[ravi-delia]

The KwH makes sense if you consider that while units are largely path-independent, mental calculations are not. At any given moment, the sensible measure of your house's electricity usage is in kW. And to work backwards to figure out consumption, the hour certainly beats the second. Sure, you could call it 3.6e6 joules, but at that what point what does it buy you?

Horsepower is clearly insane though, I have no idea why you'd bother.

[andrewla]

For most realistic conversations you'd be talking about the relative consumption of two devices. Watts/kW is totally fine for this.

Regardless, head-math or otherwise, the vast majority of people will never do this calculation at all, except maybe to weigh the relative power consumption. And if they do the math, having a calculator and having all the units be compatible with each other (so just a natural conversion of W to J/s) is totally fine.