There is a calculator on this page for a pool heater....
http://www.engineeringtoolbox.com/sw...ing-d_878.html
Maybe fill in how many BTU/hr the sun is delivering instead. I'm not sure what that number is. Possibly around 300 BTU per square foot.
98.4ft x 49.2ft = 4,841 sq ft x 300 btu sq ft = 1,452,300 btu