You can find the BTU if you want but it wont be what u are expecting, or answer the question you are asking I don't think?
The water flow, volume, rate of filtration, % of cover, amount of direct sun, and surrounding ambient environment are huge variables in this and is probably why they don't list a BTU???
As an experiment, which is kind of fun and be more direct to your question; Try rasing the adjustment on your heater 1-2 degrees and time the heater for when it shuts off. It's not fool proof, but will get you in the ballpark.
It's not straight and easy to get exact values, but if you just want the straight heater core type into BTUs you may be able to find something general, but calling the manufacurer will be way easier!