What a peak sun hour is
Sunlight at noon on a clear day is close to 1,000 watts per square metre — the reference "one sun" at which panels are rated. A peak sun hour is one hour at that intensity: 1 kWh of sunlight on one square metre. A day with 4 peak sun hours delivers as much energy as four hours of full noon sun, however it is actually spread from sunrise to sunset.
That makes the arithmetic of solar power simple: 1 kWp of panels × 4 peak sun hours ≈ 4 kWh before losses — typically 3.2–3.5 kWh after temperature, wiring and inverter losses.
Why the tilted number matters
Most tables publish peak sun hours on a flat, horizontal surface. Panels are tilted, and in winter the difference is large: at 50° N a panel at 70° can receive two to three times the December sunlight of a flat one. For an off-grid system, the flat winter figure understates what a well-tilted array will make.
Peak sun hours in twenty cities
| Place | Flat (horizontal) | At best year-round tilt | At 70° | Lowest month at 70° |
|---|---|---|---|---|
| Phoenix | 5.83 | 6.45 (31°) | 5.14 | 3.57 |
| Los Angeles | 5.44 | 6.02 (31°) | 4.83 | 3.40 |
| Denver | 4.76 | 5.51 (38°) | 4.77 | 3.84 |
| Miami | 5.04 | 5.26 (24°) | 3.93 | 2.57 |
| New York City | 3.93 | 4.43 (35°) | 3.74 | 2.89 |
| Chicago | 3.80 | 4.27 (35°) | 3.63 | 2.64 |
| Seattle | 3.48 | 3.98 (37°) | 3.42 | 1.83 |
| Toronto | 3.64 | 4.08 (35°) | 3.47 | 2.10 |
| London | 2.95 | 3.47 (40°) | 3.10 | 1.70 |
| Dublin | 2.79 | 3.33 (42°) | 3.00 | 1.49 |
| Berlin | 2.83 | 3.29 (39°) | 2.91 | 1.09 |
| Warsaw | 2.99 | 3.49 (39°) | 3.09 | 1.20 |
| Madrid | 4.72 | 5.31 (34°) | 4.42 | 3.92 |
| Rome | 4.42 | 4.96 (34°) | 4.13 | 3.20 |
| Stockholm | 2.78 | 3.42 (44°) | 3.14 | 0.61 |
| Oslo | 2.62 | 3.26 (46°) | 3.02 | 0.71 |
| Sydney | 4.65 | 5.14 (32°) | 4.18 | 3.27 |
| Perth | 5.46 | 5.84 (27°) | 4.47 | 3.55 |
| Cape Town | 5.41 | 5.90 (29°) | 4.64 | 3.72 |
| Auckland | 4.31 | 4.73 (31°) | 3.87 | 3.37 |
From peak sun hours to system size
Array (kWp) = daily use (kWh) ÷ peak sun hours ÷ 0.8
Use the peak sun hours of your design month and your real tilt. The 0.8 is a typical performance ratio for grid-tied systems; for off-grid systems with batteries use about 0.7. The off-grid calculator does the full chain.