Why pet rats? What about purposely breeding working rats (or faster rodents) for the generation of electricity. Dump waste food at the top of a huge container and encorage them to run in a hampster wheel scambling to get at that food.
Ye only have to take out the dead (if not, er, 'recycled') and, er, other mess (which could be used to make methane). Apologies to those eating their tea....
And then there is all the prisoners in jails, and those in fitness clubs. Get them to run in similar wheels. House of Commons and the Lords, the same.
There, world fuel crisis sorted.
:-)