I can only see the energy usage growing further. Because reducing the watt per GH/s will not be infinite, so far we've seen exceptional reduction <1w per gh, but we will reach a stage or phase where the reductions are so minuscule, that the overall mining power usage will grow as more miners are added.
"When one considers Koomeys Law, we can expect energy/GH to continue to half every 18 months until 2048.
This means that we can expect our current industry best efficiency of 0.733 W/GH to reach 0.0000000873804 W/GH."