Just install a low power CPU. You can get one on eBay delivered for a little more than USPS 1st class shipping. Most of my devices, including my NAS, consume 25-35w, so up to 15w additional consumption, as checked with Kill-A-Watt.

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.