On this page
Formula
System size (kW) = Annual kWh × Offset ÷ (365 × Peak sun hours × (1 − Losses)), Panels = ⌈System size ÷ Panel watts⌉
- Annual kWh
- monthly electricity use × 12
- Offset
- the share of that use you want solar to cover (100% = all of it)
- Peak sun hours
- average daily sunlight on the panels in kWh per m² per day, for your location
- Losses
- system losses as a fraction; 14% is the PVWatts default
- Panel watts
- the panel's rated (STC) power; the panel count is rounded up
What the solar panel calculator sizes
Before you collect installer quotes, a solar panel calculator gives you a rough idea of how big a system should be. The solar panel calculator works backward from the electricity you use: it finds the system size in kilowatts that would produce that much in a year at your location, then turns it into a number of panels.
You also see the installed size once the panel count is rounded up, the yearly and monthly production of that array, and the share of your use it covers. It’s a sizing estimate for a grid-tied home system, not a quote.
How to use the calculator
- Enter your monthly electricity use in kWh. Add up the last 12 bills and divide by 12, so summer and winter both count.
- Enter peak sun hours for your location. The free PVWatts calculator from the National Laboratory of the Rockies (formerly NREL) gives this as solar radiation in kWh/m²/day for any US address.
- Keep system losses at 14% unless you have shade or want to include the inverter.
- Enter the panel wattage you’re considering, the rated (STC) power from its spec sheet.
- Set the share of use to cover: 100% to match your yearly use, or less for a partial system.
How to calculate solar panel size by hand
The method has three steps: find the yearly target, find what one kilowatt of panels produces, then divide.
System size (kW) = Annual kWh ÷ (365 × Peak sun hours × (1 − Losses))
Take a home using 1,100 kWh a month in a place with 4.7 peak sun hours, the default 14% losses and 400 W panels:
- Yearly target: 1,100 × 12 = 13,200 kWh
- Yield per kW: 365 × 4.7 × 0.86 = 1,475 kWh per kW per year
- System size: 13,200 ÷ 1,475 = 8.95 kW
- Panels: 8,947 W ÷ 400 W = 22.4, rounded up to 23 panels (9.2 kW installed)
With 440 W panels, the same home would need 21 panels. Raising losses to 18% to cover the inverter pushes the size to about 9.38 kW.
System size by usage and sun hours
The table shows the kW of panels needed to cover 100% of use with 14% losses. The second column is what each installed kW produces in a year.
| Peak sun hours | kWh per kW per year | 500 kWh/mo | 750 kWh/mo | 1,000 kWh/mo | 1,500 kWh/mo |
|---|---|---|---|---|---|
| 3.5 | 1,099 | 5.46 kW | 8.19 kW | 10.92 kW | 16.38 kW |
| 4.0 | 1,256 | 4.78 kW | 7.17 kW | 9.56 kW | 14.34 kW |
| 4.5 | 1,413 | 4.25 kW | 6.37 kW | 8.50 kW | 12.74 kW |
| 5.0 | 1,570 | 3.82 kW | 5.73 kW | 7.65 kW | 11.47 kW |
| 5.5 | 1,726 | 3.48 kW | 5.21 kW | 6.95 kW | 10.43 kW |
| 6.0 | 1,883 | 3.19 kW | 4.78 kW | 6.37 kW | 9.56 kW |
| 6.5 | 2,040 | 2.94 kW | 4.41 kW | 5.88 kW | 8.82 kW |
Sunshine matters as much as usage. A home in a 6.5-hour location needs about 46% less capacity than the same home at 3.5 hours.
Getting the inputs right
Peak sun hours
This is the input that varies most. It depends on latitude, climate, and the tilt and direction of your roof. A south-facing roof in the US gets the most; east- or west-facing roofs get less. Enter your address and roof angle in PVWatts and read the annual average solar radiation; use that number here.
System losses
PVWatts’ 14% default combines soiling (2%), shading (3%), mismatch (2%), wiring (2%), connections (0.5%), light-induced degradation (1.5%), nameplate rating (1%) and availability (3%). The losses multiply rather than add, which is why they total 14% instead of 15%. Raise the figure if trees or chimneys shade the roof.
Your usage
Cutting use first shrinks the system you need. The Department of Energy’s homeowner guide points out that efficiency upgrades mean you’ll need less solar. The kWh calculator shows which appliances use the most, and the electricity cost calculator prices them.
Common mistakes
- Using one month’s bill. A summer or winter bill alone can be far from your average.
- Confusing daylight hours with peak sun hours. Twelve hours of daylight might deliver only 4 or 5 peak sun hours.
- Forgetting the rounding. You can’t buy 0.4 of a panel, so the installed size is usually a little above the target.
- Ignoring future loads. An EV or heat pump can add thousands of kWh a year; include them if they’re coming.
- Treating the result as roof space. Check that the panel count fits your roof before planning around it.
Glossary
- kW (DC): the rated size of the panels added together, under standard test conditions.
- Peak sun hours: daily solar energy on the panels in kWh/m², equivalent to hours of 1,000 W/m² sun.
- Offset: the share of your yearly use the system is designed to produce.
- Net metering: a billing arrangement that credits power you send to the grid against power you draw.
Worked examples
An average home in a sunny state
900 kWh a month, 5.5 peak sun hours, the default 14% losses and 400 W panels.
