YepCalc

Solar Panel Calculator

Estimate the solar system size and the number of panels you need to cover your electricity use. Enter your use from your electric bill and the peak sun hours where you live. The calculator shows the system size in kW, the yearly production, and the area the panels cover.

Electricity use

Find it on your electric bill. Twelve months of bills give the best average.

%

The share of your electricity use to cover with solar.

Sun and losses

hours/day

Your average daily sun in kWh/m² per day. Look up your address in PVWatts and use the yearly solar radiation.

%

Default: 20%

Covers dirt, shading, wiring, the inverter, and other losses.

Panels

Sizes are from typical panels. Check your panel's spec sheet.

Cost

Result

Enter your electricity use and peak sun hours to see how many solar panels you need.

How many solar panels you need

A 1 kW system makes about 1 kWh for each peak sun hour, less the system losses. So the system size is your yearly use divided by what 1 kW makes in a year:

Yearly output per kW = peak sun hours × 365 × (1 − system losses)

System size (kW) = yearly use × offset target ÷ yearly output per kW

Panels = system size × 1,000 ÷ panel wattage, rounded up

The calculator turns a monthly use into a yearly one by multiplying by 12, and a daily use by 365. Dividing a daily use by peak sun hours × (1 − losses) gives the same system size. This is the same calculation as the example in NREL's Technology Tips for Solar + Storage REAP Application Reviews: a 3 kW system with 5 kWh/m² of sun a day and 20% losses makes 3 × 5 × 0.80 = 12.0 kWh a day.

Because the panel count is rounded up, the system you install is a little larger than the size you need. The calculator works out the production and the offset from that installed size:

Installed system size = panels × panel wattage ÷ 1,000

Estimated production = installed system size × yearly output per kW

The production is an estimate for the first year. Panels lose some output as they age, about 0.5% a year according to NREL's PVWatts Version 5 Manual, and the calculator doesn't take that into account.

How to find your peak sun hours

Peak sun hours are the sunlight a panel gets in a day, in kWh/m², counted as hours of full sun at 1,000 W/m². One kWh/m² is one hour of full sun, so 5 kWh/m² per day is 5 peak sun hours. Panel ratings are measured at the same 1,000 W/m².

  1. Open PVWatts, the free solar calculator from the U.S. Department of Energy's national lab (pvwatts.nlr.gov).
  2. Enter your address, then the direction your roof faces (azimuth) and its tilt.
  3. On the results page, use the yearly Solar Radiation value, in kWh/m²/day, as your peak sun hours.

PVWatts's solar radiation is the sun on the face of the panels, so it already includes the direction and tilt you entered. That's why the calculator has no fields for them.

As a rough guide, NREL's REAP review tips give 3.0 to 6.0 kWh/m² per day on flat ground over most of the continental U.S. If you only have that flat-ground value, you can use it, but the result will be on the safe side: panels tilted toward the sun get more, and the calculator still takes the losses off, so it may show a panel or two more than you need.

NREL was renamed the National Laboratory of the Rockies (NLR) in December 2025, and PVWatts moved to the nlr.gov address with it.

System losses

System losses cover everything between the panel rating and the AC power the system delivers: dirt, shading, wiring, the inverter, heat, and more. The default of 20% is the overall loss from the array to the inverter output in NREL's REAP review tips.

PVWatts splits these losses in three. Its system losses default to 14.08%, combined from soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0%, and availability 3%. The inverter is a separate input at 96% efficiency, and PVWatts works out heat losses from the cell temperature. Together, the first two come to about 17.5%. The calculator uses one number for all of them, so don't enter PVWatts's 14.08% here, or you'll get too few panels.

Panels make less power as they get hotter, so raise the losses in a hot climate. Raise them too for a roof with shade from trees or chimneys.

Panel wattage and size

Most home solar panels today are about 400 to 440 W. The calculator starts with 400 W, the count you'd need for the lower end of that range. The panel sizes are from typical panels:

Panel sizes vary by model, so check your panel's spec sheet. For a different panel, choose Custom panel and enter its rated power (Pmax) and size.

The panel area is the panels alone. Your roof needs extra space around and between them, which depends on the roof shape and local rules, so the calculator doesn't work out the roof area you need.

Price per watt

Solar quotes are often compared by price per watt. Divide the total of a quote by the system size in watts, and enter that price to compare other sizes. The cost is the installed system size × 1,000 × the price per watt.

Examples

A home using 900 kWh a month

With 5 peak sun hours and 20% losses, 1 kW makes 5 × 365 × 0.8 = 1,460 kWh a year. The home uses 900 × 12 = 10,800 kWh a year, so it needs 10,800 ÷ 1,460 = 7.40 kW. With 400 W panels, that's 7,397 ÷ 400 = 18.5, rounded up to 19 panels, a 7.60 kW system. It makes about 11,096 kWh a year (925 kWh a month), 102.74% of the use, and the panels cover about 401.58 ft² (37.31 m²). At $3.00 per watt, it costs $22,800.

A Metric example

A home using 4,000 kWh a year, with 2.8 peak sun hours and 20% losses, gets 2.8 × 365 × 0.8 = 817.6 kWh a year per kW, so it needs 4.89 kW. With 430 W panels, that's 12 panels, a 5.16 kW system making about 4,219 kWh a year and covering 23.21 m² (249.83 ft²).

FAQ