Solar Panel Calculator: How Many Panels Do I Need?
A home using 10,800 kWh a year (900 a month) needs about 6.88 kW DC (18 panels) to offset all of it, with 5 peak sun hours a day, 14% system losses and 400 W panels. That is 360 sq ft of roof at 20 sq ft a panel, making about 11,300 kWh a year. This is an estimate from round numbers: PVWatts or an installer gives a site-specific figure that accounts for your roof’s direction, tilt and shading.
Figures based on the sources listed belowUpdated How we calculate
Your bill shows the month’s kWh; many utilities also print the past 12 months, which is the better number because winter and summer differ.
Add up 12 months of bills, or read the yearly total from your utility’s online account. 10,800 kWh (900 a month) is an example, not your figure.
100% sizes the system to produce as many kWh in a year as you use. Use less if the roof is small or the utility limits system size; use more if you plan an electric car or heat pump.
The yearly average of daily sunlight on the panels, in kWh per square metre per day (the same number). See the table below for typical values by city, or run your address through NREL’s PVWatts calculator.
NREL’s PVWatts default is 14%: soiling, shading, wiring, mismatch, inverter and downtime together. A shaded roof or an old system loses more.
The DC rating on the panel’s label or data sheet. 350 to 450 W is common for residential panels.
Length × width of one panel from its data sheet. Current 400 W panels are about 19 to 21 sq ft; older 60-cell panels are around 17.5.
6.88 kW DC (18 panels).
6.88 kW DC (18 panels)
- Panels: 18 panels of 400 W = 7.2 kW DC installed (6.88 kW needed, rounded up to whole panels)
- Roof area for the panels: 360 sq ft, 20 sq ft per panel; panels only, with no gaps, setbacks or walkways
- Estimated annual production: 11,300 kWh a year (7.2 kW × 365 × 5 sun hours × 0.86), 105% of the 10,800 kWh used
Shopping list
- Solar panel roof mounting rail
- MC4 solar connectors
- Solar panel cable (10 AWG)
- Home energy monitor
- Electricity usage monitor
Your project
- Electricity use is given as: Kilowatt-hours per year
- Annual electricity use: 10,800 kWh
- Share of your use to offset: 100 %
- Peak sun hours per day: 5 hours
- System losses: 14 %
- Panel wattage: 400 W
- Roof area per panel: 20 sq ft
Also pick up
- Solar panel roof mounting rail
- MC4 solar connectors
- Solar panel cable (10 AWG)
- Home energy monitor
- Electricity usage monitor
Reopen this calculation: /calculators/solar-panel/?s=eyJyb29tIjp7Imxlbmd0aEZ0IjowLCJ3aWR0aEZ0IjowfSwiZGVkdWN0aW9ucyI6W10sIndhc3RlUGN0IjowLjEsInBhcmFtcyI6eyJiYXNpcyI6MCwiYW5udWFsS3doIjoxMDgwMCwibW9udGhseUt3aCI6MTAwMCwib2Zmc2V0UGN0IjoxMDAsInN1bkhvdXJzIjo1LCJsb3NzZXNQY3QiOjE0LCJwYW5lbFdhdHRzIjo0MDAsInBhbmVsU3FGdCI6MjB9fQ
Saved projects
No saved projects yet.
Typical peak sun hours by city, and the system for the example home
| City | Sun hours a day | kWh a year from 1 kW | System for the example home | Basis |
|---|---|---|---|---|
| Phoenix, AZ | 6.5 | 2,040 | 5.29 kW, 14 panels | agrees with secondary sources citing NREL data |
| Las Vegas, NV | 6.3 | 1,978 | 5.46 kW, 14 panels | agrees with secondary sources citing NREL data |
| Los Angeles, CA | 5.6 | 1,758 | 6.14 kW, 16 panels | unverified typical figure |
| Denver, CO | 5.3 | 1,664 | 6.49 kW, 17 panels | agrees with secondary sources citing NREL data |
| Dallas, TX | 5.3 | 1,664 | 6.49 kW, 17 panels | unverified typical figure |
| Miami, FL | 5.2 | 1,632 | 6.62 kW, 17 panels | agrees with secondary sources citing NREL data |
| Atlanta, GA | 4.7 | 1,475 | 7.32 kW, 19 panels | unverified typical figure |
| Boston, MA | 4.4 | 1,381 | 7.82 kW, 20 panels | unverified typical figure |
| New York, NY | 4.2 | 1,318 | 8.19 kW, 21 panels | agrees with secondary sources citing NREL data |
| Chicago, IL | 4.1 | 1,287 | 8.39 kW, 21 panels | agrees with secondary sources citing NREL data |
| Seattle, WA | 3.6 | 1,130 | 9.56 kW, 24 panels | agrees with secondary sources citing NREL data |
System size and panels by electricity use and sun hours
| Annual use | 3.5 sun hours | 4.5 sun hours | 5.5 sun hours | 6.5 sun hours |
|---|---|---|---|---|
| 6,000 kWh (500 a month) | 5.46 kW, 14 | 4.25 kW, 11 | 3.48 kW, 9 | 2.94 kW, 8 |
| 9,000 kWh (750 a month) | 8.19 kW, 21 | 6.37 kW, 16 | 5.21 kW, 14 | 4.41 kW, 12 |
| 12,000 kWh (1,000 a month) | 10.92 kW, 28 | 8.5 kW, 22 | 6.95 kW, 18 | 5.88 kW, 15 |
| 15,000 kWh (1,250 a month) | 13.65 kW, 35 | 10.62 kW, 27 | 8.69 kW, 22 | 7.35 kW, 19 |
