There are three in the US.
East, West and Texas.
> I think that the US either has a national power grid, or has one for the West and East halves of the country
It would makes sense.
> Obviously, throwing power across the whole damn continent is far less efficient than just throwing it across a state.
For now. With the future high-temperature supra-conductive cabling we will be able to do wonders of efficiency on large distance.
I've -very recently- heard people making the claim that Texas has its own grid, but that's news to me. Maybe the situation in Texas is that they have all the power gen capacity that they need within the state, and are also hooked into the national grid. shrug
> I know heavy infrastructure like these often have a sound design respectively to their constraints (which can be absurd).
Anti-nuke hysteria is toxic to sound planning and design. :(