Arguably in the winter that 400W is also helping heat their house, and depending on what type of furnace they use, may be reducing some of their dependency on natural gas.
Unfortunately that is similarly offset by the extra need to cool in warmer weather. Though it's interesting to think about given you have to factor in different system efficiencies for different power and heat/cool types as well as climate. Figuring out where the balance actually lies is gonna be different for everybody.
Ah, but if their cooling is electric, and hence in their case, solar, and their heat is natural gas, this is still better. =)
That being said, it's entirely possible for someone who has a solar set up to also have switched to electric heat, it's just pretty uncommon in northern climates still.
Generally though, yeah, I prefer NUCs and the like over servers for home use for power and noise constraints.