Formula
The denominator is a year of peak sun, reduced by the performance ratio:
kW = annual kWh / (peak sun hours × 365 × performance ratio)
The panel page counts modules from a daily target. This page stops at the array's kilowatts.
Assumptions
- The year has 365 days.
- Peak sun hours are a daily average you type.
- The performance ratio is one number for temperature, soiling, and inverter loss.
Worked example
8,000 kWh a year, 5 peak sun hours, and a performance ratio of 0.8 needs an array of 5.48 kW.
Input definitions
- Performance ratio
- The share of ideal output you expect to keep. 0.8 means 80 percent.
- Array size
- The direct-current kilowatts of panels, before they are split into a catalog wattage.
Limitations
This is a planning estimate. The rate, sun hours, and losses are numbers you type. It is not a utility bill, a solar design, or a room-by-room load calculation.
Questions
Why not a panel count?
A count needs a module wattage. That question is on the solar panel page.
What performance ratio should I use?
0.8 is a common planning figure. Raise it only if you have a reason to expect smaller losses.