According to CSA testing, over the course of an entire swimming season in southern
Ontario, a typical 4' x 10' solar panel can produce
about 1.3 Gigajoules of heat energy
to heat your pool.
That's about 11 kbtuh a day (over a theoretical 120 day pool season). A typical 250 kbtuh natural gas pool heater puts that amount
of heat into your pool in 4 minutes.
To produce the same amount of heat energy as a conventional 250 kbtuh pool heater
in a single 7.5 hour day you would need to install
128 solar panels.
Effectiveness of solar panels for pools...