The most common question homeowners ask before going solar — and the one that gets the vaguest answers from installers with an incentive to sell you more panels than you need. Here's how to actually calculate it yourself, what the number depends on, and how to sanity-check any quote you receive.
The Short Answer
The average U.S. home needs 17–21 solar panels to cover 100% of its electricity use. But that average means almost nothing for your specific situation. The real number depends on three things: how much electricity you use, how much sun your location gets, and how efficient your panels are.
A home in Phoenix using 1,000 kWh/month might need 12–14 panels. The same energy use in Seattle might need 22–28 panels. Same house, same bill — wildly different system sizes.
Step 1: Find Your Annual Electricity Use
Pull up 12 months of electric bills and add up the kilowatt-hours (kWh) used. The kWh figure is always on your bill — it's what you're actually paying for, not the dollar amount. Add all 12 months together for your annual total.
U.S. averages by household size:
- 1–2 person household: 6,000–8,000 kWh/year
- 3–4 person household: 9,000–12,000 kWh/year (national average: ~10,500 kWh)
- 5+ person household or large home: 12,000–18,000 kWh/year
If you're planning to add an EV or switch from gas heating to a heat pump, account for that now. Adding an EV adds roughly 3,000–4,500 kWh/year depending on mileage. A whole-home heat pump replacement can add 2,000–5,000 kWh/year depending on climate and previous heating system.
Step 2: Find Your Peak Sun Hours
Peak sun hours aren't the number of daylight hours — they're a measure of total solar energy available per day, expressed as equivalent hours of peak-intensity sunlight (1,000 W/m²). This is the number solar installers and the NREL database use to model production.
Approximate peak sun hours by region:
- Southwest (AZ, NV, NM, Southern CA): 5.5–7.0 hours/day
- Southeast (FL, TX, GA, SC): 4.5–5.5 hours/day
- Mid-Atlantic and Midwest: 4.0–4.5 hours/day
- Northeast (NY, MA, CT): 3.5–4.5 hours/day
- Pacific Northwest (WA, OR): 3.0–4.0 hours/day
- Alaska: 2.5–3.5 hours/day
For a precise number for your ZIP code, use NREL's PVWatts Calculator (pvwatts.nrel.gov) — it's the same tool serious installers use.
Step 3: The Math
The formula has two parts. First, calculate the system size you need in kilowatts (kW):
System size (kW) = Annual kWh ÷ (Peak sun hours/day × 365 days × 0.80)
The 0.80 accounts for real-world system losses — heat, wiring, inverter efficiency, dust. Most systems operate at 75–85% of theoretical peak.
Then, calculate panel count:
Panels needed = System size (kW) × 1,000 ÷ Panel wattage
Modern residential panels typically range from 370–430 watts. A 400W panel is a reasonable midpoint for this calculation.
Real Examples by State
Phoenix, Arizona
Annual use: 12,000 kWh (above average due to AC). Peak sun: 6.0 hrs/day.
System size = 12,000 ÷ (6.0 × 365 × 0.80) = 6.8 kW
Panels at 400W = 6,800 ÷ 400 = 17 panels
Phoenix homeowners benefit from exceptional sun but also have high summer cooling loads. A well-sized 7 kW system typically offsets 90–100% of annual use.
Atlanta, Georgia
Annual use: 11,000 kWh. Peak sun: 4.8 hrs/day.
System size = 11,000 ÷ (4.8 × 365 × 0.80) = 7.9 kW
Panels at 400W = 7,900 ÷ 400 = 20 panels
Boston, Massachusetts
Annual use: 8,000 kWh. Peak sun: 4.2 hrs/day.
System size = 8,000 ÷ (4.2 × 365 × 0.80) = 6.5 kW
Panels at 400W = 6,500 ÷ 400 = 16–17 panels
Massachusetts has strong net metering and state incentives that make solar economics favorable despite lower sun hours.
Seattle, Washington
Annual use: 10,000 kWh. Peak sun: 3.5 hrs/day.
