On Thu, 23 May 2013 05:44:59 -0700 (PDT),
wrote:

No, there is no insulation between the pool and the ground. It doesn't get that cold down here for that kind of thing, I would think.

The ground temperature in Florida is around 70F. That
is the temperature of all those beautiful Florida springs. You want
your pool quite a bit warmer than that. Water and concrete conduct
heat well -- not like metal but well enough for insulation to make a
big difference.

In the long run, the installation cost of solar (with virtually no
operating cost) is tiny compared with the operating cost of a heat
pump, so it's worth doing even if you do the heat pump also. Every bit
of heat added by the sun doesn't have to be added by the heat pump.
And you say tile -- if you have a concrete tile roof, then you should
not be worrying about the weight of the solar heat system, which does
not have much water in it at any given time. Just make sure to use a
reputable installer so they don't damage the tile.

The larger unit will run less, but when you buy electricity, you pay
for energy used. Energy = power times time. You have a power rating in
KW -- that's power input and is what you pay for. You have another
power rating in BTUs/hour -- that's power output and is what you get
in your pool. Both the power output and the power input are different
on the two units, so a difference in run time alone doesn't mean much
by itself. The ratio of the power outputs (in kiloBTU/hour) to the
power inputs (in KW) is 20.69 for the smaller unit and 22.03 for the
larger unit. Thus the larger unit is slightly (about 6%) more
efficient. That's the figure you should use for deciding which is
better in terms of energy use. It may take a long time for the greater
efficiency to compensate for the greater initial outlay.

Other than that, everything everybody else said ...

Edward