Wattage is maximum power throughput. The only thing with heat is the more watts something uses, the more heat it creates (since electrical energy in PCs is 99% turned into heat energy). This comes out of the components that use the power, not the PSU itself. The PSU does produce heat however through efficiency loss. Cheap and/or old PSUs like that Thermaltake are only 70-75% efficient, which means 25-30% of their max 300W will be ADDED in additional power drawn from the socket, and generated as heat in the power supply. At 300W D.C. load, that's 75-90W of heat generated in the PSU. It's worth noting that a lot of new PSUs especially in europe with 220-240V supply are nuding 90% efficient for a wide range of loads. A good 850W PSU like a Zalman may only be wasting the same 75-90W at 700W load.