Useful Installer Tools · Solar & Backup Power

Battery Backup Runtime Calculator

Estimate battery backup runtime from battery voltage and Ah, series/parallel bank arrangement, usable depth of discharge, battery condition, inverter efficiency, inverter self-consumption and connected load. The tool shows bank voltage, bank Ah, nominal and usable energy, estimated DC current and practical backup time.

Battery bank & load

Enter the actual battery-bank arrangement and expected AC load.

Battery voltage (each)

Battery capacity (Ah each)

Batteries in series

Parallel strings

Battery type / usable depth

Battery health / capacity remaining

AC load (watts)

Inverter efficiency

Inverter idle/self-use (watts)

Important: Runtime is an estimate. Battery temperature, discharge rate, cable loss, BMS limits, inverter low-voltage cut-off, battery age and surge loads can shorten real backup time.

Estimated backup runtime—Enter the battery bank and load, then calculate.

Bank voltage—

Bank capacity—

Nominal battery energy—

Usable DC energy—

Usable AC energy—

Approx. DC current—

Total inverter demand—

Total batteries—

Series batteries increase voltage. Parallel strings increase Ah. Runtime uses nominal Wh × usable depth × battery health × inverter efficiency, divided by the connected load plus inverter self-consumption.

How to use it

Read the battery bank correctly before estimating runtime.

Use the battery label and actual bank wiring, not a guess.

01Enter one battery ratingUse the voltage and Ah printed on one battery or module.

02Count series batteriesSeries adds voltage while Ah stays the same. Two 12V 200Ah batteries in series become 24V 200Ah.

03Count parallel stringsParallel adds Ah. Two identical 24V 200Ah strings in parallel become 24V 400Ah.

04Choose usable depthLead-acid batteries are usually not sized around 100% discharge. Lithium systems often allow a larger usable share, subject to the manufacturer/BMS.

05Account for battery healthOlder or damaged batteries may deliver much less than their nameplate Ah.

06Enter the real AC loadUse the combined running watts of the equipment expected to stay on during backup.

07Include inverter lossesThe inverter consumes energy while converting DC battery energy to AC power.

08Check high-power loadsPumps, compressors, fridges and AC units can have starting surges that affect inverter suitability even when average runtime looks acceptable.

09Verify on siteAfter installation, compare calculated runtime with actual discharge behavior and battery/inverter monitoring data.