Swimming pool heater type, efficiency and proper sizing is most important when choosing the right heating system for your pool. Today there are gas, heat pump and solar pool heater systems available for heating your pool. All are good. Selecting the right one should be considered depending upon the application. 

propane pool heater
Natural or propane gas heaters are perfect for heating indoor pools or spas in the winter months. While a heat pump is clearly the better option for pool and spa heating Spring through Fall here in Maryland. A gas heater, at best, has about 90 percent thermal efficiency when new, then gets less efficient as it’s burner tray soots up. Pretty bad efficiency. A heat pump is over 500 percent thermal efficient; always! That means for every dollar of gas consumption a gas heater only puts .90 cents worth of heat in the pool while a heat pump puts over $5.00 worth of heat in the pool! Since heat pumps are so inexpensive to operate, solar is just not necessary. 

Solar is not as effective as a heat pump creating the hot water required to heat the pool in the earlier spring and later fall times you may want to swim as the sun is too far away here in Maryland. If you’re installing solar for heating your home, then it certainly would make sense to plumb it in to your pool or spa circulation system as it will contribute to heating. 
heat pump for pool 
The efficiency of a heat pump allows your pool to maintain a comfortable water temperature 24/7 without an extremely high energy bill at the end of the month. Maintaining comfortable swimming temperatures of 82 degrees or even higher with a gas heater is super expensive so, usually the pool owner will turn the gas heater on Friday then off Sunday night to save energy. What about the rest of the week? Cold water!
 
A gas heater only raises water temperature 2-3 degrees an hour. If the water temperature is a cold 70 degrees that means one would have to wait 6 hours or so to swim in comfortable 82 degree water.
 
The point is a pool heater should maintain 82 or higher all day and night so your pool is always ready to swim at the temperature you desire without the high operating cost. 
hayward pool heat control
Since all heaters have an automatic thermostat like your house, you can set it and forget it. When the sun keeps the pool nice and warm, the heater won’t kick on.
 
Researching the efficiency factors of any heater is important because better heat pumps will utilize lower ambient air temperatures that allow swimming in earlier and later swimming season months. Swimming in mid-April and into October is common with a heat pump. For both heat pumps and gas heaters the BTU and coefficient rating is important to be sure the unit has the heating power required to perform for the gallonage in you pool. 
 
indoor heated pool
Baltimore, MD Harbor View Hohne Pools