What Size Pool Heater Should You Buy?
Don't Get Cheap! Pool Heater Size Helps With Efficiency.
The more BTUs a pool heater produces sooner it will heat the water in your swimming pool. It is necessary to calculate the number of BTUs necessary to heat your swimming pool according to the sizeof your pool .
Variables that come into play are:

surface area  The purpose of any heater is to heat the pool water at a faster rate than heat is being lost from the surface of the swimming pool. Once the perfect temperature has been reached a heater only needs to run enough to compensate for the heat that escapes from the surface of the water. Due to this fact it is a good idea to use a solar blanket in conjunction with a pool heater, or any piece of heating equipment for that matter, in order to maximize heater efficiency. With that said, the more surface area your pool contains through which heat can escape, the more BTUs a pool heater will need to produce to heat your pool water. Figuring out the surface area of your swimming pool is easy and some sample calculations have been provided:

round pools: radius (1/2 diameter) x radius x 3.14


rectangular pools: length x width

oval pools: 1/2 length x 1/2 width x 3.14

rectangular pools w/ rounded ends: length x width x 0.8

kidneyshaped pools: length x width x 0.75

temperature rise  The temperature increase of your pool is a calculation of your perfect temperature minus theaverage ambient air temperature. The perfect temperature is the final temperature you'd like your pool water to reach. The average ambient air temperature is the average daily temperature. It should be calculated on the basis of the coldest month you plan on using your pool. For instance if I lived in an area where the average temperature in May, the month I wished to start using my pool, was 65 degrees and my desired temperature was 80 degree water, my temperature rise would be 20 degrees (80  60).
With surface area and temperature rise calculated you can refer to the following chart to find just how many BTUs will be necessary for the desired heating effect on your swimming pool:
Temp Rise (degrees)

10 deg

15 deg

20 deg

25 deg

30 deg



Surface Area (sq. ft) 
Required Heater Output in BTUs / hr:



200

21,000

31,500

42,000

52,500

63,000

300

31,500

47,300

73,000

78,800

94,500

400

42,000

63,000

84,000

105,000

126,000







500

52,500

78,800

105,000

131,000

157,000

600

63,000

94,500

126,000

157,000

189,000

700

73,500

110,000

147,000

184,000

220,000







800

84,000

126,000

168,000

210,000

252,000

900

94,500

142,000

189,000

236,000

284,000

1,000

105,000

157,000

210,000

263,000

315,000

* This chart is based upon a wind velocity of 3.5 mph. Multiply by a factor of 1.25 or 2.0 for wind speeds of 5 and 10 mph.
After finding the right amount of BTUs / hr from the chart, compare pool heater models based upon the amount of BTUs produced. The number can be found by multiplying the pool heater's input ( measured in BTUs ) by a pool heater's efficiency rating.

heater efficiency  Pool heater efficiency is represented by a percentage, according to the manufacturer, that represents the amount of energy a pool heater transfers to the water for each unit of energy (BTU) used to generate that heat. The US Department of Energy requires that all pool heaters have an efficiency rating of 78% or more.
An example of this would be a 400,000 BTU pool heater with an efficiency of 90%, this pool heater would therefore produce 360,000 BTU/hr.
