I must admit the first time I learned that, I didn't understood that computers was so heat generating. But it is. CPUs are real heat generators, even if they are not that efficient when it comes to generate heating for your house, they heat datacenters for no purposes, because computers prefer to be at a cool temperatures. And the heat is often proportional to the computing power, and not really to the size. I guess voltage is the main reason for it, but I don't know. This is how you enter inside the challenging challenge of cooling datacenters. It often involve a central cooling system, and I'm not even sure that the cooling system doesn't use as much energy as the servers themselves. To be honest I don't know.