Enter Your Details
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 | — |
| Peak Sun Hours (daily) | — |
| Derate Factor | 80% (real-world efficiency) |
| Desired Coverage | — |
| Panel Wattage | — |
| Annual Production (est.) | — |
| After 30% Federal ITC | — |
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 30% federal ITC applies to the full installed cost.
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.
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.
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