Depth of discharge (DoD): why usable capacity is not rated capacity

A battery's rated capacity and the energy you actually get out of it are not always the same number. The gap between them is depth of discharge, and it is one of the easiest specs to overlook and one of the most useful to understand.

What depth of discharge means

Depth of discharge (DoD) is the percentage of a battery's total capacity that the manufacturer lets you draw before the system stops and protects itself. Lithium batteries last longer if they are not run completely flat, so the battery management system reserves a slice at the bottom you never touch.

The math is one line:

Usable kWh = rated kWh x DoD.

A 10 kWh battery rated at 90 percent DoD gives you 9 kWh of usable energy. The other 1 kWh exists on the spec sheet but you never see it.

Why it matters more than it sounds

DoD quietly rescales every runtime estimate you make. If you size your backup from the rated number but the battery only lets you use 80 percent of it, your real backup hours come up short by that same fraction. Pair this with kWh vs kW: usable capacity sets how long, the power rating sets how much at once, and DoD is the discount you apply to the capacity before you do the runtime math.

It also shapes value. Two batteries can advertise the same rated capacity, but if one offers 100 percent usable and the other 80 percent, the first gives you a quarter more real energy for the same headline number.

The good news for 2026 buyers

Here is the part that works in your favor. Most modern LFP home batteries are rated for close to 100 percent usable depth of discharge. The Tesla Powerwall, FranklinWH, and Enphase units all publish usable-capacity figures at or very near their rated capacity, because LFP tolerates deep cycling without the lifespan penalty older chemistries paid. Tesla's Powerwall 3 datasheet lists 13.5 kWh of usable energy, and FranklinWH and Enphase publish comparable usable-to-rated figures (manufacturer datasheets, verified 2026).

That means for most current home batteries, the usable-versus-rated gap that bedeviled earlier systems has largely closed. The rated number and the usable number are nearly the same.

Where the gap still bites

The distinction still matters in two cases:

• Older or used systems. Earlier home batteries, and many NMC-based packs, were rated well below 100 percent DoD, sometimes 80 to 90 percent. If you are buying used or comparing an older model, check the usable figure, not the rated one.
• Marketing that quotes rated, not usable. Always confirm which number a quote is using. "13.5 kWh" should be the usable figure on a modern LFP unit, but it is worth asking, because the honest comparison is usable-to-usable.

The practical rule

When you compare batteries or estimate backup time, use the usable kWh, not the rated kWh. If a spec sheet only lists rated capacity and a DoD percentage, multiply them yourself. Our Worth It calculator works from usable capacity so your backup-hours estimate reflects the energy you can really draw, not a number you will never reach.