CalculateY
Quick Answer
A home using 900 kWh per month in an area with 5 peak sun hours needs approximately a 8.00 kW system, or 20 panels rated at 400 watts each, producing an estimated 10,950 kWh per year.
Check your utility bill for monthly kWh
U.S. average: 4-6 hours
Common Examples
| Input | Result |
|---|---|
| 900 kWh/month, 5 sun hours, 400W panels, $0.16/kWh | 8.00 kW system, 20 panels, ~$1,752/year savings |
| 1,200 kWh/month, 4 sun hours, 400W panels, $0.16/kWh | 13.33 kW system, 34 panels, ~$2,384/year savings |
| 600 kWh/month, 6 sun hours, 400W panels, $0.14/kWh | 4.44 kW system, 12 panels, ~$1,151/year savings |
| 1,000 kWh/month, 5 sun hours, 350W panels, $0.18/kWh | 8.89 kW system, 26 panels, ~$2,189/year savings |
| 750 kWh/month, 4.5 sun hours, 400W panels, $0.12/kWh | 7.41 kW system, 19 panels, ~$1,248/year savings |
How It Works
The Formula
Daily kWh Needed = Monthly kWh / 30
System Size (kW) = Daily kWh / (Peak Sun Hours x (1 - System Loss))
Panels Needed = ceil(System Size kW / Panel kW)
Where:
- Monthly kWh is the household electricity consumption from the utility bill
- Peak Sun Hours is the average number of hours per day that solar irradiance equals 1,000 W/m2 at the location. This varies by geography and season.
- System Loss accounts for inefficiencies from the inverter, wiring, temperature effects, shading, and panel degradation. A standard estimate is 25% (0.25).
- Panel kW is the individual panel wattage divided by 1,000
Understanding Peak Sun Hours
Peak sun hours do not equal daylight hours. A peak sun hour represents one hour of sunlight at an intensity of 1,000 watts per square meter. A location with 6 daylight hours of varying intensity might only have 4 peak sun hours. The southwestern United States averages 5-7 peak sun hours, while the Pacific Northwest averages 3-4. The National Renewable Energy Laboratory (NREL) provides detailed solar resource maps for specific locations.
System Loss Factors
The 25% system loss factor accounts for multiple sources of energy loss:
- Inverter efficiency: 3-5% loss converting DC to AC power
- Wiring and connection losses: 1-3%
- Temperature effects: 5-10% (panels lose efficiency in high heat)
- Soiling and shading: 2-5%
- System age and degradation: 0.5-1% per year
Panel Wattage Standards
Modern residential solar panels typically range from 350 to 450 watts. The standard panel used in this calculator is 400 watts, which is representative of current mainstream panels. Higher-wattage panels (430-450W) cost more per panel but require fewer panels for the same output.
Annual Savings Estimate
The estimated annual savings multiplies the system’s annual production by the electricity rate. This represents the value of electricity the system offsets. Actual savings depend on the utility’s net metering policy, time-of-use rates, and seasonal variation in production.
Worked Example
For a home using 900 kWh per month in an area with 5 peak sun hours, using 400-watt panels at $0.16/kWh: Daily kWh = 900 / 30 = 30 kWh. System size = 30 / (5 x 0.75) = 30 / 3.75 = 8.00 kW. Panels needed = ceil(8.00 / 0.40) = 20 panels. Actual system = 20 x 0.40 = 8.00 kW. Daily production = 8.00 x 5 x 0.75 = 30.00 kWh. Monthly production = 30.00 x 30 = 900 kWh. Annual production = 30.00 x 365 = 10,950 kWh. Estimated annual savings = 10,950 x $0.16 = $1,752.