> They're measuring the same thing. Except a joule is one watt-second, which is an inconveniently small unit when talking about large energy storage like an EV battery.

That's what we have SI-prefixes for.

A Watt-Hour is also pretty small. We usually use Megawatt-hours or so. Using Giga-Joule instead wouldn't be too much of a difference.

> Besides, energy has long been accounted for in hour-based units. Your power bill, for example. It's just easier to use those same units when thinking about storage, rather than rebasing everything around seconds.

Yes, tradition seems to be the real reason.

kWh makes sense as an intuitive unit because it makes it easy to work out how much your heater costs to run for an hour.

You can do the same calc in Joules but it's far less intuitive to convert a rating in J/s into J/hr into $/kJ (or worse $/MJ)

It's not so much that W : = V * A, it's that V is defined as V := W / A.

Or to be more precise and pedantic, V := J / C, Joule per Coulomb. In any case, Volt is the derived unit, and Joule is more fundamental.