However a router is energy sensitive as it runs 24/7. I wouldn't want to run a desktop for this.
Ever do a tiny load of laundry to get one shirt clean? You've just used something like a year of computer running.
Do you run your air conditioning while your refrigerator exhausts into your kitchen? I call that a kitchen heater. You're heating a cooled space.
I find for whatever reason, tech people all worry about the energy consumed by tiny devices.
Another thing to keep in mind if you're worried about power/heat is that 3.5" spinning rust can pull >5 watts per spindle. I've seen 15K drives pull close to 20. But an SSD is close to nothing and so are most 2.5" laptop spindles.