There were plenty of 400-500W power supplies in use back then, and 100W was on the low side for a non-tiny CRT. Don't forget that people commonly had multiple hard drives and/or external drives because storage hadn't saturated for the average user yet (oh, so many people learned the hard way that Iomega was not the answer…), and things like personal printers were more common since home networks weren't. I'm probably biased a bit as the home users I knew were mostly professionals rather than hobbyist but when people were speccing out UPSs they tended to assume numbers which these days we'd only see for beefy servers.

The bigger challenge is that while EnergyStar made a big improvement, it took a number of years to become something you could assume. Problems with firmware and software support meant that a lot of people disabled it to avoid problems and systems didn't spend as much time in lower-power states.