Find out exactly how many kilowatts of solar you need based on your electricity usage and your location's sun hours.
Enter Your Details
⚠️ Federal ITC Expired December 31, 2025: The federal 30% residential solar tax credit (Section 25D) expired December 31, 2025 under the One Big Beautiful Bill Act. Homeowner-owned systems installed in 2026 or later do not qualify for this federal credit. If your system was fully installed on or before December 31, 2025, you may still be able to claim it on your 2025 taxes — consult a tax professional. State and utility incentives are unaffected by this change. The cost figures below show gross cost and what the credit would have been if you qualify — use our Tax Credit calculator to check your exact eligibility.
Found on your electric bill
Most modern panels are 400–450W
% of your bill you want to cover
Your Recommended System
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System Size (kW)
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Solar Panels
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Roof Space (sq ft)
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Est. Cost (before tax credit)
Detail
Value
Monthly Usage
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Peak Sun Hours (daily)
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Derate Factor
80% (real-world efficiency)
Desired Coverage
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Panel Wattage
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Annual Production (est.)
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If Federal ITC Applies (2025-or-earlier installs only)
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Your Estimated Panel Layout
Cost range is an estimate. National average for residential solar is $2.50–$3.50 per watt installed before incentives. Your actual quote may differ based on roof complexity, labor costs, and equipment brand. The federal 30% ITC expired December 31, 2025 — it only applies if your system was fully installed on or before that date. Systems installed 2026 or later do not qualify federally; state and utility incentives are unaffected.
How System Sizing Works
1
Measure Your Usage
Check your last 12 months of electric bills and average your monthly kWh consumption for the most accurate result.
2
Account for Sun Hours
Peak sun hours vary by state — Arizona gets 6.5 hrs/day while Maine gets ~4. More sun means fewer panels needed for the same output.
3
Apply Derate Factor
Real-world systems lose about 20% due to shading, temperature, wiring, and inverter losses. We apply 0.80 to be realistic.
4
Size Your Array
System kW = (Monthly kWh × 12 × offset) ÷ (sun hours × 365 × 0.80). Then divide by panel wattage to get panel count.
Methodology & Worked Example
The four steps in the "How System Sizing Works" panel above translate directly into the formula below. Here's the same math run through a real set of numbers so you can see exactly how the calculator gets from your inputs to a recommended system size.
The formula:
Annual kWh needed = Monthly kWh × 12 × Desired offset %
System size (kW) = Annual kWh needed ÷ (Peak sun hours × 365 × 0.80 derate factor)
Panel count = ⌈(System size in watts) ÷ Panel wattage⌉
Roof space (sq ft) ≈ Panel count × 17.5
Worked example: A home in Texas (5.5 peak sun hours/day) uses 900 kWh/month and wants 100% offset with 400W panels.
This matches the calculator above — enter your own monthly usage and state to get a result specific to your home.
What this does not model: your roof's actual usable area, obstructions (vents, chimneys, skylights), local setback and fire-code requirements, shading from trees or nearby structures, or your specific inverter/equipment configuration. The $2.50–$3.50/W cost range is a national average before any federal, state, or utility incentives — see the Tax Credit Calculator to estimate your net cost.
Sources: peak sun hour estimates follow the NREL PVWatts methodology for location-based solar production modeling. National installed-cost-per-watt range reflects commonly cited residential solar industry pricing. Page last reviewed for accuracy August 13, 2026.
Common Questions
What is a good system size for the average home?
The average U.S. home uses about 877 kWh/month. Depending on your state, that typically translates to a 6–10 kW system — roughly 15–25 panels at 400W each.
Should I size for 100% offset or more?
Most homeowners target 90–100% offset. If you plan to add an EV or heat pump soon, sizing to 110% or more now is smarter than adding panels later, which costs more per watt.
Does roof orientation affect the system size?
Yes. South-facing roofs at a 30° pitch are optimal. East or west-facing arrays produce 10–20% less, meaning you may need more panels. Our derate factor partially accounts for this.
How much roof space does a typical system need?
A standard 400W panel is about 17–18 sq ft. A 10-panel (4 kW) system needs roughly 175–185 sq ft of usable roof space — not counting setbacks or obstructions.
What if my roof doesn't have enough space?
Switch to higher-wattage panels (450–550W) to produce more power per square foot. Ground-mount systems are another option if you have yard space.