Sizing Lithium-Ion Batteries: How to Convert mAh to Wh and Joules

When shopping for a smartphone power bank, an electric bike, or a lithium battery for solar storage, you will see capacity listed in two very different units: milliamp-hours (mAh) and watt-hours (Wh).

Because power banks are labeled in massive mAh ratings (like 20,000 mAh) while airline limits are listed in Wh (typically 100 Wh), it is easy to get confused. In addition, scientists and engineers measure chemical energy in Joules.

Here is the simple guide to battery capacity math: how to convert mAh to Wh, calculate total battery energy in Joules, and why nominal battery voltage changes everything.

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The Core Concept: Charge vs. Energy

The main reason for different units is that mAh and Wh measure different properties:

• Milliamp-hours (mAh) measure electric charge capacity (the volume of electrons a battery can discharge).
• Watt-hours (Wh) and Joules (J) measure energy (the actual capacity of those electrons to do physical work).

If you compare two battery cells with the same mAh capacity but different voltages, the cell with the higher voltage stores more actual energy.

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How to Convert mAh to Wh

To convert milliamp-hours to watt-hours, you must know the battery's nominal voltage (V). The formula is:

Energy (Wh) = Capacity (mAh) × Voltage (V) ÷ 1,000

Worked Example: Phone Power Bank

You bought a 20,000 mAh external battery. The internal lithium-ion cells operate at a nominal voltage of 3.7 V. What is its energy capacity in Wh?

1. Identify the values: mAh = 20,000, V = 3.7 V
2. Calculate:

$$\text = (20,000 \times 3.7) \div 1,000 = 74\text$$

This power bank is well under the Federal Aviation Administration (FAA) airline limit of 100 Wh, making it safe to carry in your carry-on luggage.

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How to Convert mAh to Joules

To find the total energy in the international scientific unit of Joules (J), you multiply the watt-hour value by 3,600 (since 1 Wh is 3,600 Joules).

The combined formula is:

Energy (Joules) = Capacity (mAh) × Voltage (V) × 3.6

Sizing the 20,000 mAh Battery in Joules:

Using the same 3.7V battery from the previous example:

$$\text = 20,000 \times 3.7 \times 3.6 = 266,400\text$$

This battery stores 266.4 kilojoules (kJ) of physical energy. You can compute this automatically using our interactive mAh to Joules Converter.

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Nominal Voltages of Common Battery Chemistries

To convert capacity from mAh to energy, you must use the nominal voltage associated with the battery's specific chemistry:

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Frequently Asked Questions

How do I convert Ah to Wh?

An Ampere-hour (Ah) is 1,000 milliamp-hours. To convert Ah to Wh, simply multiply by voltage directly (without dividing by 1,000):

• Wh = Ah × V

For example, a 12V car battery rated at 50 Ah stores 50 × 12 = 600 Wh of energy.

Why do some power banks claim 20,000 mAh but charge my 4,000 mAh phone only three times?

Because of voltage conversion losses and efficiency ratings. Power banks operate internally at 3.7V, but USB charging output operates at 5.0V (or 9V/12V for fast charging). Converting the voltage up, combined with heat generation during charging, reduces the effective transfer efficiency to about 70-80%.