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Inverter Battery Backup Calculator

Estimate how long an inverter battery lasts at your load, and the battery size for the backup you want.

By Nitin Raturi · Last reviewed

Your worksheet

How to use the Inverter Battery Backup Calculator

Estimate how long your inverter battery lasts, and the battery size you need.

  1. Enter the battery capacity, voltage and number of batteries.
  2. Enter the usable charge and the inverter efficiency from their specifications.
  3. Enter the total load in watts.
  4. Read the backup time and the size needed for your target.

Good to know

  • Switching off high-wattage appliances during a cut extends backup the most.

Backup = Ah × V × usable charge × efficiency ÷ load. Use your battery and inverter specs.

Battery capacity (Ah)

Ampere-hours of one battery, printed on it.

How to fill it in

  • Common home batteries are 100 to 220 Ah.

Battery voltage (V)

Nominal voltage of one battery.

How to fill it in

  • Most home inverter batteries are 12 V.

Number of batteries

How many batteries the inverter uses.

How to fill it in

  • A 24 V inverter uses two 12 V batteries.

Example

Two 150 Ah batteries running 600 W last about 4 h 4 min.

Usable charge (%)

How much of the battery’s charge you use before it cuts off (depth of discharge).

How to fill it in

  • Use your battery’s rated depth of discharge; lead-acid makers often advise not draining fully.

Keep in mind: Draining a battery deeper than its rating shortens its life.

Inverter efficiency (%)

Share of battery power the inverter delivers to your appliances.

How to fill it in

  • Use the figure from your inverter’s specification.

Load (W)

Total power of everything running during a cut.

How to fill it in

  • Add up the watts on each appliance, for example fans 75 W and LED lights 10 W each.

Example

One 150 Ah battery runs 400 W for about 3 h 3 min and 200 W for 6 h 7 min.

Backup you want (h)

Used to work out the battery size you need.

How to fill it in

  • Enter hours.

Example

For 4 hours at 400 W you need about 196 Ah per 12 V battery.

Backup time

3 h 3 min

1 × 150 Ah, 12 V battery running 400 W through a 85% efficient inverter, using 80% of the charge, last about 3 h 3 min. For 4 h you would need 196 Ah per battery.

Stored energy
1,800 Wh
Usable energy
1,440 Wh
Battery current
39.2 A
Ah per battery for 4 h
196.1

A steady load and a healthy, fully charged battery. Real backup is shorter with old batteries, heat, high loads (battery capacity falls at high current) and inverter standby draw.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

  1. Step 1EnergyAh × VWatt-hours stored.
  2. Step 2Usable× depth of dischargeThe share you actually use.
  3. Step 3Backupusable × efficiency ÷ loadHours at that load.

A battery’s ampere-hours times its voltage gives its energy in watt-hours. Only part of that is used before the inverter cuts off, and the inverter loses some energy turning DC into AC.

Divide what is left by the load in watts to get the backup in hours.

Worked example: a 150 Ah battery on 400 W

12 V, 80% usable, 85% efficient inverter
Hours
Ah × V × usable × efficiency ÷ load
  1. Stored energy150 × 121,800 Wh
  2. Usable energy1,800 × 80%1,440 Wh
  3. Delivered1,440 × 85%1,224 Wh
  4. Backup1,224 ÷ 4003 h 3 min

Read the result

  • 400 W3 h 3 min
  • 200 W6 h 7 min
Backup from the same battery

For 4 hours at 400 W you need about 196 Ah of 12 V battery, with the same assumptions.

  • Two batteries, 600 W4 h 4 minTwice the energy for 1.5 times the load.
  • One battery, 200 W6 h 7 minHalving the load doubles backup.

Assumptions and limits

  • Battery ageCapacity falls as batteries age.
  • High currentHeavy loads get less than the rated Ah.
  • Standby drawInverters use some power themselves.
  • Not charging timeRecharging is a separate calculation.

Backup time depends on the load in watts. Appliance use and bills are the same numbers over time.

To see what each appliance adds to your bill, use the electricity calculator. The Ohm’s law calculator relates watts, volts and amps.

Common mistakes

  • Mistake: Using the full Ah as usable.

    Instead: Multiply by the depth of discharge.

  • Mistake: Forgetting inverter losses.

    Instead: Divide the load by the efficiency.

Checking the method

A separate script drains the battery second by second at the load and finds the needed size by bisection, matching every golden value.

Read how we test our calculators, or browse more everyday tools.

Frequently asked questions

How long does a 150 Ah battery last on 400 W?

About 3 h 3 min, using 80% of the charge and an 85% efficient inverter.

And on 200 W?

About 6 h 7 min: half the load, twice the backup.

What battery do I need for 4 hours at 400 W?

About 196 Ah at 12 V, with the same assumptions.

How long do two 150 Ah batteries last on 600 W?

About 4 h 4 min. Two batteries double the stored energy.

Why is my real backup shorter?

Battery age, heat, high current and inverter standby use all reduce it.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 4 Nov 2026; next review due by . Found an error? Tell us.

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  • : Initial release.

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