Mobile & Electronics

Inverter Battery Backup Calculator

How long a home inverter runs your load, with the lead-acid depth of discharge that decides the real answer.

7 inputs Free, no sign-up

Your figures

A single inverter battery is 12 V. Two in series is 24 V, four is 48 V.

The Ah figure on the battery. Indian inverter batteries are commonly 100 to 220 Ah.

Add up everything on the inverter: fans, lights, television, router.

Leave blank for this page's 80% assumption, which covers the inverter. For a realistic lead-acid figure, use 40-45% - that folds in not discharging past half.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

The figure that decides inverter backup is not the battery rating — it is how much of the battery you are willing to use. A 150 Ah lead-acid battery holds 1,800 Wh, but discharging one below about half repeatedly shortens its life sharply, so the practical energy is closer to 900 Wh before the inverter's own losses.

That single fact halves most people's expectations, and it is why an inverter that "should" run four hours runs two.

How to use this calculator

  1. Enter the battery voltage and its Ah rating.
  2. Add up everything on the inverter. Fans, lights, a television and a router is a typical Indian household load of 200–400 W.
  3. For the optimistic figure, leave the efficiency at 80% — that is the inverter alone. For the realistic one, use 40–45%, which also allows for not flattening the battery.

The formula

Battery energy = Voltage x Capacity
Usable energy  = Battery energy x Efficiency
Backup time    = Usable energy / Load

Two separate reductions are at work and this page lets you combine them in one figure. The inverter converts DC to AC and loses 15–20% doing it. Separately, a lead-acid battery should not be taken below roughly half its capacity. Multiply the two and you get around 40–45%, which is why that is the efficiency to use for a realistic answer.

Lithium batteries change this materially: they tolerate much deeper discharge, so their usable share is far closer to the nominal figure.

SymbolMeaningUnit
V Battery bank voltage V
Ah Battery capacity Ah
Load Total household load W
Efficiency Inverter, and usable depth %

Worked example

A 12 V 150 Ah battery running a 300 W household load:

Energy = 12 x 150 = 1,800 Wh

At 80% - inverter only:
Usable = 1,440 Wh   ->  4.8 hours

At 45% - inverter plus half-discharge limit:
Usable = 810 Wh     ->  2.7 hours

The second is the number to plan around with a lead-acid battery.

Notes and limits

  • Lead-acid should not be discharged past about half. Doing so repeatedly is the commonest reason inverter batteries fail early.
  • Battery capacity is rated at a slow discharge, usually over 20 hours. A heavy load gets less than the rating, which this calculation does not model — so treat the answer as an upper bound.
  • Batteries lose capacity with age and in heat. A battery three summers old will not match a new one.
  • Reducing the load extends backup proportionally, and it is the lever you actually control during a cut. Turning off everything but fans and lights roughly doubles the time.
  • To size a battery for a target backup instead, use the required capacity calculator.

What this calculation assumes

  • The load is steady at the figure entered.
  • The efficiency you choose is where inverter loss and depth of discharge are combined.
  • Capacity lost to a fast discharge rate, to age and to heat is not modelled, so this is an upper bound.

Frequently asked questions

How long will a 150 Ah inverter battery last?

With a 300 W load, about 2.7 hours realistically - using 45% to cover both the inverter's losses and not discharging a lead-acid battery past half. The optimistic figure ignoring depth of discharge is nearer 4.8 hours, and planning on it will shorten the battery's life.

Why does my inverter give less backup than the shop said?

Usually because their figure ignored depth of discharge, or assumed a lighter load than you actually run. Battery age and a heavy discharge rate both reduce it further.

What efficiency should I use for an inverter?

The inverter alone is around 80-85%. For a realistic lead-acid answer use 40-45%, which combines that with only using half the battery. For lithium, 80% is closer to right because deep discharge is acceptable.

Is a bigger battery or a second one better?

Either adds capacity. Two batteries in series double the voltage and let the same power flow at half the current, which means less loss in the cabling; two in parallel keep the voltage and double the amp-hours. Your inverter dictates which it supports.

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