Size of residential solar panels
| Panel | Cells | Dimensions | Area (sq ft) |
|---|---|---|---|
| Qcells Q.PEAK DUO BLK-G6+, 330 W | 60 | 68.5 × 40.6 in | 19.3 |
| Panasonic EVPV400H, 400 W | 144 half cells | 1821 × 1016 mm | 19.9 |
| Solaria PowerX-400R, 400 W | 108 half cells | 1723 × 1134 mm | 21.0 |
Methodology & assumptions
The same arithmetic as NREL’s PVWatts, with its inputs reduced to a few round numbers. 1) Annual use = the yearly kWh, or the average monthly kWh × 12. 2) Energy to produce = annual use × the share to offset. 3) A 1 kW (DC) array makes 365 × peak sun hours × (1 − losses) kWh a year; with 5 sun hours and 14% losses that is 1,570 kWh. 4) System size = energy to produce ÷ that figure, in kW DC: 10,800 ÷ 1,570 = 6.88 kW. 5) Panels = system watts ÷ panel watts, rounded UP to whole panels, so the real array (18 panels × 400 W = 7.2 kW) is a little larger than the exact size. 6) Roof area = panels × the area of one panel. 7) Annual production = the installed kW × the yearly output of 1 kW, which is why it is a little above the use you asked to offset. Peak sun hours are the yearly average of the sunlight falling on the panels in kWh per square metre per day (the same number as hours of full 1,000 W/m² sun); the city table is a guide only. System losses of 14% are the PVWatts default: soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate rating and availability. PVWatts applies a further inverter efficiency and a temperature effect on the cells that this simple formula folds into that one percentage. What this is not: a design. The real output depends on which way the roof faces and how steeply it is pitched (south at about the latitude is best), shading from trees and chimneys, the inverter type, local weather year to year (a single year can differ from the average by several percent), and panel aging (output slowly declines over the years, which is not applied here). Roof area is the panels alone; fire-code setbacks from ridges and edges, vents and skylights mean a roof must be larger than that. Use PVWatts (free, from NREL) with your address, tilt and direction, or an installer’s design, before you buy. Net-metering rules, incentives and the cost per watt are not modeled.
- NREL, PVWatts Version 5 Manual (Dobos, 2014, NREL/TP-6A20-62641): total system losses are an input in percent with a default of 14%, replacing the older DC-to-AC derate factor
- NREL, PVWatts V8 API documentation: the calculator’s solar resource output, annual and monthly, is in kWh/m²/day, which is numerically the same as peak sun hours; the same page lists the inputs (tilt, azimuth, array type, system losses)
- NREL, Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors (NREL/TP-463-5607, 1994): monthly and yearly kWh/m²/day for 239 US stations at several fixed tilts, the origin of the city guide values
- Solaria PowerX-400R 400 W module listing (greentechrenewables.com, a reseller): 1723 × 1134 × 35 mm, 108 half cells, about 21 sq ft
- Qcells Q.PEAK DUO BLK-G6+ 330 W module listing (greentechrenewables.com, a reseller): 60 cells, 68.5 × 40.6 × 1.26 in, about 19.3 sq ft
- Panasonic EVPV400H 400 W module (solardesigntool.com, a third-party spec database, not a manufacturer sheet): 144 half cells, 1821 × 1016 mm, about 19.9 sq ft
- Unverified: the sun-hour figures for Los Angeles, Dallas, Atlanta and Boston are typical values this site could not check against an NREL page and are marked so in the table; the others agree with several secondary sources that cite NREL’s National Solar Radiation Database, but none was run through PVWatts for this page. The 17.5 sq ft older-panel size is a rule of thumb, and 10,800 kWh a year is an example, not an average. Always confirm sun hours for your address in PVWatts
How to size a solar panel system
- Find your electricity use. Add up 12 months of kWh from your bills or your utility’s online account, or take the monthly average. Use a full year, because use changes with the seasons.
- Choose the share to offset. Use 100% to match a year of use, less if the roof is small or the utility limits system size, more if you will add an electric car or heat pump.
- Enter peak sun hours and losses. Take peak sun hours for your location from the city table or from NREL’s PVWatts, and keep the 14% PVWatts default for system losses unless you know better.
- Enter the panel. Enter the wattage and the length × width of the panel you are considering, from its data sheet. 350 to 450 W is common.
