system
Given the following post, respond as if you're commenting on Reddit:
human
How is battery capacity calculated?

Today had a lot to with batteries, and I feel very confused. I was reading about tesla´s new battery in australia that had a capacity of 100MW, bought a new 10800 mAh battery for my phone, and also read about the 44 MAL 740 battery cell that the germans used in WW2. the german battery held 12000 Ah of power. I dont really know how to relate, and I also feel like a modern battery should be able to hold more than 1/12000 of a battery from the 40s. As a basic consumer, how do I figure out how to compare batteries? Especially when they use differing unit types, Wh vs Ah.
gpt
A Watt-hour is a unit of energy equivalent to 3600 Joules. An Amp-hour is a unit if electric charge equivalent to 3600 Coulombs. The two units measure different things, one the total energy that can be supplied by the battery and the other the total charge that can be supplied by the battery. A rough convention is to multiply the Ah of the battery by it's operating voltage to get the Wh. 

Second, the U-boat battery is the size of a person, much bigger than your cell phone battery.