About inverter battery backup calculator
During a power cut, how long a battery keeps your lights and fans running depends on its capacity and what you connect.
Formula
Backup hours = battery Ah × battery voltage × usable share × inverter efficiency ÷ load in watts. A 150 Ah, 12 V battery with 50% usable and 85% efficiency running a 300 W load lasts about 2.5 hours.
Making the backup last
- Connect only essentials such as LED lights and fans.
- Avoid heavy loads like irons and geysers on the inverter.
- Top up distilled water in flooded batteries as required.
Another worked example
A 100 Ah, 12 V lithium battery holds 1,200 Wh. With 80 percent usable and 90 percent efficiency it delivers about 864 Wh. A 300 W load of fans, lights and a router then runs for about 2.9 hours. A lead-acid battery of the same size with 50 percent usable would last about 1.7 hours.
Tips
- Add up the wattage of what you plan to run, using the label on each device.
- Include a margin, since batteries lose capacity as they age.
- Do not discharge lead-acid batteries deeply and often, which shortens their life.
