How Many Solar Panels Do I Need?
Enter your electricity usage and location — get the exact number of panels, system size, roof space needed, and estimated cost for your home.
Your Solar System Estimate
Assumes 80% system efficiency (real-world derate factor). Cost at $3.00/W installed before 30% ITC. Each 400W panel = 21.5 sq ft of roof space.
Peak Sun Hours by U.S. City
Peak sun hours determine how much electricity your panels produce. One peak sun hour = 1 hour of sunlight at 1,000 W/m² intensity. More sun hours = fewer panels needed for the same output.
| City | Peak Sun Hrs/Day | Panels for 900 kWh/mo |
|---|---|---|
| Phoenix, AZ | 6.5 | 11 panels |
| Los Angeles, CA | 5.5 | 13 panels |
| Dallas, TX | 5.3 | 14 panels |
| Atlanta, GA | 4.7 | 15 panels |
| Chicago, IL | 4.4 | 16 panels |
| New York, NY | 4.1 | 17 panels |
| Boston, MA | 3.9 | 18 panels |
| Seattle, WA | 3.5 | 20 panels |
Panel count for 900 kWh/month, 80% efficiency, 400W panels.
The Formula Explained
The panel count formula every solar installer uses:
Panels = (Monthly kWh × 12) ÷ (Sun Hours × 365 × 0.80) ÷ Panel Watts × 1,000
Breaking it down: Annual kWh need → divided by annual production per kW → gives system kW → divided by panel wattage in kW → gives panel count. The 0.80 accounts for real-world efficiency losses (heat, wiring, inverter, dirt).
How Much Roof Space Do You Need?
A standard 400W panel measures approximately 6.5 ft × 3.3 ft = 21.5 square feet. A 16-panel system requires about 344 square feet of usable roof. For most homes with 1,800–2,500 sq ft of total roof area, roughly 40–60% (720–1,500 sq ft) is usable for solar after accounting for roof orientation, setbacks, and obstructions.