Since you have to specify the voltage no matter how you measure energy, because voltage is what primarily determines if a battery will work in most consumer ele

Since you have to specify the voltage no matter how you measure energy, because voltage is what primarily determines if a battery will work in most consumer electronics, what units you specify energy in is not relevant as far as consumers comparing batteries go. If I see that two batteries have the same stated voltage and battery A has higher mAh than B I know that A should last longer in my device. But that's exactly the same if I see A and B have the same voltage but A has higher mWh. It should last longer. Even if the energy was specified in some completely ridiculous way such as how much mass that amount of energy could lift a 2 liter bottle of diet Pepsi on the moon I'd just have to look at the two numbers and pick the bigger one.