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. NREL (nrel.gov) has 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.