Inverter & Battery Backup Time Calculator

What this battery backup calculator does

This battery backup calculator tells you how long your inverter or UPS battery will keep your appliances running during a power cut. You enter your battery and the things you want to run, such as fans, LED lights, a router and a TV. Then the calculator works out the backup time, and it includes the losses that simple formulas ignore.

It also works the other way round. For example, if you need four hours of backup for two fans and a few lights, the Battery I need tab shows the capacity to buy.

• Battery types: lead-acid tubular, AGM or gel, and lithium (LiFePO4 and lithium-ion), each with a sensible usable capacity.
• Battery banks: one battery, or several in series or parallel at 12 V, 24 V or 48 V.
• Real losses: inverter efficiency, the inverter’s own power draw, and the faster drain lead-acid batteries show at high current.
• Appliance list: common household loads with typical wattages that you can edit, plus an inverter size suggestion.

How to use the battery backup calculator

1. First, choose your battery type and voltage, then enter the capacity in Ah printed on the battery.
2. If you have two or more batteries, set how many and whether they are in series or parallel.
3. Next, list what will run during a power cut. Pick an appliance, check the watts on its label if you can, and set the quantity.
4. Read your backup time on the right. Open Inverter settings if you know your inverter’s efficiency or idle draw.
5. Finally, switch to Battery I need and enter the hours you want, so the calculator shows the capacity required.

The calculator runs in your browser and remembers your appliances for next time.

How the battery backup calculator works it out

The battery backup calculator multiplies voltage by capacity to get the energy stored, for example 12 V × 150 Ah = 1,800 Wh. However, you can’t use all of it. Lead-acid batteries last much longer if you don’t drain them fully, so the calculator uses 70% by default for tubular and AGM batteries and 90% for LiFePO4. You can change this.

After that, it adds the inverter’s losses. A 90% efficient inverter needs about 111 W from the battery to deliver 100 W, and most home inverters also use 15–40 W just to run. Lead-acid batteries also give less than their rated capacity when you draw a lot of current, which is known as the Peukert effect. The calculator includes this unless you switch it off.

Because batteries age and temperatures vary, treat the result as a good estimate within about 20%. Battery University explains the chemistry in more depth.

Frequently asked questions

How long will a 150Ah battery last?

On a 12 V tubular battery, about three hours for two fans, four LED bulbs, a router and a TV (around 260 W). With only a fan and two lights, it is closer to nine hours. Enter your own appliances in the battery backup calculator for your exact figure.

How do I calculate inverter backup time by hand?

Multiply battery volts by Ah and by the usable fraction, then divide by the load in watts plus inverter losses. For 12 V × 150 Ah × 0.7 ÷ 316 W you get about 4 hours; the high-current loss then brings it down to about 3.

Is a lithium battery better for backup?

Usually yes. A lithium battery lets you use most of its capacity and loses less at high current, so the same Ah gives much more backup. It also lasts more cycles, although it costs more upfront.

Why does my backup time drop over the years?

Lead-acid batteries lose capacity as they age, especially if they are often drained deeply or kept in a hot room. So a battery that gave four hours when new may give two after a few years.

What size inverter do I need?

Add up the watts you want to run at the same time and add a margin. The battery backup calculator suggests a VA rating using a 25% margin and a 0.8 power factor.

For the full picture, read our guide How Long Will My Inverter Battery Last?