How long will my battery last? Working out backup runtime
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“How long will it last?” is the first question anyone asks about a backup battery. The answer depends on how much energy you can actually take out of the battery, and how fast you use it.
The formula
hours of runtime = usable battery energy (Wh) × inverter efficiency ÷ load (W)
Each part matters:
- Usable energy is less than the label. Multiply the battery’s rated watt-hours by the share you can use: about 80% for lithium (LiFePO4) and about 50% for lead-acid. See lithium vs lead-acid.
- Inverter efficiency is typically around 90%: about a tenth of the energy is lost converting battery power to household power.
- Load is the power everything you run uses together, in watts.
If a battery is labelled in amp-hours, convert first: watt-hours = amp-hours × battery voltage. A 200 Ah, 12 V battery holds 2,400 Wh; a 100 Ah, 48 V battery holds 4,800 Wh.
Worked examples
A lithium battery running essentials
- Battery: 5 kWh lithium, so about 4,000 Wh usable
- After inverter losses: 4,000 × 0.9 = 3,600 Wh
- Load: lights, fans, a fridge, TV and Wi-Fi averaging 400 W
- Runtime: 3,600 ÷ 400 = about 9 hours
Two lead-acid batteries on the same load
- Battery: two 200 Ah, 12 V batteries wired for 24 V, which is 4,800 Wh rated
- Usable at 50%: 2,400 Wh, after inverter losses about 2,160 Wh
- Runtime at 400 W: about 5.4 hours
The lead-acid bank has a bigger label but gives fewer hours, because you should only use about half of it.
Why real runtime is often shorter
- The fridge cycles. It draws power only when its compressor runs, so its average is lower than its rating. That helps. But heaters, kettles and irons draw their full rating the whole time they are on.
- The inverter uses power itself. Most inverters draw some power just being switched on, often tens of watts in larger units. Over a 10-hour night that adds up. The data sheet lists this as “no-load” or “idle” consumption.
- Lead-acid batteries give less at high currents. Their capacity is usually rated over 10 or 20 hours. Drain them faster and you get noticeably less out of them.
- Batteries age. Capacity falls over the years; expect less runtime from an old bank than a new one.
- Cold weather temporarily reduces the capacity of most batteries.
How to stretch your runtime
- Switch off what you don’t need during outages, including devices on standby.
- Keep heavy heating loads off the battery. One 1,500 W space heater uses as much as dozens of LED bulbs.
- Use efficient appliances: LED bulbs, inverter-type fridges and air conditioners, DC fans.
- Keep the battery charged between outages, so each one starts from full.
Working backwards: how big a battery do I need?
Flip the formula:
battery size (Wh) = load (W) × hours needed ÷ inverter efficiency ÷ depth of discharge
For 400 W over 10 hours on lithium: 400 × 10 ÷ 0.9 ÷ 0.8 ≈ 5,600 Wh, so a 5.6 kWh battery or the next size up.
The calculator works this out for your own appliances and outage length.