"How many solar panels do I need?" is the most searched solar question in the U.S. — and the answer most websites give is wrong. They use a national average that ignores your actual electricity consumption, your roof's sun exposure, and the specific panel wattage your installer quotes.
This guide walks you through the exact formula, with worked examples for 10 cities. By the end, you'll know your target system size and panel count before you talk to a single installer.
Disclaimer: Estimates are based on NREL PVWatts irradiance data and EIA consumption statistics. Actual system size should be confirmed by a licensed solar installer using a site-specific shade analysis.
Key Takeaways
- Most U.S. homes need 18–32 panels (6–12 kW) to offset 100% of electricity use — the exact count requires your annual kWh and your city's peak sun hours
- The formula: System kW = Annual kWh ÷ (Peak sun hours × 365 × 0.80). Then divide system watts by panel wattage for panel count
- Using the national average of 4.5 peak sun hours undersizes Seattle (3.6) or Boston (4.1) systems by 10–25%
- Adding an EV adds 3,200–4,000 kWh/year — size for your post-EV load from the start
The Short Answer
For an average U.S. home (10,500 kWh/year), you need roughly 22–28 solar panels using standard 400W panels. That range swings significantly based on where you live and how much electricity you use.
The two-step formula:
Step 1 — System size (kW) = Annual kWh ÷ (Peak sun hours/day × 365 × 0.80)
Step 2 — Panel count = System size in watts ÷ Panel wattage
The sections below build each step with real numbers.
Step 1 — Find Your Annual Electricity Usage
Pull your last 12 months of electric bills and add up the kilowatt-hours consumed each month. This is your annual usage baseline.
Where to find it: Most utility account portals show 12-month usage history. The number is in kWh — not dollars.
National averages by household size:
| Household Size | Avg Annual kWh |
|---|---|
| 1–2 people | 6,000–8,000 kWh |
| 3–4 people | 9,000–12,000 kWh |
| 4–5 people + large home | 13,000–18,000 kWh |
| 5+ people + EV charging | 18,000–28,000 kWh |
If you're adding an EV: Every 12,000 miles of annual EV driving adds approximately 3,200–4,000 kWh to your home's electricity load (at 3–3.5 miles/kWh). If you plan to buy an EV within three years, size your solar system for your post-EV load from the start. Adding panels later means a second permit and higher per-watt cost.
Step 2 — Find Your Peak Sun Hours
Peak sun hours are the number of hours per day, on average, that your roof receives full-intensity solar radiation (1,000 W/m²). This is not total daylight hours — it's the solar energy equivalent used for production calculations.
| City | Peak Sun Hours/Day | kWh Produced per kW Installed/Year |
|---|---|---|
| Phoenix, AZ | 6.0 | 1,788 |
| Las Vegas, NV | 5.8 | 1,730 |
| Los Angeles, CA | 5.6 | 1,674 |
| Dallas, TX | 5.2 | 1,553 |
| Miami, FL | 5.0 | 1,490 |
| Denver, CO | 5.0 | 1,490 |
| Atlanta, GA | 4.8 | 1,430 |
| New York, NY | 4.2 | 1,254 |
| Boston, MA | 4.1 | 1,224 |
| Minneapolis, MN | 4.1 | 1,224 |
| Chicago, IL | 4.0 | 1,194 |
| Seattle, WA | 3.6 | 1,074 |
Source: NREL PVWatts solar resource data.
Step 3 — Apply the System Size Formula
Formula:
System size (kW) = Annual kWh ÷ (Peak sun hours × 365 × 0.80)
The 0.80 derate factor accounts for real-world losses: panel soiling, temperature, wiring resistance, inverter efficiency, and shading. NREL uses 0.78–0.82 as the standard range; 0.80 is the typical baseline.
Worked examples:
Phoenix, AZ — 14,000 kWh/year
14,000 ÷ (6.0 × 365 × 0.80) = 14,000 ÷ 1,752 = 8.0 kW system
New York, NY — 10,500 kWh/year
10,500 ÷ (4.2 × 365 × 0.80) = 10,500 ÷ 1,226 = 8.6 kW system
Seattle, WA — 9,000 kWh/year
9,000 ÷ (3.6 × 365 × 0.80) = 9,000 ÷ 1,051 = 8.6 kW system
Seattle needs the same system size as New York despite using less electricity — it gets significantly less sun. This surprises most homeowners.
Step 4 — Convert System Size to Panel Count
Modern residential panels range from 350W to 450W, with 400–420W being the current standard in 2026.
