We are in the process of building a new pool at our new house but I can say that my old pool was about the same size and we had a 400,000 BTU unit that would heat the pool very quick...it would also heat up your wallet at the same time. The last time I calculated the cost it was around $80 to heat the pool to 85F. When running the Spa (8' dia) it would only take about 30 minutes to heat up to about 102F

I am in Houston Texas and the water isnt very cold to begin with in the winter but it is too cold for me to get into it without a heater.