Solar Battery Autonomy Calculator

About this calculator

Battery autonomy is how many days an off-grid or backup solar system can power a home's loads with zero sun input, calculated as usable battery capacity (Wh) ÷ average daily energy use (Wh/day), where usable capacity already accounts for the battery's depth-of-discharge limit — for example, a 10 kWh lithium bank usable to 90% DoD provides 9,000 Wh of usable energy, not the full 10,000 Wh.

Solar installers and off-grid homeowners size battery banks around this number because autonomy is what protects a home through cloudy stretches, not the panel array — a system with plenty of panel capacity but a small battery bank can still run out of power on the third overcast day in a row, and most off-grid designs target 1 to 3 days of autonomy depending on climate and how critical continuous power is.

This calculator takes your battery bank size and daily energy use and returns how many days of autonomy you have, so you can size a battery bank that matches your climate's typical run of low-sun days rather than guessing.