Above are the two most popular options for heating a spa. On the left you have a gas heater, either natural gas, or propane, and on the right is an electric heater. For heating a spa, which is typically 1000 gallons or less, you will often see 150,000 BTU - 250,000 BTU gas heaters and 5K - 11k electric heaters. Since there is often such a small difference in price between the smaller and larger sizes, I like to recommend the larger of the two, because they bring the water temperature up in less time. The electric heaters may end up costing you less over time to heat a spa. The price of gas can be expensive compared to the cost of electricity and depending on how often the spa is used and what time of year it is used, the cost of heating the water can differ greatly. An 11k electric heater is roughly 37,000 BTU's, which means it will take a lot longer to heat your spa than a 250,000 BTU gas heater. Though an electric heater can be cheaper to buy and cheaper to run [I have even heard some say it provides a more uniform heat] it simply cannot heat up the water as fast as the gas heaters, which is why I don't personally recommend them. 

You should know that if you have a propane system, your propane tanks are never filled all the way. Propane tanks are filled to 80% capacity. This is called the 80% rule. Leaving some air in the tank is a preventative safety measure against the fluctuations that happen inside a tank. Propane, like water, will expand when it heats up. However, propane, increases in volume nearly 17 times greater than water with the same increase in temperature. Here in Phoenix, the outside temperature can be as low as 32 degrees at night in January and as high as 103 degrees during the day in July. Considering the temperature differences through out the year and from day to night, space in the tank is necessary. If you have a 25 gallon tank, that does not mean you have 25 gallons of fuel to burn.

Typically, it takes 75 minutes to raise the temperature of 1000 gallons of water 30 degrees with a 250,000 BTU gas heater. The amount of gas you use will depend on the temperature of the water to start with and how warm you want to make it. The colder the water is, the more time and fuel it will take to raise the temperature to 100+ degrees. Lets say for example you have a 1000 gallon spa and a 250,000 BTU gas heater. During the month of January, the water temperature is often 65 degrees here in the valley. When I use a spa, I like to heat the water to 103 degrees. This is an increase in temperature of 38 degrees, which will take about an hour and a half. Now on the other hand, if it were July and the starting water temperature is 80 degrees, it would only take about 45 minutes to go up 23 degrees, requiring a lot less fuel.

An 11K electric heater will raise the temperature approximately 5 degrees an hour in a 1000 gallon spa. Under the same conditions mentioned above, it will take about 8 hours to bring the 65 degree water to 103 degrees and 4.5 hours to raise the temperature from 80 degrees to 103 degrees. 

With a 250,000 BTU gas heater, you can expect to use approximately 3 gallons of propane per hour and approximately 3 therms of natural gas per hour. The electric heaters use the kilowatts listed per hour. I have been using the Raypak 11K heater in my examples, so that is 11 kilowatts per hour of energy used.  

I cannot stress this enough...if you have a heater, you must pay attention to the water chemistry weekly through out the year. You want to make sure you are keeping your pH between 7.2 and 7.6. Scaling will form on the heat exchangers when the pH is allowed to get high, over 7.6. When the pH is left to drop below 7.2, the water is more acidic, which can also damage heat exchangers as a result of corrosion. Corrosion can also happen through oxidization, so you want to avoid putting chlorine tabs in the skimmer when you own a heater. Damaged heat exchangers due to poorly maintained water, or neglect, will void the manufactures warranty on new products.