There's no such thing as an efficient space heater. They are all the same efficiency: 100%, because all the energy goes to it's intended use, which is heat.

In Bitcoin mining, generating heat is an inefficiency. Computers generate heat as a waste product, so in effect them using the waste heat from Bitcoin mining to heat their greenhouses makes the computers into the more efficient space heaters.

Honestly that's the absolute best Roku feature hands down. Being able to just search a movie or tv show and see what service has it or where I can rent it is a fucking godsend.