System size (kW) = annual electricity use (kWh) ÷ peak sun hours ÷ 1.2 (system loss). Average US home uses 10,500 kWh/year in a 4-5 peak sun hour area = 6-7 kW system = 15-20 panels. Larger homes or cloudier climates need bigger systems.
Calculating your annual electricity use
Review your last 12 months of utility bills (kWh). Add them up. Average US home: 10,500 kWh/year (varies 5,000-20,000 depending on size and location). High users: all-electric homes (heating, cooling, water heating), large homes, or EV charging. Low users: apartments, mild climates, efficient homes. Example: bills show 800 kWh/month × 12 = 9,600 kWh/year.
Peak sun hours and your location
Peak sun hours = effective hours of full-sun-equivalent per day. Varies by location: Arizona/California = 5-6 hours. Texas/Florida = 4-5 hours. Northeast/Pacific Northwest = 3-4 hours. The U.S. Department of Energy publishes solar irradiance maps by zip code showing exact peak sun hours for your address. This is the biggest factor after electricity use.
System size calculation
Formula: kW needed = (annual kWh ÷ peak sun hours ÷ 365) ÷ 1.2. Example: 10,000 kWh/year in 4.5 peak sun hour area → 10,000 ÷ 4.5 ÷ 365 ÷ 1.2 = 6.7 kW. Rule of thumb: 1 kW per 1,000-1,200 kWh annual use. So 10,000 kWh → 8-10 kW system (conservative) or 6-7 kW (optimized).
Panel count and watts-per-panel
Modern panels: 400-450 watts each. System size in watts ÷ watts per panel = panel count. Example: 6,700 W system ÷ 400 W per panel = 16.75 panels → rounds to 17 panels. Average: 15-22 panels for US residential. Larger homes or cloudier areas: 20-30+ panels. Smaller homes: 8-12 panels.
System oversizing and underperformance
You can oversize slightly (add 5-10% more kW than calculated) to account for seasonal variation and degradation. Winter produces less; oversizing ensures year-round coverage. You cannot undersize—insufficient system size means summer net metering excess but winter energy imports from grid. Net result: higher bills and poor ROI.
Frequently Asked Questions
Does my roof size limit how many panels I can install?
Yes—most residential roofs fit 15-25 panels depending on roof angle, obstructions, and orientation. South-facing roofs are best. Your installer will assess roof suitability and recommend the max system size for your roof.
Should I oversize my system for future EV charging?
Yes—if planning an EV in next 5 years, add 2-3 kW to your system size now. Retrofitting is expensive. An EV uses 4-6 kWh/day (roughly 20-25 kW system). Account for it upfront.
What if my electricity use varies seasonally?
Solar production varies seasonally too (more in summer, less in winter). Design for year-round balance: aim for zero net billing annual (produce = use). Slight overproduction in summer funds winter use.
Do solar installers do this calculation?
Yes—all proposals include system size recommendations based on your electricity use, location, and roof. Get quotes from 2-3 installers for comparison; they may recommend different sizes (all should be within 10-15% of each other).
Can I install a smaller system now and expand later?
Maybe—if your electrical panel has capacity. Expanding costs 30-50% more per watt than original install (relabor, permitting). Better to install full size upfront if budget allows.