What “peak sun hours” actually means (and why the number differs between sites)
The day’s sunlight squeezed into hours of perfect 1,000 W/m² sun. 5 peak sun hours = 5 kWh of sunlight per square metre per day.
Sunlight intensity changes all day. “Peak sun hours” is a bookkeeping trick: take all the solar energy that lands on a surface during an average day and ask how many hours of full-strength (1,000 W/m²) sun that equals.
It depends on which surface you measure. Flat ground gets less than a panel tilted toward the sun, so a number “for Denver” can be 4.8 (flat) or 5.7 (tilted optimally) — both correct. Solar Plan Builder always says which surface a number refers to.
Rule of thumb: daily output ≈ system kW × peak sun hours × ~0.8 (the rest is lost to heat, wiring, inverter and dirt). The planner does this properly hour by hour instead.