Solar System Size Calculator 2026 — How Many Panels?
Find the solar system size you need in 2026: kW, panel count, roof space and installed cost from your monthly kWh usage and local peak sun hours.
Estimate scope & data quality
- SEO Quick Estimate Uses the inputs available in this calculator.
- Personalized Estimate Use your own bill and location information where this calculator supports it.
- Detailed Solar Economics NOT_COMPUTABLE here without a verified tariff, system design, quote, and eligibility record.
- Data quality
- NOT_AVAILABLE
- Result limitation
- Calculator output is not a source-complete customer result.
A tariff credit is distinct from annual savings. If a result is NOT_COMPUTABLE or CONFLICT, it must not be replaced with an estimate or treated as canonical.
How to use this calculator
- Enter your average monthly electricity usage in kWh (from your utility bill).
- Use the state selector to load your location's daily peak sun hours automatically.
- Set what percentage of your electricity you want solar to cover (80–100% is typical).
- Select your panel wattage (400 W is the current standard).
- See your recommended system size, number of panels, roof space and installed cost at $3.00/W — with no federal credit subtracted for 2026 installs.
Understanding your results
How system size is calculated: Required kW = (monthly kWh × 12 × offset %) ÷ (peak sun hours × 365 × 0.82). The 0.82 factor covers real-world losses. For a home using 899 kWh/month at 100% offset and the 4.5-hour US average: 10,788 ÷ (4.5 × 365 × 0.82) = 8.0 kW, or 20 panels at 400 W. Panel count is rounded up to whole panels (unless within 0.1 panel of a whole number).
The 0.82 loss factor explained: Solar systems don’t produce their nameplate rating every sunny hour. Losses come from inverter conversion, wiring, high panel temperatures, soiling and shading or mismatch. NREL’s PVWatts uses about 14% default system losses (0.86); this calculator uses 0.82 as a slightly more conservative planning figure.
What that size costs in 2026: The calculator prices systems at $3.00/W, a blended US figure (marketplace quotes are often lower; full-market medians are higher — see LBNL Tracking the Sun). A 7 kW system is about $21,000 and the 8.0 kW example about $24,000. Until Dec 31, 2025, homeowners could claim 30% of that cost under §25D; the One Big Beautiful Bill Act (Public Law 119-21, signed July 4, 2025) ended the credit for systems installed after that date (IRS FAQ), so the “installed cost” result is the full price. This is not tax advice; confirm with a tax professional.
Why size matters more without a credit: With no 30% offset, every extra kW adds about $3,000 you pay yourself. Using the site’s savings model (year-1 savings = monthly bill × 12 × bill offset, 0.75 US average), the 899 kWh/month home above pays about $164/month at 18.19¢/kWh (EIA, July 2026 year-to-date), saves roughly $1,472 in year 1, and pays back a $24,000 system in about 16.3 years. Where net metering is weak, sizing closer to daytime usage rather than 100% of annual usage can improve payback.
Roof space planning: A standard 400 W panel is about 21 sq ft including minimal spacing. For 18 panels: 18 × 21 = 378 sq ft of unobstructed roof. Skylights, vents and chimneys reduce usable area — have an installer confirm the layout.
Oversizing considerations: Sizing above 100% makes sense if you plan to add an EV or heat pump soon, since adding panels later needs new permits and wiring. However, many utilities cap net metering at 100–120% of historical usage and pay less for exports than retail, so check your utility’s policy before oversizing.
Frequently asked questions
Direct answers for US homeowners.
Related guides
In-depth articles on sizing, costs, and energy decisions.