Since lots of people may confused by the capacity of batteries. We are going to discuss more details in this article.
Battery capacity is how much energy is stored in the battery. This is often described and calculated in Wh (Watt-hours). Wh (Watt-hours) is the Voltage (V) that the battery provides multiplied by how much current (Amps) the battery can provide for a period of time (usually in hours). The formula is Voltage * Amps * hours = Wh.
For a certain type battery, generally its voltage is fixed according to its internal chemistry (Lead acid battery cell 2V, LiFePO4 battery cell 3.2V, etc). Therefore, people also use Ah (Amps*hour) to measure the (certain type) battery capacity. Certainly, mAh (1000 mAh = 1 Ah) is also common used.
To get Wh, you just need to multiply the Ah (Amps*hour) by the nominal voltage (V). Lets take a standard 3.2V 100Ah LiFePO4 battery cell as example. It has a nominal voltage 3.2V with 100 Amp-hour capacity, therefore it has 3.2V * 100Ah = 320 Wh of capacity.
In reality, most people are only looking for certain type batteries for their applications. Means the voltage of the batteries needed are certain. No matter use Ah or Wh, it will be both easy to understand.
Finally, there are also some industry experts using Ah (Amps*hour) to express the battery capacity, while using Wh (Watt-hours) to express the energy capacity.