Home / Guides / How to Calculate Inverter Battery Backup Time (Formula + Worked Example)
Backup time = (Battery Ah x Battery Voltage x Inverter efficiency x Depth of discharge) / Load in watts. For a 150Ah 12V tubular battery at 70% efficiency and 50% depth of discharge running a 156W load, that is (150 x 12 x 0.7 x 0.5) / 156 = 630Wh / 156W = 4.0 hours.
Backup hours = (Ah x V x efficiency x depth of discharge) / load in watts. Three of those four terms are where people go wrong. Voltage is the system voltage, not the mains voltage. Efficiency is the whole chain — inverter conversion, cable losses and battery age — not the number on the inverter's brochure. Depth of discharge is the fraction you are allowed to use, which for lead-acid tubular is about half.
Load is the fourth trap. Most calculators want one flat wattage. Real homes are not flat: a fridge cycles, a mixer runs in bursts, a pump fills and stops. Battery Buddy asks minutes per hour for each appliance and multiplies by that fraction.
| Step | Arithmetic | Result |
|---|---|---|
| Average load | 2 x 70W fans + 3 x 9W bulbs, all 60 min/hr | 167W |
| Nominal energy | 150Ah x 12V | 1800Wh |
| Usable energy | 150 x 12 x 0.7 x 0.5 | 630Wh |
| Backup time | 630Wh / 167W | 3 hours 46 minutes |
Manufacturers quote nominal capacity at a slow, laboratory discharge rate and at 27 degrees. An inverter pulls far harder than that, which triggers the Peukert effect and returns less energy than the label promises. Add a battery two summers old and a charger that never finishes a cycle during a short grid window, and a 'nominal' 1800Wh routinely behaves like 600-700Wh.
This is exactly why Battery Buddy defaults to 70% efficiency rather than the 85-90% you see quoted elsewhere. A conservative default that matches reality is more useful than an optimistic one that makes you buy the wrong battery.
Measure your own efficiency factor with a single discharge test, put your actual appliance list in with real minutes-per-hour, and the answer stops being a rule of thumb. Both tools are free, need no account, and produce a shareable URL you can send to your electrician.
Backup hours = (Battery Ah x Battery voltage x Inverter efficiency x Depth of discharge) / Load in watts. For a 150Ah 12V battery at 70% efficiency and 50% depth of discharge on a 156W load: (150 x 12 x 0.7 x 0.5) / 156 = 630 / 156 = 4.0 hours.
Most assume 85-90% efficiency and ignore depth of discharge or duty cycle. Battery Buddy defaults to 70% end-to-end efficiency and enforces the 50% lead-acid depth-of-discharge limit, which matches what Indian tubular batteries actually deliver after a year or two of use.
Average load for runtime, peak load for inverter sizing. They answer different questions: average hourly draw tells you how long the battery lasts, while continuous plus start-up surge tells you whether the inverter will trip.
No, only the numbers. A 24V bank of two 12V batteries in series uses the same equation with V = 24, which is why a higher system voltage carries more power at the same current.
As accurate as your inputs. Nameplate wattages and an assumed efficiency give a useful estimate; a measured efficiency factor and real minutes-per-hour figures typically land within about 10% of what you observe.
Battery Buddy is free, needs no sign-up, sells no batteries and shows every formula it uses. It is built by Flocci Technologies, an India-first AI-native software studio behind 30+ apps that share one Flocci identity.