System size = 10,000 ÷ (3.5 × 365 × 0.80) = 9.8 kW
Panels at 400W = 9,800 ÷ 400 = 25 panels
Seattle's low sun hours require a significantly larger system to produce the same output. This, combined with low electricity rates from hydropower (~10¢/kWh), makes solar economics in the Pacific Northwest less compelling than in other regions. Payback periods of 12–16 years are typical, though this changes if you add an EV.
Roof Space: Does It Fit?
Each 400W solar panel is typically about 20–22 sq ft (roughly 65" × 39"). A 20-panel system needs approximately 400–440 sq ft of usable roof space.
"Usable" means south-to-west facing, unshaded, not occupied by vents, skylights, or chimneys, and with a pitch between 15–40°. Most installers use satellite imagery software (Aurora, Helioscope) to calculate exact usable area before designing a system.
If your roof doesn't have enough south-facing space, east/west split arrays are a common alternative — they produce 10–20% less per panel but can work well on many rooftops. North-facing panels are not recommended in the U.S. (the opposite applies in the Southern Hemisphere).
Should You Size for 100% Offset?
Not always. The answer depends on your utility's net metering policy. If your utility credits excess solar at full retail rate (true net metering), sizing for 100% or slightly above makes sense — you bank credits in sunny months to draw down in winter. This is still the policy in California, New York, New Jersey, Massachusetts, and many other states.
If your utility uses "net billing" or "avoided cost" net metering — crediting excess solar at the wholesale rate (3–5¢/kWh) rather than retail (13–25¢/kWh) — oversizing your system doesn't pay. In that case, size to offset roughly 90–95% of your usage, avoiding consistent excess export. This has become more common as utilities in states like California (NEM 3.0), Nevada, and others have revised their net metering programs.
Ask your installer specifically what your utility's current net metering rate is for exported solar. The answer should drive your sizing decision.
Battery Storage: Does It Change the Panel Count?
Adding battery storage (like a Tesla Powerwall or Enphase IQ Battery) doesn't change how many panels you need to produce your target annual output — but it can change how you size the system if your goal is self-sufficiency rather than annual offset.
If you want to run through several cloudy days without grid power, you'd size your panels and battery to cover daily use plus a buffer. That typically means 20–40% more panels than the grid-tied calculation above. For most homeowners, the economics of an oversized battery-backed system don't pencil out — focus on right-sizing for your annual kWh use first.
See our Battery Storage Calculator to model whether storage makes sense for your situation.
Sanity-Checking an Installer Quote
When you receive a solar quote, verify these numbers before signing:
- Annual production estimate vs. your actual annual use: The quote should show estimated kWh production. Divide by your annual use — the ratio should be 0.90–1.05 for a properly sized system.
- Price per watt: Divide gross system cost by system wattage (e.g., $21,000 ÷ 7,000 watts = $3.00/watt). Fair range in most U.S. markets is $2.50–$3.50/watt before incentives. Significantly higher warrants pushback.
- Production software used: Ask if they used PVWatts, Aurora, or Helioscope. Any credible installer uses one of these. A production estimate produced by proprietary in-house models with no third-party basis should be questioned.
- Shading analysis: Ask if they did a shading analysis and what their shading derate is. Even small amounts of shading significantly affect output — the model should reflect your actual roof, not an idealized one.
Quick Reference: Panel Count by Annual Use and Location
Assuming 400W panels and 80% system efficiency:
- 8,000 kWh/yr in high sun (5.5 hrs): ~5.0 kW → 13 panels
- 8,000 kWh/yr in medium sun (4.5 hrs): ~6.1 kW → 15–16 panels
- 8,000 kWh/yr in low sun (3.5 hrs): ~7.8 kW → 20 panels
- 12,000 kWh/yr in high sun (5.5 hrs): ~7.5 kW → 19 panels
- 12,000 kWh/yr in medium sun (4.5 hrs): ~9.1 kW → 23 panels
- 12,000 kWh/yr in low sun (3.5 hrs): ~11.7 kW → 29–30 panels