SnapEnergyLab
Batteries

Depth of discharge and usable capacity

Depth of discharge is the difference between the number on the spec sheet and the energy you are allowed to take out.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A modular home battery stack in a daylit garage, sunlight falling across the upper modules while the lower ones stay in shadow

Quick answer

Depth of discharge (DoD) is the share of a battery’s nominal capacity you are permitted to take out before the management system stops the discharge. It is not a safety margin you can override — it is the operating window the manufacturer warranties.

Usable energy is nominal capacity × DoD, and what reaches an AC appliance is that figure again multiplied by inverter efficiency. Both terms are missing from most runtime claims.

How to read the numbersYour inputSite defaultCalculated result
Nominal capacity
10 kWh
Your input
Depth of discharge
90%
Site default
Usable to AC loads
8.28 kWh
Calculated result

Why lithium and lead-acid are treated so differently

Lead-acid batteries degrade rapidly when deeply discharged. Sizing a lead-acid bank at 50% DoD is standard practice: taking a 200 Ah bank down to 20% repeatedly can cut its life to a few hundred cycles, while holding it at 50% yields thousands.

Lithium iron phosphate (LFP) tolerates deep cycling far better, and home batteries commonly specify 90–100% usable. Where a manufacturer quotes 100% DoD, they have usually already reserved capacity internally — the “nominal” number you see is the usable one, and the true cell capacity is higher.

The practical consequence: a 10 kWh lead-acid bank and a 10 kWh LFP bank are not comparable products. The first delivers about 5 kWh; the second about 9 kWh before inverter losses.

  1. usable DC energy (kWh) = nominal capacity (kWh) × DoD
  2. usable AC energy (kWh) = usable DC energy × inverter efficiency
  3. runtime (h) = usable AC energy ÷ average AC load (kW)

Inverter efficiency is typically 90–95% at moderate load and lower at very light load, where standby consumption of the inverter itself becomes significant.

Usable AC energy from a 10 kWh nominal battery against depth of discharge02.557.560%80%100%Depth of discharge (%)Energy delivered to AC loads (kWh)
10 kWh nominal capacity at a constant 92% inverter efficiency, which is a site default. The curve is linear in DoD; in practice inverter efficiency also falls slightly at the low state of charge reached at high DoD.
Show the plotted values
Depth of dischargeUsable AC energy (kWh)
50%4.6
60%5.52
70%6.44
80%7.36
85%7.82
90%8.28
95%8.74
100%9.2

Worked example: two 10 kWh banks

  1. Lead-acid nominal10 kWh at 50% DoDYour input
  2. Usable DC10 × 0.50 = 5.0 kWhCalculated result
  3. After inverter at 92%5.0 × 0.92 = 4.6 kWhCalculated result
  4. LFP nominal10 kWh at 90% DoDYour input
  5. Usable DC10 × 0.90 = 9.0 kWhCalculated result
  6. After inverter at 92%9.0 × 0.92 = 8.28 kWhCalculated result
Difference in delivered energy1.8×

Backup reserve is a third deduction

Systems configured for grid backup usually hold a reserve state of charge — often 10–20% — so that a power cut does not find the battery empty after a day of self-consumption. That reserve sits inside the usable window, so your day-to-day available energy is smaller again.

Read a system’s specification carefully: “10 kWh usable” with a 20% backup reserve gives you 8 kWh for arbitrage and 2 kWh held for outages, not 10 kWh for both.

Model your own DoD and inverter efficiency

The calculator exposes depth of discharge and inverter efficiency as editable inputs, and shows the delivered AC energy separately from the nominal capacity so you can see exactly what each assumption costs you.

Open the battery runtime calculator

Assumptions and limitations

  • DoD figures by chemistry are conventional design values, not manufacturer specifications for any particular product. Always use the DoD in your own battery's datasheet and warranty terms.
  • Inverter efficiency varies with load: at loads far below the inverter's rating, its own standby consumption can dominate and effective efficiency drops well below 90%.
  • Capacity fades with age and cycles. A battery several years into service delivers less than its nameplate, whatever the DoD setting.

Sources

  1. IEC 62620 — Secondary lithium cells and batteries for industrial applications — International Electrotechnical Commission

    Supports: Standardised definitions of rated capacity, depth of discharge and cycle life testing conditions.

  2. Battery test manual for electric vehicles and stationary storage — U.S. Department of Energy / Idaho National Laboratory

    Supports: Relationship between depth of discharge and cycle life in lithium-ion cells.

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