One good resource for this kind of question is David MacKay's book "Sustainably Energy - Without the Hot Air" (free online: http://www.withouthotair.com/ http:/

One good resource for this kind of question is David MacKay's book "Sustainably Energy - Without the Hot Air" (free online: http://www.withouthotair.com/ http://www.withouthotair.com/). He goes through each type of sustainable energy source and estimates how much power it can provide. At least in the UK, the total potential for geothermal energy is quite small, much less than other sources such as wind and solar. But its a nice short-term supplement, we can construct them today. Having working fusion would be an amazing thing. The raw materials (lithium and deuterium) are plentiful enough that they will basically never run out, it would provide energy forever.