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Batteries

Why a 10 kWh battery does not deliver 10 kWh

Depth of discharge and inverter conversion take their share before a single appliance sees any energy.

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

A home battery and hybrid inverter mounted on a house wall with conduit running toward the consumer unit

Quick answer

A 10 kWh battery does not deliver 10 kWh because two deductions happen before an appliance sees anything: the depth-of-discharge limit set by the battery management system, and the DC-to-AC conversion loss in the inverter.

At typical values — 90% DoD and 92% inverter efficiency — a nominal 10 kWh pack delivers about 8.28 kWh. Configure a backup reserve, or put the pack somewhere cold, and it delivers less again.

How to read the numbersYour inputSite defaultCalculated resultIllustrative model
Nominal
10 kWh
Your input
Delivered to AC loads
8.28 kWh
Calculated result
Missing
17%
Calculated result

Where the kilowatt-hours go

Deductions from nominal battery capacity to delivered AC energy
StageEnergy remainingWhat happened
Nominal capacity10.00 kWhThe number on the box.
After depth of discharge (90%)9.00 kWhThe management system stops the discharge here to protect cycle life.
After inverter conversion (92%)8.28 kWhDC to AC conversion loss, delivered at the socket.
After a 10% backup reserve7.28 kWhHeld back for outages, if the system is configured that way.
After cold-weather derating (0.9)6.55 kWhApplies only when the pack is cold. Illustrative factor.
  1. delivered AC energy = nominal × DoD × inverter efficiency × (1 − reserve) × temperature factor
  3. 10 × 0.90 × 0.92 = 8.28 kWh (typical indoor, no reserve)
  4. 10 × 0.90 × 0.92 × 0.90 = 7.45 kWh (with a 10% backup reserve)
  5. 10 × 0.90 × 0.92 × 0.90 × 0.90 = 6.71 kWh (and a cold pack)

Every factor is a multiplication, so the deductions compound. Adding them as percentages overstates the remaining energy.

Deduction one: depth of discharge

No battery management system lets you run a pack to genuinely zero. Doing so damages cells and, for lithium, can make them unrecoverable. The permitted window is the depth of discharge: typically 90% or more for LFP home batteries, and around 50% for lead-acid.

Some manufacturers quote “usable” capacity that already has this deduction applied. If a spec sheet gives both a nominal and a usable figure, use the usable one and do not deduct DoD twice.

Deduction two: the inverter

Batteries store DC. Household appliances need AC. The inverter converting between them is 90–95% efficient at moderate load, and worse at light load, where its own standby consumption of 10–50 W becomes a significant share of throughput.

On a long, low-load backup discharge — the fridge and some lights for two days — inverter standby alone can account for a kilowatt-hour or more. DC loads wired directly to the battery, such as 12 V lighting, skip this deduction entirely, which is why off-grid systems often keep critical circuits on DC.

Worked example: what 10 kWh really runs

  1. Nominal capacity10 kWhYour input
  2. Usable after 90% DoD9.00 kWhCalculated result
  3. Delivered after 92% inverter8.28 kWhCalculated result
  4. Fridge, freezer and lighting160.5 W averageIllustrative model
  5. Runtime on essentials8.28 ÷ 0.1605 = 51.6 hCalculated result
  6. If you had assumed 10 kWh10 ÷ 0.1605 = 62.3 hCalculated result
Runtime you would have over-promised≈ 10.7 hours

See the deductions applied to your own battery

The calculator reports delivered AC energy separately from nominal capacity, with DoD and inverter efficiency both editable, so nothing is hidden inside a single runtime number.

Open the battery runtime calculator

Assumptions and limitations

  • DoD and inverter efficiency defaults are typical values, not specifications for your equipment. Use your own datasheet figures.
  • The cold-weather factor shown is illustrative. Manufacturer derating tables differ, and the effect depends on discharge rate.
  • Capacity fades with age. After several years of service, the nominal figure itself is optimistic before any of these deductions are applied.

Sources

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

    Supports: Rated capacity is a cell-level declaration at defined conditions, distinct from usable system energy.

  2. IEC 62040 — Uninterruptible power systems — International Electrotechnical Commission

    Supports: Conversion efficiency is specified separately from stored energy in AC backup systems.

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