Solar array sizing

The math

For off-grid sizing, the question is: how many watts of panel can produce enough energy in a day to cover your load, accounting for everything that wastes power between the panel and the load.

array_watts = daily_load_Wh / (peak_sun_hours × system_efficiency)

Peak sun hours is not "hours of daylight". It's a normalized number representing how many hours the sun would have to shine at full standard test conditions (1000 W/m²) to produce the same total energy as the actual day. Most of the US sits between 4 and 5.5. Cloudy regions like the Pacific Northwest dip to 3.0-3.5. The Southwest hits 6+.

System efficiency is the multiplier accounting for losses. The default of 70% accounts for: charge controller (~95%), battery charge/discharge (~85%), wiring (~97%), temperature derate (~90%), soiling (~95%), age (~95%). Multiply those together and you land around 70%. For a freshly installed, well-designed system in temperate climate you can push to 80%. For a hot dusty climate with marginal wiring, 60%.

Use NREL's PVWatts for grid-tied systems

If you have grid connection and net metering, NREL's PVWatts calculator uses your actual location's weather history and gives a more rigorous output number for any array size. It's the right tool for that question. This calculator is for the off-grid sizing question (working backward from a load to a required array size), which PVWatts doesn't do.

What this calculator doesn't do

It assumes you're sizing for daily energy production, not for instantaneous peak power. If your loads spike (pump startup), the inverter handles that, not the panels. It also doesn't optimize panel tilt, orientation, or shading. For those, use NREL's tools or your local solar installer's site survey.