beyond the responses above..you have to ask yourself:

-how quickly you want the pool to heat up?
-will you use it to gain a longer swim season?


i have for 12,000 gallons a 125,000 btu natural gas. it will heat up about a degree per hour on a 70 plus degree day..but overnight right now for example with colder air temps, it only heats up about 1/2 to 2/3 a degree per hour.

i had no interest in extending my season beyond current (mid may to mid sept) as once the air temp is below 70 the kids have no interest in swimming even if the pool is hot (air feels too cold to them when they get out)

i would say my 125,000 btu heater for my pool size and situation is just enough. from what i understand a bigger heater will simply heat up the pool quicker the bigger you go..in turn, running for a shorter cycle.

the problem i ran into is heaters cost a fortune in canada compared to the us, so i bought from the u.s. even so, the difference in price for a 125,000 btu to 175,000 btu was about 50% more and i didn't feel i would get my money's worth for the bigger unit..frankly, when i reviewed prices at the places i was looking at to buy, for another 10% more cost going from the 175K to a 250K, i would have gone that route..

check the prices out carefully, do your research, figure the bang for your buck.