Even with free electricity, the embodied energy of the computers used is basically wasted. A simple notary service that signed transactions with a timestamp and published a record to a public data storage bucket could handle worldwide bitcoin transaction volume running on a single machine, if you traded proof-of-waste for a simple root trust store.

(I mean, if I wrote it it would probably take several hundred machines but I'm sure Satoshi or some other brilliant programmer could fit it on a single machine. It only processes what, 5 transactions per second? My first dumb cellphone might be able to do that many signatures per second if someone could figure out how to compile the relevant libraries to run on it.)