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Inverter Battery Backup Time Calculator

Calculate battery backup duration in hours and minutes based on load in Watts, Battery Ah, Voltage, and Efficiency.

Ah
V
W

Discharging below 50% shortens life considerably.

%
Nominal energy
1,800 Wh
Usable energy
765 Wh
After 50% depth of discharge and 85% efficiency
Backup duration
2.5 h
2 h 33 min
Runtime at half load
5.1 h

How this is calculated

Usable Wh = 150 Ah × 12 V × 1 × 0.5 DoD × 85% = 765 Wh

Depth of discharge is applied as well as inverter efficiency. Ignoring it, as simpler calculators do, roughly doubles the predicted backup on a lead-acid bank.

Inverter Battery Backup Time Calculator: how it works

Backup time is not battery capacity divided by load. Depth of discharge and inverter efficiency between them roughly halve the usable energy.

The realistic formula

Backup hours = (Ah × V × DoD × efficiency) ÷ Load in watts

DoD = depth of discharge, 0.5 for lead-acid
Efficiency = inverter conversion loss, typically 0.85

Depth of discharge by chemistry

Battery typeUsable DoDNote
Lead-acid, flat plate50%Deeper discharge shortens life sharply
Lead-acid, tubular50–60%Better suited to regular cycling
Lithium LiFePO₄80–90%Tolerates deep discharge well

Discharging a lead-acid battery below 50 percent repeatedly can halve its service life. The 50 percent limit is not a technical impossibility but an economic one.

Worked example

Worked example · 150 Ah, 12 V, 300 W load, lead-acid

Given

  • Capacity: 150 Ah
  • Voltage: 12 V
  • Load: 300 W
  • DoD: 50%
  • Efficiency: 85%

Working

  1. 1Nominal energy = 150 × 12 = 1800 Wh
  2. 2Usable = 1800 × 0.5 × 0.85 = 765 Wh
  3. 3Backup = 765 ÷ 300

2.55 hours, or about 2 hours 33 minutes

Sizing your load

  • LED bulb: 9 to 12 W. Ceiling fan: 50 to 75 W.
  • Television: 60 to 150 W depending on size.
  • Refrigerator: 150 to 300 W running, with a much higher starting surge.
  • Motors and compressors draw three to five times their running current at startup — size the inverter for the surge, not the average.

Frequently asked questions

Multiply amp-hours by voltage for nominal energy, then by depth of discharge and inverter efficiency for usable energy, then divide by the load in watts.

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