I thought I had made a mistake when I calculated the cost of 100W incandescent lighting to be the awfully coincidental number of almost exactly 100€/year. Finding this to be correct was quite the revelation: makes estimating the cost of anything in the house so easy because I already knew the wattages :)

(The landlord had installed these sensor-activated ancient bulbs in the hallway, where I pass through to to the cellar / power meter, and I was trying to track down this mysterious 100W that seemed to be always running, without fail. Turns out, it was only running when I was checking the meter! We then did the math with a better runtime estimate and still went out to buy LED bulbs at our earliest convenience. They're brighter than before (we erred on the high side), just as warm light, and use 2.5x less power.)

No matter how common it is, I never know what "2.5x less than some reference number" means. Is it "divide the reference number by 2.5"?

Correct, i.e. 40W instead of 100W. It sounded more impressive than "40% of the original value" so I went with "2.5x less". Not the best measure to choose one's words by, admittedly.

40W LED? Wow that's big! I think the biggest LED bulb I have is 11W

2x20 (notice plural 'bulbs' in the original message a few steps up the thread), and this is actually measured whereas iirc the box said a bit less

And yes, in my opinion we erred on the high side, but it's not far off from what the original incandescent (which apparently was 2x50W, measured).

You managed to say that immediately. Were there other serious explanations that came to mind? If not then you need to have more self-confidence because you do know!