How Long Will My Inverter Battery Last? Backup Time Explained

When the power goes out, the only question that matters is how long your inverter battery backup will last. The label on the battery says 150 Ah, the inverter says 1,000 VA, and neither tells you whether the fans will still be running at 3 a.m. This guide explains how backup time really works, with worked examples, so you can plan loadshedding hours and buy the right battery.

If you just want your number, our free backup calculator works it out in seconds.

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The basic inverter battery backup formula

Backup time is simply stored energy divided by the power you use. First, find the energy in the battery:

energy (Wh) = battery voltage × capacity (Ah)

So a 12 V, 150 Ah battery stores 1,800 Wh. Then divide by your load. For example, a 180 W load would in theory run for 10 hours. In practice, however, you get far less, for three reasons.

Why real backup time is shorter

1. You can’t use the whole battery

Lead-acid batteries, including the tubular batteries most homes use, age quickly if you drain them completely. Most inverters therefore cut off before the battery is empty, at around 70–80% depth of discharge. As a result, a 1,800 Wh battery really gives you about 1,260 Wh. Lithium (LiFePO4) batteries, by contrast, let you use about 90%.

2. The inverter uses power too

Converting battery DC to household AC wastes energy. A typical home inverter is 85–92% efficient, and it also draws 15–40 W just to stay on. So to run 260 W of appliances, the battery actually supplies about 315 W.

3. Heavy loads drain lead-acid batteries faster

A battery rated 150 Ah gives that capacity only at a gentle current, usually spread over 20 hours. When you pull 25 A or more, it delivers noticeably less. Engineers call this the Peukert effect. It matters most for lead-acid batteries and much less for lithium.

Inverter battery backup in real examples

Here is what a single 12 V, 150 Ah tubular battery gives with different loads, using 70% usable capacity, 90% inverter efficiency and 25 W of inverter draw:

What’s running	Load	Approximate backup
1 fan + 2 LED bulbs + router	107 W	about 8 hours
2 fans + 4 LED bulbs + router + TV	262 W	about 3 hours
3 fans + 6 LED bulbs + router + TV + laptop	407 W	about 1 hour 50 minutes
Inverter AC (1 ton)	900 W	about 45 minutes

Notice that doubling the load more than halves the time, because the high-current loss grows too. That is also why an air conditioner is rarely practical on a single home battery. You can test your own mix in the free calculator.

How to get more battery backup time

• Switch to efficient appliances. An inverter (DC) ceiling fan uses around 35 W instead of 75 W, so on backup it nearly doubles the fan time for free.
• Add a second battery in parallel. Two 150 Ah batteries at 12 V give 300 Ah, and backup more than doubles because each battery works less hard.
• Move to 24 V for bigger loads. Higher voltage means lower current, so less heat and less loss, which is why larger inverters use 24 V or 48 V.
• Keep batteries charged and cool. A battery that never fully recharges between cuts gives shorter backup each time. Heat also shortens battery life.
• Top up water in tubular batteries. Low electrolyte exposes the plates and permanently reduces capacity.

Why inverter backup time falls as batteries age

Every lead-acid battery loses capacity over time. A tubular battery typically lasts three to five years, and by the end it may hold half its original charge. So if your backup has shrunk from four hours to two, the battery is probably worn, not the inverter faulty. Deep daily discharges and hot storage rooms speed this up.

Lithium batteries fade much more slowly and handle thousands of cycles, which is why they increasingly make sense despite the higher price. Battery University has a detailed comparison of both chemistries.

How long it takes to recharge

Backup is only half the story, because the battery has to refill before the next cut. Most home inverters charge at 10–15 A. So a 150 Ah battery drained to 70% needs roughly eight to ten hours of grid power to recover fully, since charging also slows down near the top. If power cuts come every few hours, the battery never fills up, and each backup gets shorter than the last. In that case, a bigger battery or an inverter with a stronger charger helps more than anything else.

Check your battery’s real capacity

An old battery can look fine and still hold half its charge. To test it, first charge it fully overnight. Then, during the next cut, run a known load such as two fans and a few lights, and time how long the inverter lasts. Next, enter the same load in the calculator. If your real time is less than about 60% of the estimate, the battery has lost most of its capacity and replacing it will do more than any other fix. Some inverters also show the battery voltage; a rested 12 V lead-acid battery below about 12.2 V is not being fully charged.

Which battery should you buy?

Start from the hours you need, not the battery on offer. First, list the appliances you must run during a cut and add their watts. Then decide how many hours of backup you want. Finally, use the Battery I need tab in the calculator to get the capacity.

As a rule of thumb, a single 150 Ah tubular battery suits a small flat with two or three fans and lights. For a whole house with several fans, a fridge and a TV, plan for 200–300 Ah at 24 V, or a lithium battery. Also check that your inverter can charge the battery you choose, because very large banks need a bigger charger.

What the numbers can’t tell you

To be honest about the limits: these are estimates. Real backup depends on the battery’s age, how full it was when the power went out, room temperature and the exact wattage of your appliances, which often differs from the label. Expect your real result to land within about 20% of the calculation. If it is much lower, the battery probably needs replacing.

Also remember that appliance labels show the maximum rating, not the typical draw. A fridge, for example, switches its compressor on and off, so its average use is far below the number on the sticker. Measuring with a cheap plug-in power meter gives you real watts, and your inverter battery backup estimate becomes much more accurate as a result.

Inverter battery backup in short

Inverter battery backup equals usable battery energy divided by what you run, plus the inverter’s losses. A 12 V, 150 Ah tubular battery gives about three hours for two fans, lights, a router and a TV, and about eight hours for one fan, two lights and a router. Use efficient fans, add batteries in parallel, keep them charged, and check your own setup with the free calculator.