and a 96% efficient inverter. Multiply a panel count by 0.4 for the system size in kW.

Monthly use3.5 h4 h4.5 h5 h6 h
500 kWh151312109
750 kWh2219171513
863 kWh (U.S. average)2522201815
1,000 kWh2925232017
1,250 kWh3632282521
1,500 kWh4338343025
2,000 kWh5750454034

Per kW and per panel, with the same assumptions:

Peak sun hours1 kW, kWh a yearOne 400 W panel, kWh a year
3.51,055422
41,205482
4.51,356542
51,507603
5.51,657663
61,808723
6.51,959783

How to find your peak sun hours

Peak sun hours are not hours of daylight. They are the day's solar energy expressed as hours at full strength, so they equal the daily solar radiation in kWh per square meter. Penn State's course points to NREL's PVWatts data for U.S. values.

  1. Open PVWatts and enter your address.
  2. Enter the panels' tilt in degrees and the direction they face.
  3. In the results, read the solar radiation in kWh per square meter per day. PVWatts gives it for each month and for the year; enter the yearly figure here.

The EIA notes that "Locations in lower latitudes and in arid climates generally receive higher amounts of insolation than other locations," and that seasonal swings grow with distance from the equator. That's why the calculator works with yearly averages, for the sun and for your bills.

What the 14% system losses cover

These are the default losses in NREL's PVWatts Version 5 manual:

LossPVWatts default
Soiling2%
Shading3%
Snow0%
Mismatch2%
Wiring2%
Connections0.5%
Light-induced degradation1.5%
Nameplate rating1%
Age0%
Availability3%
Total14%

The manual says "The total loss is not the sum of the individual losses"; each one reduces what's left, which works out to 14.08% (the plain sum would be 15%). With the 96% inverter on top, about 17.4% of the rated output is lost in all (1 − 0.86 × 0.96).

Heat is not in that list: PVWatts models panel temperature separately, and this calculator doesn't. A University of Arizona Cooperative Extension example multiplies by 0.88 for temperature losses; folded into the 14%, that's 1 − 0.86 × 0.88 = 24.32%. At 6.5 sun hours, entering 24% gives 15 panels instead of 14. Raise the losses the same way for heavy shade.

How much roof space do solar panels need?

The calculator uses a 400 W REC Alpha Pure panel: 71.7 × 40 in, which REC lists as 19.9 sq ft. REC's Alpha Pure-R measures 68.1 × 44.0 in at 400 W, about 20.8 sq ft, so check your own panel's datasheet. Twenty panels at 19.9 sq ft come to about 398 sq ft.

That's the panels only; fire codes keep parts of the roof clear. Larimer County, Colorado, summarizes section R329.6 of the 2024 International Residential Code: at least two 36 in wide pathways on separate roof planes, one on the array's plane, and an 18 in clear setback on both sides of a horizontal ridge when the array covers up to 33% of the roof (36 in above that). Local amendments differ. To add up the roof planes that face the sun, use the square footage calculator.

Tips and common mistakes

  • Using one month's bill. A summer or winter bill alone can be far from your average; use 12 months.
  • Entering hours of daylight. Peak sun hours count the day's sunlight as hours at full strength (1,000 W per square meter), not the hours the sun is up.
  • Skipping the roof check. DOE says, "If you expect to need a new roof within the next few years, you may want to consider making that improvement before installing solar." The roofing calculator estimates the shingles.
  • Sizing before cutting waste. DOE also says homeowners "should investigate their energy use and consider potential efficiency upgrades" first. Every kWh you stop using is a kWh the panels don't have to make; the attic insulation calculator is one place to start.
  • Forgetting that panels age. REC's Alpha Pure datasheet lists 0.25% annual degradation and 92% of the rated power in year 25 under its warranty. At 92%, the default system would make about 9,981 kWh a year, 96% of the use.
  • Treating the estimate as a design. DOE says "PV systems should be installed by an appropriately licensed installer," recommends at least three bids, and names NABCEP (the North American Board of Certified Energy Practitioners) as the industry standard certification. The installer gets the interconnection permit from your utility. Ask the utility before sizing above 100% of your use.

Frequently asked questions

How many solar panels do I need to power a house?

Divide your yearly kWh by what 1 kW of panels makes in a year where you live, then by the panel's kW. At the 2024 U.S. average of 863 kWh a month and 4.5 peak sun hours, that's about 20 panels of 400 W, an 8 kW system.

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

About 23 panels of 400 W (9.2 kW) at 4.5 peak sun hours, with 14% losses and a 96% efficient inverter. At 5 peak sun hours it's 20 panels; at 3.5, it's 29.

How much electricity does one solar panel produce?

A 400 W panel makes about 542 kWh a year, or about 45 kWh a month, at 4.5 peak sun hours with 14% losses and a 96% efficient inverter. At 6 peak sun hours it makes about 723 kWh a year.

What are peak sun hours?

The number of hours a day the sun would have to shine at 1,000 W per square meter to deliver that day's solar energy. The figure equals the daily solar radiation in kWh per square meter, which NREL's PVWatts calculator gives for your address.

How much roof space do solar panels need?

A 400 W REC Alpha Pure panel is 71.7 x 40 in, about 19.9 sq ft, so 20 panels cover about 398 sq ft. Fire codes also keep pathways and a strip along the ridge clear, so plan on more roof than the panels alone.

What do the 14% system losses include?

NREL's PVWatts default combines soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate rating, age, and availability. It leaves out the inverter (96% efficient by default) and panel heat, which PVWatts models separately.

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