The difficulty increases or decreases to maintain the target block interval. The only way that impacts mining revenue is if the actual block interval deviates significantly from the target—for example, if the hash rate suddenly increased such that blocks were being solved in five minutes instead of ten, that would double the revenue rate (and thus the economical power consumption) temporarily until a difficulty increase brought the interval back to ten minutes per block.

What does cause significant changes in revenue is the periodic halving of the block reward. Absent an opposing increase in price and/or transaction fees, when the reward drops from 12.5 BTC/block to 6.25 BTC/block in May of this year the energy budget for profitable mining should be cut roughly in half.