Depth of Discharge

What it actually measures

Depth of discharge is the flip side of state of charge. If a battery has 100 kWh of usable capacity and you’ve drawn 70 kWh from it, the DoD is 70%. The state of charge at that point is 30%.

The reason DoD gets its own term - rather than just saying “how full the battery is” - is that battery manufacturers specify warranty and cycle life in terms of DoD. A battery warranted for 6,000 cycles at 80% DoD is telling you something specific: if you regularly drain it to 80% and recharge, it should last 6,000 cycles before degrading to the warranted minimum capacity (usually 70–80% of original).

Why it matters more than total capacity

Cycle life is not a fixed number - it depends on how deeply you cycle the battery. The same lithium iron phosphate (LFP) cell might deliver:

• 6,000+ cycles at 80% DoD
• 8,000+ cycles at 50% DoD
• 10,000+ cycles at 20% DoD

Cycling to shallower depths repeatedly accumulates less wear per cycle. This is why the BMS (battery management system) in most home batteries imposes a floor - it stops discharge at a point that protects longevity rather than letting you drain to true zero.

How manufacturers use it to set warranty terms

When a manufacturer quotes “10,000 cycle life,” that figure almost always has a DoD qualification. Read the fine print on any battery warranty to find both the cycle count and the DoD it applies to.

For home storage, most manufacturers rate their products at 80–100% DoD. LFP chemistry handles deep cycling significantly better than older NMC cells, which is partly why LFP has become dominant in residential storage - it tolerates the daily full-cycle use case without degrading quickly.

The practical ceiling

Most residential batteries never actually reach their rated DoD in normal operation. A BMS typically leaves a buffer of 5–10% at the bottom for protection, and many systems have a user-configurable reserve for blackout backup. A 10 kWh battery with an 80% DoD rating and a 500 Wh backup reserve effectively delivers around 7.5 kWh of usable daily discharge in normal use - which is the figure that matters for calculating self-consumption savings.