The Mac mini draws 5 W when on, and 0.5W when sleeping

Is the 5 watts on an M series Mac?

This is short-sighted. The amount of extra power the computer draws at a high power state while performing boot-up tasks makes the sleep power draw a better option. Not to mention the sleep power draw happens at off-peak hours where the grid can provide more green power vs. the dirty mid-morning peak power. The break-even point that I’ve calculated across the machines I’ve plugged into my meter is approximately 3-4 days. With a big ol’ “it depends” sticker slapped on top.

Edit: and my lazy methodology doesn’t even account for the extra energy used by the machine throughout the day when it has to cold-start various programs and tasks without any caches.

Double-edit: if you want to go the extra mile you can use the “hibernate” feature of windows after force-enabling it or the “pmset sleepmode {whateverthefuckitis} “ of macOS to split the difference. Or you can take a shower that’s literally 10 seconds shorter because heating shower water for literally only 10 seconds will use more power than any of these things. I strongly implore you to calculate your trip to the grocery store in kilowatt/hrs as well. Optimize where it matters!