That device&company seems like a joke. Costs around $1000, multiyear payback (~3years) if you use it every day, you only get 400w from the computer(the rest just from a regular heater also built in), and the payback is "up to" $28, which I would assume would be the payback for 24/7 use.

At my rate of using space heaters, I'd expect probably a 10-20 year payback period (pay schedule is not clear, so it is a guess, and generous). Drop $1000 on a computer I don't use, hoping that the company stays in business for decades? No thanks. I'll run a pihole or a server or something and at least get my own compute out of it if I want heat from compute.

But a great answer to the question. Computers are around $1000 for anything approaching heavy compute, you are probably running around a 400 to 750 watt power supply, so you get a few hundred watts of heat out it (depending on load, obviously). Vs the $10 or so you'd pay for that much space heater (you get 1500 for ~20-30). Loooonnnng payback period.