I picked up an old used SFF (Small Form Factor) office PC a couple years ago to use as my daily driver instead of my gaming PC. Based on a Kill-o-watt power usage meter, my gaming PC would idle at around 40w, while the office PC (with a fairly power efficient i7-7700 and integrated graphics) idles at 10w, so 3x more efficient. Now I only ever boot up the gaming PC when I actually need its performance. As a side bonus, the DDR3 RAM it uses is still very affordable on the used market. It browses the web and plays videos really well, and even plays most games I’m interested in nowadays :)

For anyone else interested in doing that, Dell, HP, and Lenovo are the big makers of those SFF PC’s, and they can generally be had pretty cheaply on eBay, but you can also sometimes find them locally at recycling facilities or the local dump drop-off area. You may want to avoid anything older than the 6000 series of Intel CPU’s, since the older ones don’t support modern video codecs (meaning they’ll use more power playing youtube videos, decoding them on the CPU itself instead of the build-in decoder), and they aren’t as power efficient overall.

I also found that reducing your monitor’s brightness to around 50 to 60% reduces power usage significantly, though that may not be viable if you are in a room that gets very bright.