As I understand it, (and I could be wrong here) what is actually absolutely required to spend energy for, is the output.  IOW, you could at least in theory design a system that answers the one-bit question, "is there a nonce meeting the difficulty target within " by actually spending the energy to write exactly one bit.  Everything else can be reversible, so the greater the amount of computation you can do without any external effects required the more of it can be done "free" (albeit at ridiculously high complexity) but no matter what, you have to write the output.