Adessi 10:10 - Does Electricity Grow on Trees?

After watching this video on the BBC news website, Jude posed a question to the Adessi Leeds office (well, just me, because I'm the only other person in today):

By using an electric car you are not using petrol to run your car, but you are using electricity...where does that electricity come from?

I had no idea.

I thought about it some more, and neither of us really know if having an electric car is the best solution to helping the environment. Our point is that you will need to charge it, so unless you know for definite that all of the electricity you are using to charge your car has come from renewable sources, it is still affecting our supply of non-renewable energy.

Is this right? Can anyone offer any knowledge on the subject?

Laura