Check your math. You seem to have made an error in that calculation.
That electricity is being used for much more than just "confirming a single transaction".
Do you mean in the philosophical sense?
If the actual, it's not too far off. Rounding some numbers:
Current network hash rate = 13.4 EH/s
13.4 EH / 14 TH per S9 = 960K units
960K * 1.4 KWatt = 1.34 GWatt
Average block time = 1/6 hours
Average transactions per block = 3000
1.34 GWh / 6 / 3000 = 74 kWh.
I assume not everyone uses the S9, so let's add 25% to get 90 kWh/tx.
When a miner adds a block to the blockchain, it doesn't ONLY add 1 confirmation to the transactions in their block. It ALSO adds 1 confirmation to every transaction in the blockchain.
New transactions are only possible because the earlier transactions are immutable. The earlier transactions are only immutable because of the continuing proof-of-work process. This is why empty blocks are still valuable and useful. The electricity being used is sustaining an international permissionless trustless immutable decentralized record keeping system. It is securing every transaction that has ever happened, and making possible all future transactions that ever may happen.
So, if you want to use:
Average transactions per block = 3000
Then to be intellectually honest about it, at a minimum, you need to also use:
Total number of blocks in the blockchain: 499273 (as I'm typing this)
Therefore the electricity used should be divided by 3000 X 499273 and not just by 3000.