Inverter Battery Backup Time Calculator
Inverter Battery Backup Time Calculator
How many hours your inverter will run on its battery: from the bank’s Ah and voltage, the usable depth of discharge for your chemistry, the inverter’s efficiency and the load in watts.
Backup time
150 Ah tubular battery at 12 V, 60% usable, 85% inverter efficiency, 300 W load
Backup time from usable energy
- Ah
- total bank capacity: capacity of one battery × strings in parallel
- V
- nominal bank voltage (12, 24 or 48 V)
- DoD
- usable share of the rating — 50% flat plate, 60% tubular, 80% LFP here
- η
- inverter efficiency as a fraction (0.85 = 85%)
- P
- the AC load in watts, and the inverter’s own no-load draw
Worked example
150 Ah tubular battery at 12 V, 60% usable, 85% inverter efficiency, 300 W load
Usable energy = 150 × 12 × 0.60 = 1,080 Wh
Battery-side power = 300 ÷ 0.85 = 352.94 W (52.94 W of it lost in the inverter)
t = 1,080 ÷ 352.94 = 3.06 h, about 184 minutes
That is 29.41 A out of the battery — check your cable and fuse for it
Backup hours from a 150 Ah tubular battery (60% usable, 85% inverter)
| Load | 12 V, one battery | Battery current | 24 V, two batteries |
|---|---|---|---|
| 100 W | 9.18 h | 9.8 A | 36.72 h |
| 200 W | 4.59 h | 19.6 A | 18.36 h |
| 300 W | 3.06 h | 29.4 A | 12.24 h |
| 500 W | 1.84 h | 49.0 A | 7.34 h |
| 800 W | 1.15 h | 78.4 A | 4.59 h |
| 1000 W | 0.92 h | 98.0 A | 3.67 h |
How long an inverter battery lasts
Backup time is stored energy divided by the power being drawn. A 150 Ah battery at 12 V holds 1,800 Wh on paper, but you cannot use all of it: draining a lead-acid battery flat repeatedly ruins it within months. At 60% usable, a tubular battery offers 1,080 Wh. The inverter then takes its cut — at 85% efficiency a 300 W load pulls 353 W from the battery, so the bank lasts 3.06 hours, about 184 minutes.
Efficiency is not the datasheet headline. Manufacturers quote a maximum efficiency measured near the inverter’s best load point: Victron’s Phoenix range publishes 92–95%. Running a 1,000 VA inverter at 100 W is nowhere near that point, and the inverter’s own no-load draw — 8 to 35 W on the same datasheet — is then a large share of the total. Enter that standby figure separately if you know it, and you will see why a lightly loaded inverter flattens a battery overnight with nothing switched on.
Volts, amps and cables. Series batteries raise the voltage; parallel strings raise the amp-hours. The same 300 W load draws 29.41 A from a 12 V bank and half that from 24 V, which is why larger systems are built at 24 or 48 V: thinner cable, smaller losses, smaller fuses. Size the battery cable for the current shown above with the cable sizing calculator, and size the inverter itself with the inverter/UPS size calculator.
What this page does not model. Lead-acid batteries deliver less charge when discharged fast — Peukert’s law — and this page deliberately leaves that out so the arithmetic stays transparent. If your load draws more than about C/10 (15 A from a 150 Ah battery), use the battery life calculator, which applies Peukert’s exponent and handles duty-cycled loads that sleep between bursts. Cold cuts capacity further; heat, the usual problem in India and the Gulf, shortens life rather than capacity. Charging is the other half of the story: the battery charging time calculator estimates how long the bank takes to fill again.
Frequently asked questions
How long will a 150Ah battery run a 300W load?
About 3.06 hours from a 12 V tubular battery used to 60% with an 85% efficient inverter: 1,080 Wh usable ÷ 353 W drawn from the battery. Two such batteries at 24 V give about 6.12 hours.
How do I calculate inverter backup time?
Multiply capacity (Ah) by battery voltage and by the usable fraction to get watt-hours, then divide by the load divided by the inverter’s efficiency. Add the inverter’s standby watts if you know them.
Why does my inverter give less backup than the calculation?
Three usual reasons: the battery has aged and no longer holds its rating; the load is higher than you think (a fridge cycles, a pump starts); or the discharge is fast enough for Peukert’s law to bite. An old battery can hold half its label.
What depth of discharge should I use?
As a starting point, 50% for flat-plate lead-acid, 60% for tubular and 80% for LFP lithium. Victron’s Energy Unlimited notes that ageing accelerates disproportionately beyond about 80% depth of discharge for lead-acid.
Does a bigger inverter give longer backup?
No. Backup time comes from the battery. A bigger inverter lets you run more appliances at once, and a much oversized one wastes a little more in standby.
Is it better to use 12 V or 24 V?
For anything above roughly 1,000 W, 24 V or 48 V. The energy is the same for the same batteries, but the current halves or quarters, so cables, fuses and connection losses all get easier.
Related calculators
References
- Victron Energy. Phoenix Inverter 1200 VA – 5000 VA datasheet (SinusMax). Continuous output at 25 °C: 1,000 W from the 1,200 VA model and 4,000 W from the 5,000 VA model (VA × 0.8); peak 2,400 W and 10,000 W; maximum efficiency 92–95%; zero-load power 8–35 W.
- Vader R. Energy Unlimited. Victron Energy. Charge efficiency “will range in between 80% and 95%”; battery ageing advances “disproportionately fast” once roughly 80% depth of discharge is exceeded.
- Rolls Battery (Surrette). Flooded Battery Capacity & Temperature: cooler operating temperatures prolong cycle life, while low temperatures reduce the available capacity (cold-temperature capacity multiplier table).
- IEEE Std 1013-2019. IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic (PV) Systems.