I am posting this for future reference:
The goal is to have a heater that has enough capacity to heat the pool to your desired level in a reasonable amount of time. Maintaining that temperature is a lot easier once the pool is up to temp.
First, the capacity of most pool heaters are rated in BTU's or British Thermal Units. One BTU is the amount of heat required to raise the temperature of one pound of water, one degree F. And since there are 8.33 gallons of water per pound, it takes 8.33 BTU's to raise one gallon of water, one degree F.
Now let's calculate:
1. Determine the number of gallons in your pool (G).
2. Determine the amount in temperature that you want to raise the pool temp (the easiest way to figure this is to use the air temp as the minimum and the desired pool temp as the maximum) (Delta T).
3. Calculate the number of BTU's needed (BTU) per gallon of water by multiplying step 2 x 8.33. Multiply this number times the gallons of water in your pool (G). This is the number of BTU's to raise your pool temp from the minimum to desired temp.
4. Determine how FAST you want to be able to go from the minimum temp to the desired temp (in hours) (H).
5. Divide the total number of BTU's from step 3 by the hours in step 4 to yield the BTU'S per hour that your heater will need to deliver.
6. Multiply Step 5 by a error factor of 20% and add to step 5. This is approximately the size of heater that you will need.
1. Assume pool volume, G = 10,000 gallons 2. Assume 80F desired pool temp and 60F air temp. Delta T = 80 - 60 = 20F 3. BTU's per gallon x Delta T: 8.33 x 20 = 166.6 BTU's per gallon. BTU/Gal x Gallons (G): 166.6 x 10,000 gal = 1,666,000 Total BTU's. (Wow, seems like a lot!) 4. Assume I want to be able to warm the pool in two days of continuous operation = 48 hours. 5. Total BTUs / Hours: 1,666,000 / 48 = 34,708 BTU's per hour. 6. (BTU/Hr x 20%) + BTU/Hr: 34,708 x 0.20 = 6,941 BTU's 6,941 + 34,708 = 41,649 BTU's per hour.
Now, most pool heaters come in a round number of BTU's like 50,000, 100,000, 150,000, etc. You should select the closest size to your needs (usually on the higher side is best).
Hence, for this example, a 50,000 BTU heater would be more than adequate.
Obviously, you could also work these calculations backward to determine what Delta T you could handle given a certain size heater and pool size.