There are lots of choices when it comes to heating your water. In our humble opinion, when in doubt, get the larger capacity heater. The higher the BTU rating on you heater the quicker your water will heat. You might think that larger heaters use more gas and you would be right but most people don't consider that your overall heating time is significantly reduced thereby compensating for the additional gas consumption. If the cost differenced is not a consideration, we recommend always going for the larger heater! See available gas heaters (link to gas heaters in shopping cart). For the most efficient heating, especially during the colder more windy months, try using a solar cover for the pool/spa surface. It will stop the wind from reducing the water temperature and it will stop the heat from rising up and escaping from the water surface. Solar covers are relatively inexpensive.
Electric heaters are, by far, the least efficient means of heating your water. If natural gas service is not available in your area, please consider installing a propane tank to power a gas heater and compare the overall costs to the use of an electric heater. You might find the efficiency and long term savings are worth the initial investment into a propane system. A little research into propane might pay off. Some propane providers will sometimes give you the propane storage tank and connect it to your heater for free just to get you as a customer. Make a list of all your local propane providers and see which ones might be willing to take care of you!
If you are heating only a spa, an electric heater, while still costly to operate is still a viable option. Before you make a purchase, have a certified or licensed electrician do a "load calculation" on your house electrical system to see if you have enough available electrical service to power the heater. For an average size spa, we have found the best results using an 11KW heater. It just takes too many hours to heat it with anything smaller.
Solar heat is, without a doubt, the most efficient heating system ever. There are a few drawbacks though. First, it is not likely that you will be able to achieve the hot temperatures you might want in the spa except during the hot months where you might not want the water too hot. During the off season when you have cloudy and days, you might find the solar system to be ineffective. Second, you will need a south or west bound facing roof line to attach the collector panels. If you are into aesthetics and you have a beautiful house, you might find the solar panels to be unattractive and industrial looking. Over time, you will get a great value and will extend your swimming season using a solar heating system.