The energy consumption in Bitcoin's case depends not on the number of transactions but rather on the difficulty or, in other words, on the number of nodes. I read somewhere that, theoretically, several nodes would be enough to process all the transactions and that we have a lot of nodes not because they are necessary but because people want to receive the mining rewards. So, theoretically, we might not need more energy consumption if all transactions processed now by banks were in blockchain. CMIIW.