Formula: Panel count = System size in watts ÷ Panel wattage
| System Size | 370W Panels | 400W Panels | 430W Panels |
|---|---|---|---|
| 5 kW | 14 | 13 | 12 |
| 7 kW | 19 | 18 | 17 |
| 8 kW | 22 | 20 | 19 |
| 9 kW | 25 | 23 | 21 |
| 10 kW | 28 | 25 | 24 |
| 12 kW | 33 | 30 | 28 |
City-by-City Reference Table
3-person household, 10,500 kWh/year, 400W panels:
| City | Peak Sun Hrs | System Size | Panel Count | Est. Installed Cost (2026) |
|---|---|---|---|---|
| Phoenix, AZ | 6.0 | 7.5 kW | 19 | $19,000–$24,000 |
| Los Angeles, CA | 5.6 | 8.0 kW | 20 | $23,000–$28,000 |
| Dallas, TX | 5.2 | 8.7 kW | 22 | $20,000–$26,000 |
| Miami, FL | 5.0 | 9.0 kW | 23 | $21,000–$27,000 |
| Denver, CO | 5.0 | 9.0 kW | 23 | $22,000–$27,000 |
| Atlanta, GA | 4.8 | 9.5 kW | 24 | $22,000–$28,000 |
| New York, NY | 4.2 | 10.8 kW | 27 | $31,000–$39,000 |
| Boston, MA | 4.1 | 11.0 kW | 28 | $32,000–$40,000 |
| Chicago, IL | 4.0 | 11.3 kW | 29 | $26,000–$33,000 |
| Seattle, WA | 3.6 | 12.6 kW | 32 | $31,000–$40,000 |
For a full cost breakdown including state incentives, use our Solar ROI Calculator.
Does Your Roof Have Enough Space?
Each 400W panel occupies approximately 21 square feet (3.4 ft × 6.1 ft). Add 20% for mounting clearances and walkways.
| Panel Count | Panel Footprint | With 20% Clearance |
|---|---|---|
| 15 panels | 315 sq ft | ~380 sq ft |
| 20 panels | 420 sq ft | ~504 sq ft |
| 25 panels | 525 sq ft | ~630 sq ft |
| 30 panels | 630 sq ft | ~756 sq ft |
Roof orientation and output:
| Orientation | Output Relative to South-Facing |
|---|---|
| South-facing | 100% — optimal |
| East or West-facing | 80–85% — viable |
| North-facing | Not recommended for primary array |
Shading from trees, chimneys, or adjacent buildings is the single biggest real-world performance factor. A professional shade analysis — typically included free with an installer quote — is worth getting before you commit.
Should You Size for 100% Offset?
Conventional advice is "size to cover 100% of annual usage." In 2026, this is more nuanced.
Arguments for 100% offset:
- Maximizes long-term savings and payback speed
- Net metering credits carry excess summer production forward to winter bills
- Protects against future electricity rate increases
Arguments for 80–90% offset:
- Net metering has been significantly reduced in California (NEM 3.0), Hawaii, and several other states — excess export is now worth much less per kWh
- Panel prices continue to fall; microinverter systems can add panels later
- Oversizing for 100% offset increases payback period in low-sun states
California NEM 3.0 note: Export credits dropped to ~$0.04–$0.08/kWh (down from $0.30+). Sizing for 80–90% offset and pairing with battery storage typically yields better ROI than oversizing for full export.
Panel Technology: What to Specify
Monocrystalline vs. Polycrystalline
In 2026, all reputable residential installers use monocrystalline panels. Polycrystalline technology is obsolete for residential use — don't accept it in a quote.
Bifacial Panels
Bifacial panels capture light from both sides and are worth specifying for:
- Ground-mounted systems with reflective ground cover
- Rooftop installations with light-colored roofing material
- Installations where maximum output per panel matters more than upfront cost
Bifacial panels cost 5–15% more and can add 5–20% output depending on installation.
Brand Considerations
Panel brand matters less than installer quality and warranty. SunPower (Maxeon), REC, Canadian Solar, Silfab, and Qcells are all Tier 1 manufacturers (Bloomberg classification). Any of them from a qualified installer is a sound choice.
Common Sizing Mistakes
Using the National Average Sun Hours
The U.S. national average is ~4.5 peak sun hours. If you're in Seattle (3.6) or Boston (4.1), using the national average undersizes your system by 10–25%. Use your city's actual figure.
Ignoring Planned Load Changes
LED lighting upgrades, a new electric dryer, or an EV charging schedule change all affect your annual kWh. Size for where you expect to be in 3 years, not where you are today.
Forgetting Seasonal Variation
A system sized for 100% annual offset produces excess energy in summer and a deficit in winter. In northern states, some grid backup in winter months is expected regardless of system size.
Not Accounting for Roof Azimuth
A due-west-facing roof in Phoenix still produces well. The same roof orientation in Boston loses more relative output and should be sized up accordingly.
The Fastest Way to Get Your Number
Use our Solar ROI Calculator. Enter your state, monthly electricity bill, and home size — it runs the full NREL PVWatts calculation for your location and returns:
- Recommended system size (kW)
- Estimated panel count
- Estimated installed cost range (2026 pricing, no federal credit)
- Estimated annual savings
- Payback period and lifetime ROI
Takes 60 seconds and gives you numbers you can use when evaluating any installer quote.
Bottom Line
Most U.S. homes need 18–32 solar panels (6–12 kW) to offset 100% of their electricity use. The exact count depends on your annual kWh consumption and your city's peak sun hours — not the national average. Use the formula in this guide, or run our calculator, before contacting any installer. It takes 5 minutes and ensures you can recognize whether a quote is properly sized for your home.
Related Guides
- Home Solar Panels: The Complete 2026 Guide — Full overview of costs, financing, installation, and ownership once you know your system size.
- Best Solar Panels for Home in 2026 — Which panel brand to request from your installer — efficiency, warranty, and value compared.
- Solar Panel Installation Process: Step-by-Step Guide (2026) — What happens after you sign a contract, from permit to Permission to Operate.
- Solar Panel Cost by State in 2026 — Installed price benchmarks for your state with every active incentive program.
