It's easier for a consumer running a 100 watt appliance for 2 hours to calculate the cost.

Without knowing anything specific about this, but knowing a lot about how the world works, I'm fairly confident this convention predates the wide acceptance of the Joule unit.

Looking at https://en.wikipedia.org/wiki/General_Conference_on_Weights_..., the Watt and the Joule were formalized at the sa,e conference (in 1948).

Of course, that doesn’t rule out that your claim is true, but it does make me wonder whether it does.

I would guess kWh is used because it makes it easier for customers to verify that they aren’t scammed, given that appliances are tagged with Watts. I guess appliances are tagged with Watts and not Joule/s because it is easier to use a single unit than two.

Yes, but note my "wide acceptance of" weasel phrase. My impression is that the Joule only started being used widely in the 70s.

I did try to find out when it was defined but failed, so thanks for digging that fact up!

There’s also https://books.google.com/ngrams/graph?content=Watt%2CJoule&y....

That is for books and must cover mentions of both Watt’s and Joule’s name and of the unit (and, likely, way more of the names. The units typically are abbreviated), so I don’t think that proves or disproves much about wide use of the units.

The difference before 1860 probably is there because James Prescott Joule was less than a year old when James Watt died in 1819 (at the age of 83)

Edit: https://books.google.com/ngrams/graph?content=kJ%2CkWh%2Ccal... seems to support the claim that “Joule” came up relatively late.

I think because consumer devices are rated in watts, and consumers can easily guess how many hours they use a particular device.

Probably because power consumption of utilities are measured by watt, so watt/h gives more intuition on how much energy was consumed.