Solar System Size Calculator
Calculate how many solar panels you need from annual electricity usage and local sun hours.
Get system size in kW, panel count, and estimated roof space needed.
Recommended System Size
How Solar System Size Is Calculated
Sizing a solar array correctly ensures your system meets your electricity needs without wasting money on excess panels. The calculation starts with your daily energy consumption.
Solar System Size Formula:
System Size (kW) = Daily Energy Use (kWh) / (Peak Sun Hours × System Efficiency)
Where:
- Daily Energy Use = annual kWh from utility bill ÷ 365
- Peak Sun Hours = average hours/day of full-strength sunlight at your location (4–6 for most of US/Europe)
- System Efficiency = 0.75–0.85 (accounts for inverter loss, wiring, shading, temperature)
Worked Example: Home using 11,000 kWh/year in Phoenix, AZ (6.0 peak sun hours):
- Daily use = 11,000 / 365 = 30.1 kWh/day
- System size = 30.1 / (6.0 × 0.80) = 30.1 / 4.8 = 6.27 kW
- Round up: 6.5 kW system (13 panels × 500W each)
Number of Panels:
Panels = System Size (W) / Panel Wattage
Peak Sun Hours by Location:
- Phoenix, AZ: 6.0–6.5 hours
- Denver, CO: 5.5 hours
- New York, NY: 4.5 hours
- Seattle, WA: 3.5–4.0 hours
- London, UK: 2.5–3.5 hours
- Sydney, AU: 5.0–5.5 hours
Cost Reference (2025):
- Installed cost: $2.50–$3.50/W in the US
- 6.5 kW system: approximately $16,000–$23,000 before incentives
- US federal tax credit (ITC): 30% through 2032