Well, if its 1500W, and he is making his own coil out of PEX that size he is going to be in for a surprise before he even gets to extracting the heat from the ground. I do know that.You don't really know that. It's obvious to me that the OP likely has more experience in knowing how much energy it takes to keep HIS tank from freezing in HIS environment based on HIS experience. What he lacks is understanding how much heat he can pull out of the ground with this method. Which in my opinion is not much.
Relevant Heat Transfer Equation:
This is the point about trying to compute the losses from the tank. If it's not 1500 W perhaps it could require significantly less tube...to get the heat into the tank (at least).
Overall I suspect its not going to be feasible, but hey...that's what we are trying to figure out for them. Perhaps there are configurations that could be considered so that it could work. I don't know yet...because we haven't tried to compute the actual losses from the tank ( 100 gal - which is not huge as far as tanks go).