Battery capacity comparison from MacBook to Xiaomi Powerbank to Eneloop.

All portable devices need a battery for their power source. We learned basic electric units such as Watts, Ampere, Voltage, Watts-Hour, etc. However, when the device specification listed 2700mAh, or 54Wh battery, we get somewhat confused what it literally means. Here I want to compare those different units and the actual capacity of the batteries.

First of all, there are two different units the manufacturers use to describe the battery capacity: Watts and Amperes. You remember the equation to calculate Watts from Ampere.

Watts (W) = Ampere (A) * Voltage (V)

Apple uses Watts, instead of Ampere, to describe their charger capacity instead of Ampere. For example, Apple's 12W iPad charger is equivalent to 2.4A adapter, as the output of this charger is 5V (standard USB voltage). Similarly 5W iPhone charger outputs 1A at maximum. This is why you can't use iPhone charger for iPad as the output currents is too low for iPad, while you can use iPad charger to charge iPhones.

Similarly Watts-hours can be calculated with Ampere-hours because the voltage is constant regardless of time for the device.

Watt-hours (Wh) = Ampere-hours (Ah) * Voltage (V)

Most mobile devices nowadays use lithium-ion battery. A single cell of the lithium-ion battery has around 3.7 Volts, which is applicable to the most mobile phones and tablets. Therefore, iPhone 6s' 1715 mAh battery is equivalent to 6.35 Wh. We can use Wh or mAh interchangeably for the most case if the device is equipped with 3.7V single cell (or multi-cell with parallel connectivity) battery.

However, laptops needs more power than other portable devices to run the many integrated functions, which is the reason why MacBook Air's battery has higher voltage: 7.4V. This is actually equivalent to the dual cell lithium ion battery in serial connection. Therefore, 54 Wh MacBook Air battery's Ampere-hour is 7300 mAh, not 14,600 mAh.

The attached 'hours' in these unit mean that it can constantly provide such amount of Watts or Amperes for one hour. For example, 54 Wh means the battery can provide 54 Watts power for an hour. If the device consumes only 5.4 Watts, this device can survive for 10 hours with the 54 Wh battery. Power adapter does not use these 'hours' units, as they can supply unlimited power, whereas batteries can provide only for a certain amount of hours.

MacBook Air 13": 54 Wh = 7.4 V * 7,300 mAh
MacBook Pro 13": 74.9 Wh = 7.4 V * 10,120 mAh

iPhone 6s: 1,715 mAh * 3.7V = 6.35 Wh
iPhone 6s Plus: 2,750 mAh * 3.7V = 10.18 Wh
Nexus 5X: 2,700 mAh * 3.7V = 9.99 Wh
Nexus 6P: 3,450 mAh * 3.7V = 12,77 Wh
iPad Air 2: 7,340 mAh * 3.7V = 27.16 Wh
Xiaomi Powerbank 16000mAh: 16,000mAh * 3.7V = 59.2 Wh

Eneloop AA (NiMH): 2000mAh * 1.2V = 2.4 Wh

Note that Xiaomi's 16000 mAh powerbank specifies 16000 mAh capacity based on the battery cell, not the output level. Its output Volt is 5V, not 3.7V, thus it has 11840 mAh output capacity.