The amount of energy to power a plastic shredder will more then be made up when you can grind 50 lbs of PLA into pellets to be used again. I havent done the math, but a very slow high torque motor, assuming thats what is being used to shred the stuff, will not need that much electricity.
Sorting is part of what I do anyway when prints fail. Would you rather people throw away the plastic and just buy new rolls? How about shipping those rolls to your house? And where is the failed print going to go? Into the ocean probably. Pick your poison, I pick to recycle the plastic because I am a little optimistic.
As 3D printing emerges more and more, there will be tons more plastic being thrown out, and that is more wasteful then the 60watts of electricity used to grind the plastic back into pellets. Just my thoughts.