Glossary
Depth of discharge (DoD)
Depth of discharge (DoD) is the percentage of a battery's total capacity that can be used in each cycle without shortening its life. A 10 kWh battery with a recommended DoD of 90% gives you 9 usable kWh. Today's LFP batteries allow very deep discharges; older chemistries and lead-acid allow far less.
What depth of discharge is
A battery should not be fully drained: discharging past a certain point accelerates wear and, in some chemistries, causes permanent damage. The manufacturer defines how far it is safe to go in each cycle. That figure is the recommended depth of discharge, and its practical result is usable capacity: the energy in kWh you can actually draw.
Many manufacturers now publish usable capacity directly, which is more honest. Others advertise nominal (total) capacity and leave the DoD in the fine print.
Why it matters in Florida
When the battery is for outage backup, usable capacity is what decides how many hours you keep the fridge, the modem and a window AC running. If a proposal says “13 kWh” and the usable figure is lower, your backup hours are fewer than you calculated. The system also typically reserves a percentage to restart the inverter, which is not available either.
LFP batteries tolerate nearly full discharges, which is why they dominate the residential market. Lead-acid batteries, still offered in some cheap systems, allow using only about half, which doubles the size you need.
Common mistake
Sizing backup with nominal capacity. Always ask for the guaranteed usable capacity, both at the start and at the end of the warranty (because the battery loses capacity over the years). With those two numbers you can do the real autonomy calculation. Details in how long solar batteries last.
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