How much energy a solar array makes comes down to three things: its size, your local sunshine, and the real-world losses between the panels and the meter. Multiply the three and you have the daily kWh — the inverse of sizing an array from your usage.
The formula
Daily energy (kWh) = system size (kW) × peak sun hours × performance ratio, where the performance ratio is 1 minus the system losses (typically ~0.8). A single 450 W panel at 4.5 peak sun hours produces about 0.45 × 4.5 × 0.8 ≈ 1.6 kWh a day — roughly 590 kWh a year. A 5 kWp system at the same figures makes about 18 kWh a day, or around 6,570 kWh a year.
What are peak sun hours?
The equivalent number of hours per day of full-strength (1 kW/m²) sunshine your site receives, averaged over the year — most places fall between 3.5 and 6. It is not daylight hours: a 12-hour day may only deliver 5 peak sun hours because morning and evening light is weak.
Solar Panel Output Calculator
Enter panel or system size and your sun hours for the kWh produced per day, month and year.
Why panels beat their rating rarely
The nameplate rating is measured in laboratory conditions the roof rarely sees. Heat is the big one — panels lose output as they warm — plus dust, wiring and inverter losses, and imperfect orientation. That is the ~20% system loss in the calculation, and why a “5 kW” array rarely shows 5 kW on the meter.
What’s a good annual yield?
| Region | Specific yield (kWh/yr per kWp) |
|---|---|
| Cloudier north, Pacific NW | 1,000–1,200 |
| Sunny SW, Arizona, California, Texas | 1,400–1,700 |
The specific yield — kWh per year per kWp installed — is how installers compare sites. If your system delivers well below the local norm, look for shading or faults.