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Watts Hour and Ampere Hour

March 10th, 2012

The capacity of a battery life is specified either in Ampere Hour or in Watts Hour. Let us understand the two terms and the inter relation between them.

The Ampere Hour is more intuitive term for expressing the battery life. If a battery is specified at 20 Ampere hour then the implication is simple - A 20 Ampere hour battery life will last for 20 hours if it draws 1 Ampere of current. It will last for 10 hours if it draws 2 Ampere of current. Simple ?

The problem with the Ampere hour specification is that it does not specify the Voltage at the battery. Let us say you have two batteries. Both the batteries A and B has a Ampere Hour rating of 10 Ampere Hour. However, Battery A is at 3.6 Volts while the Battery B is at 7.2 Volts. Now if you draw 1 Ampere of current from both the batteries, both will last 10 hours. But the Battery B is delivering 1 Ampere at 7.2 Volts while the battery A is delivering it at only 3.6 Volts. I strictly technical terms we say that battery B has double the capacity of the battery A.

This is where the Watt hours come in play. If a battery is rated 20 Watt hours, it will mean that it will last for 20 hours if 1 Watt power is drawn from it. Now watt is defined at the product of the Volt ( at the battery) and Ampere ( drawn from the battery).

In the previous example, the 20 Ampere Hour 7.2 Volt battery would be rated as 1440 Watt Hour while the 20 Ampere Hour 3.6 Volt battery would be rated at 720 Watt Hour.

You can use the battery life calculator to calculate the life of a battery. This calculator just calculates the battery life if you know the Ampere Hour rating and the current drawn from the battery.


  1. Hannes Riedel
    April 29th, 2013 at 02:28 | #1

    "In the previous example, the 20 Ampere Hour 7.2 Volt battery would be rated as 1440 Watt Hour"
    could you help me with this part?
    20 Ah * 7,2 V = 1440 Wh
    20 * 7,2 = 144 ... I am missing a factor of 10

  2. Md.Mourad Hossain
    January 10th, 2014 at 05:24 | #2

    i don't understand the factor 10 because 20Ah*7.2 V = 1440Wh how ? please detail explain

  3. RebTurtle
    January 10th, 2014 at 18:09 | #3

    @Hannes Riedel
    10 hours

  4. Richard Zachary
    August 19th, 2016 at 13:24 | #4

    20 Ah * 7.2 V = 144 Wh
    not 1440 Wh

  5. Jason Lever
    November 1st, 2016 at 13:03 | #5

    Could you please tel me if 8 x 225ah 6v deep cycle batteries will run 900watts for 6 hours through a inverter

  6. sazu
    September 12th, 2018 at 23:55 | #6

    @Hannes Riedel
    1440wh is the maximum Battery capacity of that full charged battery.

  7. KP
    July 18th, 2019 at 06:46 | #7

    I would like to know : How long the battery last if It has 40V with 4.0 ampHr.
    or the battery with 18V with 2.6ampHR.

  8. Paul Snell
    November 17th, 2019 at 10:26 | #8

    I have 3ea 12v 100 ah deep cycle sealed lead acid batteries. I have a potential 700 watt draw which I calculate 56 amps. Is that correct ?? How long will the batteries last. 1/2 day??

    I do have a 100 watt solar panel to charge the batteries. They go into float charge daily with little usage.

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