What depth of discharge is
Depth of discharge (DoD) is the share of a battery's rated capacity that you take out before recharging. Draw 60 Ah from a 100 Ah battery and you have discharged to 60% DoD — equivalently, 40% state of charge. When a calculator asks for a "planning DoD", it is asking: how deep do you intend to go on a normal cycle?
Why chemistry sets the limit
Every discharge cycle wears a battery, and how fast depends on chemistry and depth. Lead-acid plates sulfate when left deeply discharged, so AGM and flooded banks are planned around 50% DoD. Lithium chemistries tolerate depth far better: LiFePO4 is routinely planned at 80–90%. That difference compounds: for the same usable energy, a lead-acid bank must be nearly twice the nominal size.
| Chemistry | Planning DoD | Usable from 1,200 Wh nominal |
|---|---|---|
| LiFePO4 (LFP) | 90% | 1,080 Wh |
| Li-ion (NMC) | 80% | 960 Wh |
| Gel | 60% | 720 Wh |
| AGM / flooded | 50% | 600 Wh |
What it does to runtime
Take a 100 Ah, 12 V battery powering a 120 W load through a 90%-efficient inverter. As LiFePO4 at 90% DoD it runs about 8 h 06 min; the identical rated capacity as AGM at 50% DoD gives 4 h 30 min. Same amp-hours on the label, almost half the useful runtime — which is why comparing batteries by rated Ah alone is misleading.
What it does to bank sizing
Sizing works the same equation backwards: required bank size = usable energy needed ÷ planning DoD. A system needing 2,667 Wh of usable storage takes a 2,963 Wh LiFePO4 bank — or a 5,333 Wh AGM bank. The Battery Bank Sizing calculator shows this chemistry comparison live for your own numbers.
Choosing your own figure
The defaults are planning figures, not laws. If you want maximum lead-acid lifespan, plan at 30–40% instead of 50%. If a LiFePO4 bank only bridges rare outages, 100% of rated capacity is defensible. Every Watful calculator lets you override the DoD — set it to match your priorities and your battery's datasheet, and be consistent between runtime and sizing calculations.
Frequently asked questions
Is discharging to 100% ever acceptable?
A LiFePO4 battery survives an occasional full discharge, but planning on it leaves no reserve and shortens cycle life. For lead-acid, deep discharges below 50% cause lasting damage. Plan with margin.
Does a lower DoD always mean the battery lasts longer?
Broadly yes — cycle life rises steeply as DoD falls. But total energy throughput matters more than cycle count alone: a bank cycled shallowly needs more cycles to deliver the same energy.
What is the difference between DoD and state of charge?
They are mirror images: 70% depth of discharge is the same as 30% state of charge. DoD counts what you have taken out; SoC counts what is left.