Why are larger (house, car) rechargeable batteries specified in (k)Wh but smaller batteries (laptop, smartphone) are specified in (m)Ah?


I get that, for a house/solar battery, it sort of makes sense as your typical energy usage would be measured in kWh on your bills. For the smaller devices, though, the chargers are usually rated in watts (especially if it’s USB-C), so why are the batteries specified in amp hours by the manufacturers?

In: 7

Tradition of using mAh for one and progress of using proper unit of energy for the other. Also lying to customers.

mAh is not a unit of battery capacity. If you see a battery with 200 mAh and another battery with 300 mAh this is not enough information to say which one has bigger capacity.
To get the capacity from mAh you need to multiply it by the voltage.
A 200 mAh battery with 10 V output has capacity of 200*10 = 2000 mWh.
A 300 mAh battery with 5 V output has capacity of 300*5= 1500 mWh.

If you compare batteries of same type (same voltage) then mAh is enough to compare them with. But in general it is useless number on its own.

For cheap electronics a big part is also using this nonsense to lie to the consumer because it allows listing big numbers for the product that do not mean anything. So if any product that is not just a bare battery lists its capacity in mAh you can usually completely disregard that number as worthless marketing blubber.
For example a quick check on battery bank listings on a single shop I found these two:

* Product 1: Advertised as 30000 mAh. Actual capacity 111 Wh.
* Product 2: Advertised as 26000 mAh. Actual capacity 288 Wh.
* Many products that do not list their Wh capacity at all.

For general batteries the voltages can be whatever depending on the battery construction. And there may be circuits to step the voltage up or down. So using real unit of capacity is the only proper way to label them.

Force of habit, and it’s a bad habit.

Using Ah was a habit formed when everyone has the same voltage, which is no longer the case now. Using Ah at this point could and has caused confusions.

IMO mAh doesn’t makes sense as a unit of storage. That’s like saying this water bottle has a discharge rate of something instead of saying how much liters is it.

> rated in watts (especially if it’s USB-C), so why are the batteries specified in amp hours

Those are different units completely.

Watts is a unit for POWER, Amp Hour (or watt hour) is a unit of CAPACITY. See the difference as how much water a hose can deliver vs how much water a bucket can hold.

The main ‘feature’ of a battery is how much capacity they can hold so while it is relevant to know what kind of power it can deliver its still more important to know how much capacity fits in there.

mAh doesn’t properly indicate capacity, since it doesn’t tell you the voltage, and therefore not the power. However, almost every phone and powerbank uses a 3.7V battery. Since the voltage is the same, you can use mAh to compare. However, large batteries often have different voltages, so they use kWh or Wh.

I guess the use of mAh is just a habit and no-one sees a need to change. And to the uninitiated, 5000 mAh probably sounds better than 18.5 Wh, even though it’s the same at 3.7V.