Checked 30 Oct 2026

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Inverter battery backup calculator: how many hours will it last?

To work out inverter battery backup, multiply the battery's Ah (its capacity) by its voltage and the inverter's efficiency, then divide by the load in watts. A 150 Ah, 12 V battery running two fans, four LED bulbs and a Wi-Fi router (144 W) lasts about 11.3 hours at 90%, fully charged and healthy.

Work out your backup time

Capacity of one battery, Ah

Battery voltage

Inverter efficiency, %

How many of each will run

Ceiling fan, regular, 48 W

Ceiling fan, BLDC (low-watt type), 33 W

LED bulb, 9 W

Wi-Fi router (adapter rating), 12 W

Laptop (adapter rating), 30 W

Fridge, 242 L frost-free (average over a day), 30 W

Anything else, watts

Load144 W

Backup11.3 hours

At 90% inverter efficiency. At 80% it is 10.0 hours; at 95%, 11.9 hours.

Formula: Livguard, how to calculate inverter battery backup time; Tata Green Battery, how to calculate inverter battery backup time. Appliance watts: appliance guide.

By Raviraj Ramavarapu. Formula as battery makers publish it; appliance watts from their labels and ratings. Checked 30 Oct 2026.

Inverter battery backup calculator, in short

Backup hours = battery Ah x battery volts x inverter efficiency / load in watts.
A 150 Ah, 12 V battery at 90% efficiency lasts about 5.4 hours at 300 W and 8.1 hours at 200 W.
At 24 V the volts in the formula double, so the same Ah and load last twice as long.
An older battery gives fewer hours than the formula says, Livguard notes.
Two Havells ceiling fans take 96 W of a 144 W example load; the BLDC model (a fan type that draws fewer watts) is rated 33 W against 48 W for the regular 1,200 mm fan.

How long will an inverter battery last? 100 Ah, 150 Ah and 200 Ah

Battery	Load	Backup at 90%
100 Ah, 12 V	200 W	5.4 hours
150 Ah, 12 V	300 W	5.4 hours
150 Ah, 12 V	144 W	11.3 hours
200 Ah, 12 V	400 W	5.4 hours
150 Ah, 24 V	450 W	7.2 hours

Battery makers differ on efficiency: Livguard works its example at 95%, and Tata Green Battery leaves efficiency out of its formula and says it is around 80 to 90%. We use 90%. Use your inverter's own figure if you have it.

These hours assume a fully charged battery giving its full rated Ah. Tata Green Battery says the actual backup may vary with the inverter's efficiency and other factors, and Livguard says older batteries give less backup.

Use your own watts and your inverter's efficiency in the calculator above.

Livguard, how to calculate inverter battery backup time; Tata Green Battery, how to calculate inverter battery backup time.

How to make the backup last longer

Run fewer watts. Halving the load roughly doubles the hours.
Swap to a BLDC fan. The Havells pair we used: 33 W instead of 48 W for each fan.
Use LED bulbs. A 9 W LED bulb is a small load; in the example, most of the load is the fans.
Expect less from an older battery. Battery health is one of the factors Livguard lists; an old battery holds less.
Keep the AC, geyser, iron and pump off the inverter. Each draws hundreds of watts or more; see the appliance guide.

The fridge figure is its label's 267 units a year spread over the day, so it is an average: the compressor switches on and off, and draws more as it starts. Whether the inverter can run a load at all depends on its VA rating: Luminous's example divides the load in watts by 0.8 to size one, and notes the real figure varies. Check it before you add a large appliance.

Fridge: LG GL-N292RDSY, 242 L frost-free refrigerator; starting draw and VA: Luminous, how to determine inverter capacity for your home, 3 Sep 2026; battery age: Livguard, how to calculate inverter battery backup time; heavy loads: appliance guide (Havells Fabia Slim storage water heater (10, 15 and 25 L), Philips GC181/80 dry iron, Crompton Mini Master I pump, Panasonic CS/CU-SU18BKY3W, 1.5 ton 3 star inverter split AC (2026)).

TNPDCL lists planned power cuts on its TNPDCL Planned Power Outages page. How to find yours: see power shutdown today.

Inverter battery backup calculator: questions people ask

How do I calculate inverter battery backup time?

Multiply the battery's Ah by its voltage and by the inverter's efficiency, then divide by the load in watts. A 150 Ah, 12 V battery running 144 W at 90% efficiency lasts about 11.3 hours. The calculator takes one battery's Ah and the system voltage your inverter uses.

How do I calculate 150Ah battery backup time?

150 x 12 x 0.9, divided by your load in watts. On a 12 V battery at 90% efficiency that is about 5.4 hours at 300 W, 8.1 hours at 200 W and 16.2 hours at 100 W. The fewer watts you run, the longer it lasts.

How many hours is a 200Ah battery backup?

200 x 12 x 0.9, divided by your load in watts. On 12 V at 90% efficiency that is about 5.4 hours at 400 W and 7.2 hours at 300 W.

Can an inverter run an AC?

Only if its VA rating, the inverter's size rating, covers the AC. Luminous sizes an inverter in VA: its example divides the load in watts by a power factor of 0.8, so a 400 W load needs about 500 VA, and it notes the real figure varies. Use the AC's rated input watts if the maker gives them and divide by 0.8. The compressor also draws more as it starts, so leave headroom, and check the inverter maker's recommended load before you connect it.

What efficiency should I use?

Efficiency is how much of the battery's energy the inverter passes on. Use your inverter's own figure, from its box or spec sheet. Battery makers quote 80 to 95%; we use 90% unless you change it.

Sources

Livguard, how to calculate inverter battery backup time. Backup hours = Ah x V x efficiency / load W; "Most inverters work at 90 to 95% efficiency"; example 150 Ah, 12 V, 290 W at 0.95, around 6 hours (5.9 unrounded).
Tata Green Battery, how to calculate inverter battery backup time. Backup hours = Ah x V / load W; efficiency "should be around 80-90%"; examples 150 Ah, 12 V, 300 W about 6 hours; 150 Ah, 24 V, 450 W, 8 hours.
Luminous, how to determine inverter capacity for your home, 3 Sep 2026. VA = load in watts / power factor, 0.8; 400 W needs 500 VA; motors and compressors draw more at startup.
TNPDCL Planned Power Outages page. Official list of planned power shutdowns.
Appliance watts: Havells Reo Utsav ES ceiling fan; Havells Unnovate BLDC ceiling fan; Havells LED Plus 9 W B22 lamp; TP-Link Archer C6 router; Apple MacBook Air tech specs; LG GL-N292RDSY, 242 L frost-free refrigerator.

Sources last checked 30 Oct 2026. Found a mistake? Tell us, and see how we correct it.