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Solar · Calculator

Solar array sizing

Daily energy load, your location's peak sun hours and the system losses, worked back to the panel wattage you need.

Solar array sizing
Total from the daily load calculator, or your own number.
PNW 3.0-3.5, Midwest 4-4.5, Mid-Atlantic 4.5-5, Southwest 5.5-6.5 as annual averages. Size for the worst month you need full output in: winter runs roughly half of these.
Charge controller + battery efficiency + wiring + temp derate + soiling. 30% is the default for off-grid, 20% for a well-designed system.
Most modern residential panels are 350-450W. Used only to count how many you need.
Required array -- watts (DC)
Panel count --
Daily production estimate --
Notes --
Estimate, not commitment. PV output varies with seasonal sun angle, snow cover, shading, and panel age. Build in 10-20% extra array if you can afford it; panels are the cheapest part of the system per watt.

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 30% losses (70% efficiency) is what a decent off-grid system comes out at once charge controller, battery round trip, wiring, temperature, dust and age are all in: roughly 0.95 × 0.85 × 0.97 × 0.92 × 0.97, call it 0.7.

Grid-tied is a different problem

This calc works from the load back to the array, which is how off-grid sizing goes. If you have a grid connection and net metering, the question flips to how much a given array will produce over a year in your weather, and that needs hourly irradiance data for your location. Use a production estimator built on that data, or your installer's site survey; this page will oversize a grid-tied array.

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.

Related tools

Further reading