I’m a bit below 20W. But I custom-built the computer a long time ago with an energy-efficient mainboard and a PicoPSU. I think other options for people who don’t need a lot of harddisks or a graphics card include old laptops or Mini-PCs. Those should idle ad somewhat like 10-15W. It stretches the definitiin of “desktop pc” a bit, but I guess you could place them on a desk as well 😉