Battery Backup Calculator
Tell us what you want to keep running during a power cut. We'll add up the load and suggest a battery that covers it — with the maths shown, so you can check our working.
Wattages are typical values — edit any of them to match your own appliance ratings.
How much backup do you need?
Estimated load
380W
Running everything selected, for 4 hours.
Common questions
How is backup time calculated?
Backup time is the usable energy in the battery divided by your load. Usable energy is the rated capacity (Ah) multiplied by the voltage, then reduced for depth of discharge and inverter conversion losses. The calculator shows every assumption it uses.
Why can't I use 100% of the battery's rated capacity?
Discharging any battery all the way down shortens its life. LiFePO4 handles deep discharge far better than lead-acid — around 90% usable versus roughly 50% — which is why a lithium battery of the same Ah rating delivers noticeably more real backup.
Does the calculator account for surge loads?
No. Motors in fridges, pumps, and coolers briefly draw several times their running wattage on start-up. The calculator uses steady running watts, so size your inverter with headroom for those surges.
Not sure where to start? Try the Find My Battery wizard or compare long-term cost.