President Obama set the ambitious goal of having 1 million electric vehicles on US roads by 2015. Whether automakers will hit this target is the subject of much debate. But the possibility of the widespread adoption of electric vehicles raises an interesting problem.

It’s easy to imagine the number of electric cars increasing but it’s much harder to see the electricity generating capacity increasing the same rate at the same time. Generating capacity will eventually catch up with this extra demand. But in the meantime, how can all these cars be charged?