Solar Heating for Hot Tubs and Spas
Hot tubs and spas actually require a much smaller collector area than that needed for a large swimming pool (in spite of the fact they are heated to a higher temperature). Typically, a glazed collector will be required, as these heat to higher temperatures.
Hot tubs require a lot of energy to heat, especially if you change the water frequently. The use of a solar pool heater or even a solar thermal water heater makes good energy sense, when you take a good look at it.
Take the example of a hot tub 5 feet in diameter holding up to 500 gallons of water. This hot tub will require about 87 kilowatt-hours of electricity before start-up. That will add up to nearly $7.00 if your rate is 7.5 cents per kilowatt-hour. That's just to heat the water. Keeping it warm for days and months on end will of course cost much more.