[hollerith]

You are assuming that power used by the network is directly proportional to the rate of transactions processed. I doubt that is a sound assumption.

For example miners use energy to compete for the mining reward which is awarded every ten minutes. I can think of many ways of modifying the network to increase the rate at which transactions can be processed that do not increase this mining reward. (I do not understand why none of those ways have been adopted.)

One of the primary reasons that reward is not lower is probably because the amount was set when Bitcoin was created, and people worry that changing it would set a precedent making other changes easier.

[hwillis]

The amount of work is what keeps transactions secure. The difficulty of computation is directly proportional, but the power usage depends on the demand for mining.

Lighting attempts to decrease this load by effectively batching transactions, but its not a huge difference compared to the orders of magnitude differences that exist already.