How to use this battery backup calculator
- Total load, enter the combined wattage of everything you want to power (watts).
- Backup time, enter how many hours you want that load to keep running.
- Battery voltage, pick 12, 24 or 48 V to match your battery bank.
- Capacity per battery, enter the rated amp-hours (Ah) of one battery.
- Battery type, choose lead-acid or lithium so the right depth of discharge is used.
- Inverter efficiency, leave it near 90% unless you know your inverter's figure.
The results update instantly as you type. Nothing is submitted or stored: the numbers never leave your device.
How inverter battery backup is calculated
The starting point is the energy your load actually needs. Energy in watt-hours is yourload in watts times the hours of backup, divided by inverter efficiency: Wh = (watts × hours) ÷ efficiency. Dividing by efficiency matters because an inverter converts stored DC into AC and loses some energy as heat, so a 90% efficient inverter forces the battery bank to hold roughly 10% more than the raw load suggests.
Next comes the usable energy in a single battery. A battery's rated capacity is volts × amp-hours, but you should never drain it to empty. The usable figure is volts × Ah × depth of discharge. Depth of discharge (DoD) is the safe fraction you can pull out: a flooded or sealed lead-acid battery is usually held to about50% DoD to protect its lifespan, while lithium (LiFePO4) chemistries comfortably reach about90% DoD. That is why a 100 Ah lithium battery delivers far more real backup than a 100 Ah lead-acid one.
The number of batteries you need is simply the required energy divided by the usable energy per battery, rounded up. In reverse, the backup time from a single battery is its usable energy times efficiency, divided by your load. This makes the tool genuinely useful in load-shedding regions: when the grid drops, you can quickly check whether your current battery will keep the fridge, fans and lights on through an outage, or how many extra batteries you would need to ride out a longer cut.
This is an estimate, please read
These numbers are a planning estimate, not a guarantee. Real backup time shifts with battery age, temperature, wiring losses, surge loads when motors start, and how steady your draw is. Battery capacity also fades over the years. Size your system with some headroom rather than running it right at the calculated limit, and check the manufacturer's recommended depth of discharge for your exact batteries.
Frequently asked questions
How do I calculate inverter battery backup time?
Work out the energy you need (load in watts times hours, divided by inverter efficiency), then divide by the usable energy in your bank (volts × Ah × depth of discharge). That gives the batteries needed, and in reverse how long one battery lasts.
Why does battery type change the result?
You should not drain a battery fully. Lead-acid is usually limited to about 50% depth of discharge, while lithium can safely reach about 90%, so lithium gives more usable energy from the same rated capacity.
How many batteries do I need to run my home during a power cut?
Add up the watts you want to run and the hours of backup you want. A fridge, fan and lights need far fewer batteries than an air conditioner or water pump.
Why divide by inverter efficiency?
Inverters waste some energy as heat. A 90% efficient inverter loses about 10%, so the battery bank must store more than the raw load. Dividing by efficiency builds that loss in.
Is this calculator exact?
No, it is a planning estimate. Battery age, temperature, wiring losses and surge loads all affect real backup, so leave some headroom.