In the end it doesn't matter how efficient the mining ends up being. All that matters is the price of electricity. Assuming efficiency gains level out and thus

In the end it doesn't matter how efficient the mining ends up being. All that matters is the price of electricity. Assuming efficiency gains level out and thus the cycle of buying new hardware to keep up approaches infinity length, the only major cost left is electricity use. So in that case the energy used is dependent on (transaction fees * bitcoin price) / (energy price) So to get a % of all energy usage, you need to know the future energy usage, the future energy price, the future bitcoin price and the future amount that will be spend on transaction fees. I'm sure the future energy usage and price have good forecasts available. Then you're left with the bitcoin price and fees (both which are dependent on popularity (which in turn is dependent on price and fees)), but good luck forecasting that.