If you put two 12V batteries in series you’ll get 24V but amp hour value will stay the same. If you connect them in parallel, voltage stays the same, but amp hour value will be doubled.
If you multiply amp hours times battery voltage, you’ll get Watt hours.
Amphours were commonly used for batteries. It’s because back in the day most devices were simple like lightbulbs which draw constant current.
how does that differ from watt hours?
If you put two 12V batteries in series you’ll get 24V but amp hour value will stay the same. If you connect them in parallel, voltage stays the same, but amp hour value will be doubled.
If you multiply amp hours times battery voltage, you’ll get Watt hours.