What about the storage devices (the S in NAS)? My external HDD (just for reference) has an AC adaptor that outputs 12V at 1.5A, so that's another 18W. And most people who care enough about storage to have a NAS have multiple disks...

I'm not saying it isn't under 40W - as you note, if it's idling properly, it's pretty easy to have a NAS including disks under 10W idle. But the electricity cost is not negligible when you are running something 24/7. Even 10W24hr30day is 7.2kWh per month, and most places cant count on 8c/kWh electricity anymore.

> My RPi 4b runs on a 18W power supply, which would put it at 432Wh per day

Which would be 13 kWh per month. If we're comparing to monthly fees for cloud storage, it's important to look at the same time frame.