Work out how long your inverter or UPS battery will run a load. Enter the battery capacity, voltage, the load in watts and an efficiency factor for real-world losses.
Backup time = (Ah × Volts × efficiency) ÷ load in watts. Efficiency (typically 70–85%) covers battery depth-of-discharge and inverter losses. Real runtime falls as batteries age and if the load changes.
A battery's backup time depends on how much energy it stores versus how fast your appliances draw it. The stored energy in watt-hours is the capacity (in amp-hours) times the voltage; multiply by an efficiency factor for real-world losses, then divide by the load in watts to get the runtime in hours. This works for inverter, UPS and solar battery banks.
How to calculate battery backup time
Backup hours = (capacity Ah × voltage V × efficiency%) ÷ load W
150 Ah, 12 V, 80% efficiency
Backup time
200 W load
7.2 hours
300 W load
4.8 hours
500 W load
2.88 hours
800 W load
1.8 hours
Worked example
A 150 Ah, 12 V battery at 80% efficiency stores 150 × 12 × 0.8 = 1,440 usable watt-hours. Running a 300 W load: 1,440 ÷ 300 = 4.8 hours (4 h 48 min).
Sizing a battery bank
To go the other way — sizing a battery for a target runtime — rearrange the formula: required Ah = (load W × hours) ÷ (voltage × efficiency). Two 12 V batteries in series make 24 V (same Ah, double the energy); in parallel they stay 12 V but double the Ah. Always leave headroom, since discharging lead-acid batteries fully shortens their life.
Frequently asked questions
How long will a 150Ah battery last?
At 12 V and 80% efficiency it stores about 1,440 Wh — roughly 4.8 hours on a 300 W load, or 7.2 hours on 200 W.
What efficiency should I use?
Around 80% is typical for an inverter and a healthy lead-acid battery; use 70% to be conservative or for older batteries.
How do I size a battery for a set backup time?
Required Ah = (load in watts × hours) ÷ (voltage × efficiency). E.g. 300 W for 5 h at 12 V, 80% = 156 Ah.
Does connecting batteries in series add backup time?
Series adds voltage (and total energy); parallel adds amp-hours. Either way total watt-hours — and runtime — increase.
