The proper sizing
Many factors come into consideration, including the amount of shade on the pool, and the collector area, the wind conditions and the use of a solar pool cover, which generally helps keep the water 6 to 8 F warmer. Given the same climatic conditions and the use of a cover, the design of the collectors will account for important differences in the number of collectors.
Though 10 feet is the norm for most manufacturers, most
roofs can accommodate the more efficient 12-14 foot
collectors, which means you can install a larger, more
potent system. When comparing Btu ratings, one should only
compare collectors of the same length, or obtain Btu/square
foot/day figures from the SRCC (Solar Rating
Certification Corporation) at http://www.solar-rating.org.
If the salesperson can supply these numbers with certification, it tells you the collectors have been evaluated. If the salesperson cannot supply these numbers, the collectors may not have been evaluated, or the ratings are too low to be published. How then can the salesperson guide you to the proper number of collectors?
Testing also has its limits and margin of errors, and only experienced salespeople can explain why. In any event, all plastic (polypropylene) collectors are virtually equal, with no more than 5% discrepancy. As a rule of thumb, polypropylene collectors should amount to 50% of the pool area, in the Windsor to Quebec City corridor.
Neoprene collectors can be up to 50% less efficient, due to the thickness of the material, a reduced fluid flow rate, and somewhat fewer channels for the water to run through, therefore exposing less water to the radiant sun. You could need as many as twice the number of neoprene collectors, to get the same performance as with its competition. The salesperson may suggest, through field experience, a number of collectors. But you should be the one to decide: get at least 3 references of pool owners who have a similar pool set-up as yours. Call them, and visit them, then decide how many collectors you need. If you can't get 3 references, draw your own conclusions. It will give you a way out from purchasing "on that first night".