Solar Panels for a 3,500 sq ft House

How Many Solar Panels Do You Need? The Sizing Formula

Once you know your annual usage, the standard sizing formula is:

System size (kW) = Annual kWh ÷ (Peak sun hours per day × 365 × 0.80)

Peak sun hours are the daily average of full-sun-equivalent hours at your location. The 0.80 factor is a common allowance for real-world losses such as inverter conversion, wiring, soiling, and heat. For precise results, model your address in NREL’s PVWatts calculator. As a rough guide from NREL solar-resource data, the desert Southwest averages above 6 sun hours a day, while the Pacific Northwest averages closer to 3.5–4.

Panels needed to offset 18,000 kWh per year, by peak sun hours (400W panels)

Chart summary: For a home using 18,000 kWh a year, sunlight alone changes the panel count from 45 (3.5 sun hours) to 24 (6.5 sun hours). That is a swing of 21 panels for identical electricity use, which is why quotes for similar houses in different regions can look very different.

Panels needed at 4.5 peak sun hours, by annual usage (400W panels)

A 400W panel produces about 1.4 kWh per day at 4.5 sun hours under this formula. Panel output declines slowly over time; NREL’s review of field data finds a median degradation rate of about 0.5% per year. Our 3,200 sq ft guide and 3,800 sq ft guide show how the numbers shift for nearby home sizes.

Is There Still a Federal Solar Tax Credit in 2026?

No, not for a system you buy with cash or a loan and place in service in 2026. The 30% Residential Clean Energy Credit (Section 25D) was ended by the One Big Beautiful Bill Act, signed July 4, 2025, for expenditures made after December 31, 2025. The IRS lists the credit as unavailable for property placed in service after that date.

• Installed and operating by December 31, 2025: you can still claim the credit on your 2025 federal return using IRS Form 5695.
• Bought in 2026: the federal homeowner credit is $0, so use the no-credit cost and payback figures in this guide.
• Leases and power purchase agreements (PPAs): industry sources report that the commercial credit (Section 48E) can still apply to third-party-owned systems that meet its deadlines. The provider claims it, and any benefit reaches you only through contract pricing. Read the terms carefully and confirm with a tax professional.
• State, local, and utility incentives: these still exist in many places. Check your state in the DSIRE database, maintained by the NC Clean Energy Technology Center.

This guide is general information, not tax advice.

Does Your Roof Have Enough Space for 24–48 Panels?

Many 400W-class panels measure roughly 65 by 40 inches, or about 18 sq ft. That means:

• 35 panels need about 630 sq ft of usable roof area.
• 24 panels need about 430 sq ft, and 48 panels about 870 sq ft.

Those figures exclude setbacks, vents, chimneys, skylights, and shaded areas, which vary by local code and roof design. Usable space depends on your roof’s shape, pitch, orientation, and shading, so there is no safe rule for how much of a 3,500 sq ft house’s roof will qualify. Check three things:

1. Orientation and shading. Roofs facing east or west generally produce less than south-facing roofs. PVWatts lets you test tilt and orientation for your address.
2. Panel wattage. Higher-wattage panels reduce the number of panels you need for the same kW, which helps on complex roofs.
3. Ground mounts. If the roof cannot fit the system, a ground mount on your property is an option. It generally costs more than a roof mount, so ask installers to price both.

If you cannot fit a full-size system, a smaller one still offsets part of your bill. How much a partial system saves depends on your utility’s net metering or net billing rules.

Sources and Methodology

• Electricity use: U.S. Energy Information Administration, Residential Energy Consumption Survey (RECS) 2020 — average US household use of 10,566 kWh per year. The 15,000–22,000 kWh and 18,000 kWh figures for a 3,500 sq ft home are our planning assumptions.
• Electricity prices: EIA Electric Power Monthly, June 2026 residential averages (US average 18.34¢/kWh).
• Installed cost: SEIA/Wood Mackenzie U.S. Solar Market Insight, Q3 2026 edition (about $3.36/W); EnergySage marketplace data (about $2.58/W).
• Federal tax credit status: IRS Residential Clean Energy Credit page; One Big Beautiful Bill Act (Public Law 119-21), signed July 4, 2025.
• Degradation: NREL review of photovoltaic degradation rates (median about 0.5% per year).
• Sun hours and production modeling: NREL PVWatts and solar-resource data.
• State incentives: DSIRE, NC Clean Energy Technology Center.
• Our calculations: System size uses kW = kWh ÷ (sun hours × 365 × 0.80), panels rounded up at 400W. Payback is simple payback (cost ÷ yearly electricity value) with flat prices and no financing, maintenance, or incentives. The 25-year figure applies 0.5% annual degradation.

Prices, policies, and utility rules change, so verify current details with licensed installers and your utility before you buy.