Smart Energy Calculators
for Every Household

How Solar Panel Sizing Actually Works

Sizing a solar system comes down to one question: how much energy do you use in a day, and how much can your panels realistically produce given your location’s sunlight? Most homeowners either oversize (wasting money on panels they don’t need) or undersize (running out of power on cloudy days), because they skip the actual math.

The rough formula looks like this:

1. Add up your daily energy use in kilowatt-hours (check your utility bill, or add up appliance wattage x hours used)
2. Divide that by your area’s average peak sun hours per day (typically 4 to 6 hours in most of the US)
3. Divide again by your panel’s wattage, then add roughly 25% to cover system losses from wiring, inverter efficiency, and dust

A household using 20 kWh a day in a region with 5 peak sun hours would need roughly 5 kW of solar capacity, which usually works out to around 12 to 14 panels rated at 400W each. The Solar Panel Size Calculator handles this automatically once you enter your usage and location.

Typical daily energy use for common household appliances