Battery Backup Runtime Calculator

Estimate battery backup runtime from battery-bank voltage and amp-hours, usable depth of discharge, system efficiency and average load.

Estimate battery backup runtime

Estimate usable battery energy and runtime from voltage, amp-hours, depth of discharge, efficiency and load.

Do not use nominal Ah alone to guarantee runtime. For critical backup sizing, use battery/inverter manufacturer discharge curves and system design limits.

Battery bank nominal voltage (V)

Battery bank capacity (Ah)

Usable depth of discharge (%)

Inverter/system efficiency (%)

Average AC load (W)

Nominal battery energy—

Estimated usable AC energy—

Estimated runtime—

Approx. DC current at load—

How the Battery Backup Runtime Calculator works

Nominal battery energy is voltage × amp-hours, expressed in watt-hours. The calculator multiplies nominal energy by the allowed depth of discharge and system/inverter efficiency to estimate usable AC energy, then divides by average AC load to estimate runtime. It also approximates DC current needed at the entered load.

How to use this battery backup runtime calculator

Enter the battery bank’s effective nominal voltage and Ah capacity as configured, not a single battery value if several units form the bank. Set a usable depth-of-discharge appropriate to the battery/BMS design and an inverter/system efficiency representative of your load range. Use average load rather than only nameplate peak power.

How to interpret the result

Runtime is an energy estimate. A 4.8 kWh nominal battery bank is not the same as 4.8 kWh delivered to AC loads because reserve/DoD limits and conversion losses reduce usable energy. The result is useful for scenario planning and comparing load reductions, but it should not be treated as a guaranteed outage duration.

Assumptions and limitations

Battery capacity can fall at high discharge rates, low temperature and with aging. Lead-acid batteries are especially affected by discharge-rate behavior; lithium systems may have BMS reserve/cutoffs. Inverter standby consumption, surge loads and battery voltage variation are also omitted. Critical systems need manufacturer curves and engineered design.

Practical example and workflow

A 48 V, 100 Ah bank has 4.8 kWh nominal energy. At 80% usable DoD and 90% system efficiency, estimated AC energy is about 3.46 kWh. An 800 W average load therefore gives roughly 4.32 hours in the simplified model before real-world derating.

Frequently asked questions

How do I convert battery Ah to Wh?

Multiply the battery-bank voltage by amp-hours: Wh = V × Ah.

Why multiply by depth of discharge?

Many systems intentionally reserve part of nominal capacity to protect the battery or meet BMS limits, so not all nominal energy is usable.

Why can actual runtime be lower?

Battery aging, temperature, discharge rate, inverter losses, standby draw and cutoff voltage can all reduce delivered energy.

Can I size critical backup from this result alone?

No. Use manufacturer discharge data, surge requirements, redundancy and professional design for critical loads.