Did a little bit more digging, the radioactive decay account for about half the heat being given off 20+TW. The rest is the heat from earth formation and the huge pressures created at the core by earth mass and gravity.

What you have to remember is the earth is really big, and when the earth first formed and was a hot molten ball, most of the heavy elements like iron, uranium and thorium sank deep into the core.

Iron makes up 80% of the inner and outer core, but isn't anywhere near that common on the surface, same goes for Uranium and thorium, while they aren't that common on the surface they are much more so in the core, thus the core has a lot more radioactive decay.

The thermal output of the core was calculated by drilling hundreds of deep boreholes and calculating the heat transfer through the earth crust.

While 47TW may seem like a lot, it's nothing compared to the 173,000TW of energy the sun baths the earth in. (thus the reason why greenhouse gases trapping even a tiny percentage more of the suns energy is very bad)