Shop batteries OEM & Enterprise
Track order Register warranty sales@halovolt.com
Request a quote
Added to cart View cart
Free tool

Battery backup calculator for inverters

See how long a battery runs your load, or which battery you need. The formula and worked examples are below.

Backup time

How long will my battery last?

A 12V lithium battery is really 12.8V, so it is listed that way.

Backup time
2 h 34 min
Battery energy 1.28 kWh, counting 80% of it as usable.
Battery size

How much battery do I need?

Choose what you want to run during a power cut and for how many hours. We recommend a battery to match.

Load740 W Energy2.8 kWh See pick
LED bulb
10 W
10
Ceiling fan
75 W
5
LED TV
100 W
1
Refrigerator
150 W
1
Wi-Fi router
15 W
1
Laptop
65 W
0
Air conditioner (1.5 t)
1500 W
0
Water pump (1 HP)
750 W
0
Running load
740 W
Energy needed
2.8 kWh
Recommended

Inverter: at least .

Connect in parallel for this backup.

Add at least one appliance to see a recommendation.

Battery backup time formula

Backup time (hours) = battery voltage × capacity (Ah) × 0.8 ÷ load (watts)

Voltage times amp-hours gives the energy the battery stores, in watt-hours. We count 80% of it as usable, the same depth of discharge our cycle ratings use, then divide by the load you run.

For example, a 12V 100Ah lithium battery runs a 400 W load for 12.8 × 100 × 0.8 ÷ 400 = 2.56 hours, about 2 hours 34 minutes.

A tubular lead-acid battery should not be run down fully, or it wears out fast, so count only about half its rated capacity: choose "12V tubular lead-acid" above to see its backup.

Backup time of 100Ah, 150Ah and 200Ah lithium batteries

Backup at three common loads: 200 W is a few fans, lights and a TV; 400 W adds a fridge; 800 W adds a water pump or more of everything.

Battery200 W load400 W load800 W load
12V 100Ah1.28 kWh lithium 5 h 07 min 2 h 34 min 1 h 17 min
12V 150Ah1.92 kWh lithium 7 h 41 min 3 h 50 min 1 h 55 min
12V 200Ah2.56 kWh lithium 10 h 14 min 5 h 07 min 2 h 34 min
24V 100Ah2.56 kWh lithium 10 h 14 min 5 h 07 min 2 h 34 min
24V 150Ah3.84 kWh lithium 15 h 22 min 7 h 41 min 3 h 50 min
24V 200Ah5.12 kWh lithium 20 h 29 min 10 h 14 min 5 h 07 min

Which inverter battery should I buy?

Add up the watts of what you run during a power cut, then read the backup from the table or the calculator. A 12V home inverter takes a Shakti 12 (100Ah) or Shakti 12 Plus (200Ah); a 24V inverter, common in larger homes, shops and offices, takes a Shakti 24 or Shakti 24 Plus.

Our guides explain how to choose an inverter and battery for home and how to size an inverter for a shop or office.

See the lithium inverter batteries and inverters with their prices, or ask support and we will size it for you. No inverter yet? See the Prakash 1100 and Prakash 2200.

FAQ

Battery backup time questions

How do I calculate inverter battery backup time?

Multiply the battery voltage by its amp-hours and by 0.8, then divide by your load in watts. A 12.8V 100Ah lithium battery at 400 W gives 12.8 × 100 × 0.8 ÷ 400, or about 2.5 hours.

How long will a 200Ah inverter battery last?

A 12.8V 200Ah lithium battery stores about 2.56 kWh. At 400 W it runs for about 5 hours, and at 800 W for about 2.5 hours.

What size battery do I need for a 1 kW load?

For 4 hours at 1 kW you need about 5 kWh of rated energy, since 80% of it is counted as usable. The Surya 51 and Shakti 24 Plus each store 5.12 kWh.

Does the calculator include inverter losses?

No. It counts 80% of the rated energy as usable and has no separate figure for inverter losses, so treat the result as a planning figure.