And I would argue your point that geothermal heat comes from the sun. It comes from the Earth. Remember, the earth used to be a big glob of magma and has very slowly cooled over lots and lots of years (even though the sun has been shining on it). eventually it got cold enough that the outside of that ball of magma solidified, but the inside remained molten because it hasn't cooled enough. That is where geothermal energy comes from and is why it is warm underground in the winter, even when the top part is frozen. If you dig through permafrost, you will still find warm ground once you get deep enough. If the sun stopped shining, you would still find heat below the Earth's surface for a long long long long time.