The strength of a pool heater is measured in BTUs, or British Thermal Units. This is an indication of how much heat it can generate, and thus how quickly it can raise the temperature of the water. To determine how long it will take to heat your own pool, perform a quick calculation based on the volume of water it holds.
British Thermal Units

A BTU is a measurement of heat energy. A single BTU is the amount of heat it takes to raise the temperature of one pound of water by one degree Fahrenheit.
BTU Ratings

Many appliances, such as air conditioners, fireplaces, and, of course, pool heaters are given a rating in BTUs. For heaters, this is the amount of heat the device can generate over 60 minutes. Thus, if your pool contains 80,000 pounds of water, it would take exactly an hour to raise its temperature by one degree Fahrenheit.
Weight of Water

Though the density of water fluctuates slightly depending on its temperature, 1 pint weighs roughly 16 ounces, or 1 pound. Since pool capacity is usually measured in gallons, and a gallon contains 8 pints, the weight of the water in pounds contained in your pool can be determined by simply multiplying the number of gallons it holds by 8.
Final Calculation

To find how long an 80,000 BTU pool heater will take to raise the temperature of the water by one degree Fahrenheit, simply divide the weight of the water in pounds by 80,000. The result is the number of hours it will take to heat the pool. Multiply this number by 60 to convert your answer to minutes.
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