You cannot really compare traditional payment systems to cryptos. Bitcoin does not compute / use energy for a transaction, bitcoins network constantly runs computation to secure the ledger and to process transactions. It could be one, it could be thousands. Also theres no fixed "x power per tx" as the power used by the btc network scales by the amount of miners, not by the amount of txs. You could shut down 90% of the current miners and the transaction troughput (After the lowered amount of miners has propagated trough the network) would be exactly the same as it was before with 10x the miners. Obviously, it is in the interest of miners to be as energy efficient as possible, hence miners that manage to increase hashrate and decrease powerusage hit the market all the time. The only reason the power usage goes up is because of more people entering the market - but they are not needed as per specification. Theres also various different technologies, e.g. minerless coins like Nano where everyone "mines" their transactions as they send them as well as bitcoins very own lightning network where you can "combine" multiple smaller payments in a way where you do not need to confirm them upon sending them (While they are still trustless) but they will be mined as one single transaction as you close the channel (When you run out of funds in the lightning channel)