- Read the system, then check the site. The result gives the kW DC, the number of panels, the roof area and the yearly output. Then run your address, roof direction and tilt through PVWatts, or ask an installer for a site design.
Worked examples
| Project | Result |
|---|---|
| 10,800 kWh a year, 5 sun hours, 14% losses, 400 W panels | 6.88 kW DC (18 panels), 360 sq ft, about 11,300 kWh a year |
| 600 kWh a month, offsetting 80%, 4 sun hours | 4.59 kW DC (12 panels), 240 sq ft, about 6,027 kWh a year |
| The same example home in a 6.5-sun-hour city (Phoenix) | 5.29 kW DC (14 panels), 280 sq ft |
| The same example home in a 3.6-sun-hour city (Seattle) | 9.56 kW DC (24 panels), 480 sq ft |
| The example home offsetting half its use | 3.44 kW DC (9 panels), 180 sq ft |
| The example home with 350 W panels of 18 sq ft | 6.88 kW DC (20 panels), 360 sq ft |
| 14,000 kWh a year (with an electric car), 4.5 sun hours | 9.91 kW DC (25 panels), 500 sq ft, about 14,126 kWh a year |
Frequently asked questions
- How many solar panels do I need for my home?
- Take the kWh you use in a year, divide by the kWh one kW of panels makes in a year (365 × sun hours × (1 − losses)), and divide by the panel wattage. For 10,800 kWh, 5 sun hours and 14% losses that is 6.88 kW, or 18 panels of 400 W. Cloudier places need more: the same home in a 3.6-sun-hour city needs 9.56 kW DC (24 panels).
- What are peak sun hours?
- Peak sun hours are the yearly average of the sunlight on the panels, written as the number of hours of full-strength sun (1,000 watts per square metre) it equals. It is the same number as NREL’s kWh/m²/day. It is not the length of the day: a place with 14 hours of daylight in summer may have 5 peak sun hours averaged over the year. The table on this page gives guide values for several US cities; PVWatts gives one for your address.
- What system losses should I use?
- 14% is the default in NREL’s PVWatts, and is what this calculator starts with. It covers dirt on the panels, shading, snow, mismatch between panels, wiring, connections, first-year light-induced degradation, the nameplate rating and downtime. Use more for a partly shaded roof; use less only if you have a reason, since with no losses the same home needs 5.92 kW DC (15 panels) instead of 6.88 kW DC (18 panels).
- How is the system size in kW worked out?
- System kW = annual kWh × share to offset ÷ (365 × peak sun hours × (1 − losses)). For 10,800 kWh: 10,800 ÷ (365 × 5 × 0.86) = 6.88 kW. The number of panels is that in watts ÷ the panel wattage, rounded up, so 18 panels at 400 W (7.2 kW installed).
- How much roof space do solar panels need?
- About 17.5 to 21 sq ft a panel. A current 400 W panel is around 20 sq ft, so 18 panels take about 360 sq ft. That is the panels alone: the roof needs more for the setbacks from ridges and edges that fire codes require, for vents and chimneys, and for the shaded or poorly aimed parts that should stay empty. Enter your panel’s length × width from its data sheet in the area field for a closer figure.
- Should I offset 100% of my use?
- Not necessarily. 100% produces as many kWh in a year as you use. Offsetting half takes 3.44 kW DC (9 panels), against 6.88 kW DC (18 panels) for all of it. Roof space, the utility’s limit on system size and how it credits extra power all affect the best share. If you plan an electric car or a heat pump, size for your future use: 14,000 kWh a year at 4.5 sun hours is 9.91 kW DC (25 panels).
- What if I only know my monthly bill?
- Choose “Average kilowatt-hours per month” and enter the average of a year of bills, which the calculator multiplies by 12. Use 12 months, not one: summer air conditioning or winter heating can make a single bill much higher or lower than the average. A home averaging 600 kWh a month, offsetting 80% at 4 sun hours, needs 4.59 kW DC (12 panels).
- Why is the real system different from this estimate?
- This uses one sun-hours number and one loss percentage for the whole year. A real roof has a direction and a pitch (south-facing at about the latitude makes the most; east, west or flat roofs make less), shade that moves through the day and year, and weather that varies from year to year. PVWatts models each hour of a typical year for your address; an installer measures the roof. Get a PVWatts run or an installer’s design before you buy.
- Does this tell me the cost or the savings?
- No. It sizes the system and its yearly output only. Cost per watt, incentives, battery storage, your utility’s rate and its net-metering rules vary too much to put one figure on, so ask installers for quotes that show the system size in kW DC and the estimated yearly kWh, and compare them with the figures here.
Handy tools
- Electricity Cost Calculator: Price a kWh of what the panels offset: turn watts and hours into a monthly bill.
- Voltage Drop Calculator: Check the wire run from array to inverter: it gives the voltage lost over its length.
- Conduit Fill Calculator: Find the conduit size for the array wiring from the wire count and gauge.