It would be more interesting to calculate the amount of power used by the bitcoin network per transaction. Sure, central banks have huge server farms but the amount of transactions they handle each day are enourmous compared to bitcoin.
If I remember correctly then there was a study that said that each bitcoin transaction eats the power that 1.XX average US-households use per day.
They will probably include the cost of running each node as a standalone functiion. I'm running core 0.12 as a background task whilst I do some other work on the computer. How much extra electricity is that going to consume?