The actual amount of energy consumed by one bitcoin transaction is actually pretty exactly the amount of money you pay as a fee times the cheapest you can get e

The actual amount of energy consumed by one bitcoin transaction is actually pretty exactly the amount of money you pay as a fee times the cheapest you can get electricity in the world. So if you pay $0.2 in fees, you will burn around $0.2 / ($0.1/kWh) = 2kWh of electricity. Note that this is an upper limit since it ignores miner profit (but that will tend towards zero over time). This is ignoring the block subsidy, but that is independent of the number of transactions will forcibly solve itself over time.