San Jose 60 - 100% of surface of pool depending on covering, shade and wind exposure, and if you use a pool cover
Pacific beaches 40 - 50% if the pool is in the sun most of the time and up to 100% if pool is covered
The number of panels need depends on the that figure above. Say you are in Montezuma, you would need 40% of the area of your pool solar panels on the roof or some other place in the sun to assure you have a warm pool.
And in San Jose with no shade but a windy spot and no pool cover then you are looking at 120% of the area of the pool.
Prices are roughtly 120 - 135 a square meter of determined solar area (as described above)