Definition
battery (Wh) = daily use × days of autonomy ÷ depth of discharge ÷ inverter efficiency
Three days of autonomy means a full battery carries the load for three sunless days before it reaches its lowest allowed charge.
How many days to plan for
| Situation | Days |
|---|---|
| Sunny winters, generator on hand | 1–2 |
| Typical continental climate, array sized for the darkest month | 2–3 |
| Maritime climate, long overcast spells | 3–5 |
| Critical loads: medical, telecom, remote monitoring | 5–7 |
Panels are cheaper than batteries
Autonomy only helps if the array can refill the battery between cloudy spells. In the darkest month an undersized array never gets the battery full, and a bigger battery simply takes longer to empty. Size the array for the worst month first; add autonomy for weather, not for season. An array that is 20–30% larger than the bare minimum usually shortens outages more cheaply than doubling the battery.
Snow and dark days
Overcast days are not zero days: a steep array still makes 10–30% of its clear-sky output under thin cloud. The dangerous case is snow on the panels — which is one more reason to tilt off-grid arrays steeply.
Size both halves with the off-grid calculator or the battery alone with the battery bank calculator.