Even if the computer does perfectly-efficient computer things with every Joule, every single one of those Joules ends up as one Joule of waste heat.
If you pull 100W of power out of an electric socket, you are heating your environment at 100W of power completely independent of what you use that electricity for.
If you pull 100W of power out of an electric socket, you are heating your environment at 100W of power completely independent of what you use that electricity for.