So, is driving an EV (like the Tesla Model S) actually sustainable? Does it not matter how the actual power/energy was created? Does this matter?
short answer: yes, it's better than driving a fossil-fuel car.
Long answer: it may seem to make less sense if you only consider a very small range of impacts, where all of the costs are visible, but only some of the benefits. Only when you zoom out to the long-term system level do you capture all of the benefits. And there are even better alternatives.
carbon in motion
Even if the extra electricity is provided by coal, an electric vehicle is so much more efficient than a fossil car, that there will carbon savings: electric cars are typically 80-90% efficient, compared to about 20% for a fossil car. So even with a coal plant at 33% efficiency, there' still a carbon saving from the electric.
Electric vehicles tend to be lighter, too (except the Rolls Royce Electric Phantom), so there can be lower impact on materials for the chassis. The batteries are materials-intensive; they are currently so valuable that a lot of attention is paid to end-of-life reuse and recycling of the materials.
Electric vehicles can help with the market penetration of exogenously-variable renewables such as wind and PV. They release some of the stranglehold that oil companies have on the economy and politics. They make for quieter streets, and quieter car journeys. They eliminate the local pollution from car exhausts (tail pipes).
Patterns both of car ownership and use have changed hugely in the last 4 decades. And the change in the 4 decades before that was huge too. 4 decades is enough for really big changes in transport patterns. For several reasons. It's 1.5-2 generations, giving plenty of time for cultural attitudes to shift. It's long enough for disruptive technological innovations to gain big market share. And it's long enough for a sufficient amount of land-use to be reshaped around new transport patterns.
Current car ownership doesn't make a lot of sense, from lots of perspectives. Most cars spend almost all their time stationary. Quite a lot of dwellings have different cars for different purposes. Most cars are able to drive hundreds of miles on a single refuelling, but very rarely if ever do so.
It's possible we will see completely different patterns of ownership in the future, with various different forms of shared ownership.
It's also possible that the growing obesity epidemic in parts of the developed world, the internalising of negative externalities into energy prices, and a better public understanding of the prerequisites for and importance of a vibrant public realm, lead to a resurgence in walking and cycling, negating the need for a big proportion of current car ownership and use.