Ah, Wh and mAh

Battery Capacity: Watt-hour (Wh), Ampere-hour (Ah), mAh

Understanding Watt-hour (Wh), Ampere-hour (Ah), and mAh

A snapshot: Watt-hour (Wh) is the energy capacity of a battery, ampere-hour (Ah) is the charge capacity of a battery, and milliampere-hour (mAh) indicates one-thousandth of an Ah. Ah, or mAh is mostly seen on the bodies of batteries while Wh is rarely seen. However, Wh gives a more accurate definition of the energy stored in a battery.


You must be wondering what Watt-hour (Wh), ampere-hour (Ah), and milliampere-hour (mAh) are all about. Well, they all indicate the capacity of a battery and are pretty much connected but are different. Now, have a look at what they are about.

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Explanation of Ampere-Hour (Ah)

Ampere-hour is also called amp-hour. It is the unit of a battery capacity that measures the amount of charge that can be available in a fully charged battery. This amount of charge is what determines the amount of current that can be discharged from the battery and its discharge time. This is why it is also called a charge or coulometric capacity.

Note: The coulometric is derived from the unit of charge, Coulombs.

The Ampere-hour is given by the formula: Q=It. Where Q is the amount of charge, I is the Discharge current (in Ampere) and t is the discharge time (in hours).

For example, a 200Ah battery means that a current of 100A can be discharged through the battery for 2 hours. That is, 100A *2hours = 200Ah

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Another example: 1200mAh battery means that theoretically, a current of 120mA can flow the battery for 10 hours. That is, 120mA * 10 hours = 1200mAh.

Note: Ampere-hour is a unit of electric charge, Q but not the Standard International (SI) unit. The SI unit of Q is Coulombs.

The discharge current and time from the above example is for ideal batteries. In reality, the discharged current and time aren’t that stable. They can vary based on certain conditions.

The Ampere-hour can be seen in both high and low capacity batteries. However, in low capacity batteries, it comes as a mAh.

Key points: Ampere hour is the unit of a battery capacity that determines how much charge can be available in a fully charged battery. In turn, the available charge determines how much current can be discharged and its discharge time. Q = It

What is milliampere-hour (mAh)

mAh stands for milliampere-hour. It has just one difference with Ah. And, that difference is the ‘Milli’.

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Milli means 0.001. It is a unit prefix that represents thousandths. So, when you see mAh, it actually means thousandths of Ah.

For example, a 3000mAh means a thousandth of 3000Ah. That is, 0,001 * 3000Ah = 3Ah. So, 3000mAh is the same as 3Ah.

So, to convert mAh to Ah, you multiply the mAh by 0.001.

And, to convert from Ah to mAh, you multiply the Ah by 1000

Most of your rechargeable devices have their battery rated in mAh. Even your power bank. This is because they are of lower capacity compared to the batteries used in more energy-consuming applications (e.g. solar power systems backup batteries).

Note: Higher capacity batteries can also be rated in the form of mAh

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Simply, the higher the mAh or Ah, the more the battery capacity, Thus, the longer the battery takes to discharge.

Note: The time it takes for a battery to discharge also depends on the load that is connected to it.

Key points: mAh is an acronym for milliampere-hour. It is simply a smaller version of Ah. Milli-ampere-hour stands for the thousandths of ampere-hour. So, mAh = 0.001 * Ah.

Understanding Watt-hour (Wh)

Watt-hour is abbreviated as Wh. It is referred to as the energy capacity of a battery as it more accurately determines the energy that can be stored in a battery.

Generally, Watt-hour is a unit for energy. And it is represented in the formula: Energy = Power * time. Where power is measured in Watt and time is measured in hours.

Power = Current * voltage

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So, energy = current * voltage * time. Where the current is measured in A, the voltage measured in V, and time in hours.

So, energy = A * V * hr = Ahr *V

Wh gives a more accurate capacity of batteries because it includes the voltage and the ampere-hour.

In as much as amp-hour determines how much charge is in a battery, you can’t compare batteries by just using it. I mean, you have to consider both the amp-hour and voltage.

For example, a 12V, 200Ah battery isn’t the same as 24V, 200Ah battery. You can know from their watt-hour.

So, 12V, 200Ah battery =12V * 200Ah =2400Wh

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And 24V, 200Ah battery = 24V * 200Ah = 4800Wh. You see their energy capacity is not the same, right?

Read why batteries are rated in Ah

Key points: Wh is an acronym for watt-hour. It is the energy capacity of a battery. Watt-hour is the multiplication of the battery’s ampere-hour and voltage.


Frequently Asked Questions

How to convert voltage to ampere-hour?

Voltage can’t be converted to ampere-hour (Ah). Theoretically, they are units that don’t depend on each other. Batteries come with designated rated voltages and Ah.

What is the net ampere-hour of batteries (each with the same Ah) connected in series?

When batteries of the same ampere-hour and voltage are connected in series, the ampere-hour remain the same while voltage increases. And the energy capacity of the batteries will also increase.

Kindly share your thoughts through the comment section below.

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5 thoughts on “Battery Capacity: Watt-hour (Wh), Ampere-hour (Ah), mAh”

  1. Hi,

    Thanks for the post.
    Please let me do the following question:

    “When batteries of the same ampere-hour and voltage are connected in series, the ampere-hour remain the same while voltage increases. And the energy capacity of the batteries will also increase.”

    I have an usb (5v output) li-ion battery with these specs: 26800mah, 96.48wh. I’ve been told that the mAh the manufacturer specifies is relative to cell voltage (that is, 3.7v). Does it mean that this battery is composed by two 26800mah 3.7 cells? If so, the global energy stored should be 26800 * 5 = 134Wh, which is not the specified 96.48Wh. So I guess my assumption is wrong and the cells are 19296mah (96.48 / 5) –> 19296 x 5 = the specified 96.48wh.

    Is this reasoning ok?

    Thanks in advance!

    1. First off, the cells are most probably connected in parallel, thus voltage remains the same. So, the ampere-hour of the cells summed up to be 26800mah. This doesn’t indicate that there are two cells. They are probably more than 2 cells.

      Note: Even the specifications on the battery are all estimated.

      The battery, on a full charge, will probably have close to 5v. But, on discharge, it could reduce to that 3.7v or less.

      Since the mah is relative to the 3.7v, global energy = 3.7 * 26800 = 99.16wh. Approximately.

      Happy with this explanation?

    2. Hi; sorry I missed the reply. Thanks for the explanation!
      I’ve been reading about this, and you’re right. The batteries are probably connected in parallel. It seems they have a step up circuit to make a 5v output, and this consumes energy (intensity); so yes, the global energy at 5v is lower than indicated.

      Thanks!

What are your thoughts?