Electrical · Free Calculator
Battery Backup Time Calculator
Estimate how long a battery bank can power a load, or how much capacity you need for a target backup time.
RuntimeRequired AhDepth of dischargeInverter efficiency
Ah
V
W
%
%
Backup time5.184 h
Backup time311 min
Battery current23.15 A
Usable energy2.592 kWh
How this result was calculated
- t = Ah × V × η × DoD ÷ W = 150 × 24 × 0.9 × 0.8 ÷ 500 = 5.184 h
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Battery Runtime Formula
t (h) = Ah × V × η × DoD ÷ Load (W)
Ah = Load × t ÷ (V × η × DoD)
Ah = Load × t ÷ (V × η × DoD)
Worked example
A 24 V, 150 Ah bank at 90 % efficiency and 80 % usable discharge runs a 500 W load for 150 × 24 × 0.9 × 0.8 ÷ 500 = 5.2 hours.
Key insight: this is an ideal estimate. Lead acid capacity falls at high discharge rates (Peukert effect) and in cold rooms, so add 20 to 25 % margin for critical systems.
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Frequently Asked Questions
How do I calculate battery backup time?
Multiply Ah by voltage to get Wh, apply efficiency and usable depth of discharge, then divide by the load in watts.
What depth of discharge should I use?
About 50 % for long life lead acid and 80 to 90 % for lithium iron phosphate.
Why is real backup shorter than calculated?
High discharge currents, low temperature and battery ageing all reduce available capacity.
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