Work out how long your batteries will run a load, or how big a battery bank you need for a set number of hours. It accounts for the usable depth of discharge and the inverter efficiency.
Real backup time is shorter for old batteries, high discharge rates and cold weather. Parallel batteries add Ah, series batteries add volts. Here the voltage field is the voltage of the whole bank.
Formulas and assumptions
- Backup time (hours): Ah times number of batteries times volts times usable fraction times inverter efficiency, divided by load watts
- Battery size needed (Ah): load watts times hours, divided by (volts times usable fraction times inverter efficiency)
- Usable fraction: 0.5 for lead acid and tubular batteries, 0.9 for lithium
- Inverter efficiency: 85 percent unless your inverter data sheet says otherwise
Example
One 12 V 100 Ah lead acid battery has 100 times 12 times 0.5 times 0.85, which is 510 Wh of usable energy. Running a 200 W load, it lasts about 2.5 hours (153 minutes).
Frequently asked questions
How long will a 12V 100Ah battery last?
At a 200 W load with a lead acid battery and a typical inverter, about 2.5 hours. The usable energy is roughly 12 times 100 times 0.5 times 0.85, which is 510 Wh.
Why only 50 percent of a lead acid battery?
Draining lead acid batteries deeply shortens their life. Most makers advise using about half the rated capacity for a long life.
Does wiring batteries in series or parallel change the backup time?
Series raises the voltage and parallel raises the Ah. The total energy is the same either way, but your inverter needs the matching system voltage.
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