Inputs: Monthly electricity use = 900 kWh/month; Peak sun hours per day = 5.5 h/day; System losses = 14 %; Panel wattage = 400 W; Share of use to cover = 100 %
Yearly target = 900 kWh × 12 = 10,800 kWh Yield per kW = 365 × 5.5 h × (1 − 14%) = 1,726.5 kWh per kW per year System size = 10,800 ÷ 1,726.5 = 6.256 kW Panels = 6,256 W ÷ 400 W = 15.64 → round up to 16
Answer: To cover 100% of 900 kWh a month with 5.5 peak sun hours and 14% losses you need about 6.26 kW of solar: 16 panels of 400 W (6.4 kW).
A large home in the Northeast
1,250 kWh a month with 4.2 peak sun hours and 430 W panels.
Inputs: Monthly electricity use = 1250 kWh/month; Peak sun hours per day = 4.2 h/day; System losses = 14 %; Panel wattage = 430 W; Share of use to cover = 100 %
Yearly target = 1,250 kWh × 12 = 15,000 kWh Yield per kW = 365 × 4.2 h × (1 − 14%) = 1,318.4 kWh per kW per year System size = 15,000 ÷ 1,318.4 = 11.378 kW Panels = 11,378 W ÷ 430 W = 26.46 → round up to 27
Answer: To cover 100% of 1,250 kWh a month with 4.2 peak sun hours and 14% losses you need about 11.38 kW of solar: 27 panels of 430 W (11.61 kW).
Half the bill in a shaded yard
Covering 50% of 700 kWh a month at 4.8 sun hours, with losses raised to 20% for partial shade.
Inputs: Monthly electricity use = 700 kWh/month; Peak sun hours per day = 4.8 h/day; System losses = 20 %; Panel wattage = 360 W; Share of use to cover = 50 %
Yearly target = 700 kWh × 12 × 50% = 4,200 kWh Yield per kW = 365 × 4.8 h × (1 − 20%) = 1,401.6 kWh per kW per year System size = 4,200 ÷ 1,401.6 = 2.997 kW Panels = 2,997 W ÷ 360 W = 8.32 → round up to 9
Answer: To cover 50% of 700 kWh a month with 4.8 peak sun hours and 20% losses you need about 3.00 kW of solar: 9 panels of 360 W (3.24 kW).
A tiny cabin (edge case)
An off-grid cabin using 60 kWh a month: the math says 0.38 kW, so one 400 W panel is enough.
Inputs: Monthly electricity use = 60 kWh/month; Peak sun hours per day = 6 h/day; System losses = 14 %; Panel wattage = 400 W; Share of use to cover = 100 %
Yearly target = 60 kWh × 12 = 720 kWh Yield per kW = 365 × 6 h × (1 − 14%) = 1,883.4 kWh per kW per year System size = 720 ÷ 1,883.4 = 0.382 kW Panels = 382 W ÷ 400 W = 0.96 → round up to 1
Answer: To cover 100% of 60 kWh a month with 6 peak sun hours and 14% losses you need about 0.38 kW of solar: 1 panel of 400 W (0.4 kW).
Assumptions and limitations
- Production per kW of panels = 365 × peak sun hours × (1 − losses). This is the simple sizing method; it doesn't model temperature, tilt or hour-by-hour weather the way PVWatts does.
- Peak sun hours are entered by you, as an annual daily average for your site and panel orientation. No location data is assumed.
- The default 14% system loss is the PVWatts default (soiling, shading, snow, mismatch, wiring, connections, light-induced degradation, nameplate rating, age and availability). PVWatts models the inverter separately, so add about 4 points if you want this simple method to include inverter losses.
- Sizing targets yearly totals. Summer surplus and winter shortfall are assumed to balance through net metering; battery and off-grid sizing are not covered.
- Panels are rounded up to whole panels, and the installed size and production shown use that rounded count.
- Costs, incentives, roof area and electrical design (strings, inverter sizing) are not included.
Frequently asked questions
How many solar panels do I need?
Divide your yearly kWh by 365 × your peak sun hours × 0.86 to get the system size in kW, then divide by the panel wattage in kW and round up. Using 9,600 kWh a year, 5 sun hours and 420 W panels gives 6.12 kW and 15 panels.
How do you calculate solar panel size from kWh?
System kW = annual kWh ÷ (365 × peak sun hours × (1 − losses)). Each kW of panels then produces 365 × sun hours × 0.86 kWh a year with the default losses, about 1,413 kWh at 4.5 sun hours.
What are peak sun hours?
The day's total sunlight on the panels expressed as hours of full sun at 1,000 watts per square meter. Four peak sun hours means the panels received 4 kWh per square meter that day. PVWatts reports this as solar radiation in kWh/m²/day.
How much electricity does one solar panel produce?
Multiply the panel's watts by the peak sun hours and by 0.86 for losses. A 400 W panel in a spot with 5 peak sun hours makes about 400 × 5 × 0.86 = 1.72 kWh a day, or roughly 628 kWh a year.
What size solar system does an average home need?
The EIA's average US home used about 899 kWh a month in 2022. With 5 peak sun hours and 14% losses, that's a system of about 6.9 kW. The Department of Energy notes that NREL uses an average residential size of 7.15 kW, within a 3–11 kW range.
Should I size for 100% of my bill?
Not always. Utilities differ in how they credit power you export, so producing more than you use may earn little. If credits are low, or your budget or roof is limited, enter a smaller share in the offset box to size a partial system.
Sources
- PVWatts Calculator — National Laboratory of the Rockies (formerly NREL), U.S. Department of Energy. Checked October 10, 2026.
- PVWatts Version 5 Manual (Dobos, 2014) — National Laboratory of the Rockies (formerly NREL). Checked October 10, 2026.
- Homeowner's Guide to Solar — U.S. Department of Energy. Checked October 10, 2026.
- How much electricity does an American home use? — U.S. Energy Information Administration. Checked October 10, 2026.