Commercial Solar System Design Calculator

Peak sun hours (PSH) is a standardized measure of total daily solar energy at a location, expressed as the equivalent number of hours the sun would need to shine at a reference intensity of 1,000 W/m² to deliver that same total energy. It's not literal daylight duration -- a location can have 12 hours of daylight but only 4-5 peak sun hours once cloud cover, sun angle, and atmospheric losses are factored in. NREL publishes PSH data by location, and it typically ranges from around 3 in cloudier northern US regions to 7 in the sunniest desert Southwest locations.

System efficiency (or derate factor) captures every real-world loss between a panel's nameplate rating and the AC power actually delivered: inverter conversion losses, resistance in wiring and connections, dust and dirt accumulation (soiling), panels losing output as they heat up above their rated test temperature, and slight mismatch between individual panels in a string. A typical commercial system lands around 75-85% overall efficiency -- meaning a nameplate 100 kW DC array might realistically deliver closer to 75-85 kW of usable production under real operating conditions.

The required system size is a raw calculation from your energy consumption that can land on any fractional kW value, but real solar panels only come in fixed wattage increments, so the calculator rounds the panel count UP to the next whole panel. That means actual installed DC capacity is typically slightly higher than the bare-minimum required size -- a small, expected rounding margin, not a design overshoot.

A DC:AC ratio above 1.0 (commonly 1.1-1.3 for commercial systems) means the DC array's nameplate capacity slightly exceeds the inverter's AC output capacity. This is standard, deliberate practice: the array rarely produces its absolute peak rated output simultaneously across every panel (real-world irradiance, angle, and temperature conditions are almost never all optimal at once), so a modestly undersized inverter relative to DC capacity captures nearly all of the array's real production while costing less than an inverter sized to a peak the array will essentially never actually hit.

This calculator estimates total roof area using an allowance per panel that includes not just the panel's own footprint but also the spacing between rows that tilted racking systems need to avoid self-shading each other, plus service access space -- which is why the area-per-panel figure used here is meaningfully larger than a single panel's physical dimensions alone. Actual roof area needed also depends on roof shape, obstructions (HVAC units, skylights, vents), and the specific racking system chosen, so treat this as a planning-level estimate to confirm against an actual roof layout.