Right, but this is a Fermi problem. We have no idea what the true output of each generator is, or that there are only two per centre. All we know is anecdotal e

Right, but this is a Fermi problem. We have no idea what the true output of each generator is, or that there are only two per centre. All we know is anecdotal evidence from reporters who visited the sites and whatever VISA will publicly admit. As an order of magnitude estimate, it's good enough. And yes - it does highlight the problem with bitcoin well: 3 orders of magnitude less electricity to process 3 orders of magnitude more transactions.