Do you have any guidelines to figure how many collectors you may need for a given size pool?Thank you for a very good question which I did not address in the plans.
There is no specific formula for sizing a solar heater to a pool that I know of.
I relied myself on the advice given by the installers of solar pool heaters which is somewhat vague. They say (and they are talking to pool owners in sunnier climes than mine) that you should have from 1/3 to 1/2 of your pool's area in the area of solar collecting surface. Perhaps for my Canadian location I should have planned for a solar area EQUAL to that of my pool!
The propane pool heater that I retired was rated at 150,000 BTU/hr. I have measured the output of my array as almost 50,000 BTU/hr. I could assume that the 150,000 BTU heater was the recommendation of installers in this area. In that sense, I don't have enough.
I have a 50,000 gallon pool and built a solar array with a measured output of 50,000 BTU/hr. One BTU is the heat required to raise one pound of water one degree F, One gallon weighs about eight pounds, so on a great eight hour day, I would expect my pool water temperature to rise by about one degree from the solar heater. It actually goes up about three-four degrees but that is because the pool itself gathers heat from the sun. I use a pool cover and hopefully don't loose all of the heat buildup overnight.
How much heater area can you afford to build or have time to build? How much area do you have available for the heater? How hot do you want the pool to get? How much sun is available? (last year here was particularly disappointing in that regard)
I have a good amount of space behind the pool to expand. I built one bank of 13 collectors with one motor drive. If I was to add more, I would build another array parallel to the first with another motor drive since I think that the current size works well but is probably at the limit for one motor drive.
Getting the pool too warm leads to increased problems with algae and increased chemical usage. Personally I don't like the water warmer than about 82F and I like the water clean with no chemicals unless there is a problem. The copper collector tubes seem to help control the algae.
A hot tub would be another matter.
I hope this helps rather than confuses the issue.