Peak Oil challenges a lot of people on the traditional Left because they assume that:

(a) Capitalism can only be surpassed in a society of material plenty for all; and

(b) Admitting that energy consumption is way past a sustainable level and has to be cut would bar the way to socialism.

IMO, he's wrong in that (a) actually says a good deal less than he thinks because the concept of "plenty" is actually a social construct; and (b) is plain incorrect.

At a guess, his idea of "socialism" would probably have a lot more in common with the economics of the late, unlamented USSR than mine does, though I'm certainly not accusing him of supporting the political regime that existed there.