The size of your heater depends mainly on the size of your pool and your local weather. You can calculate the amount of water in your pool to better understand what size heater you need.
In addition to the length and width of your pool, measure the average water depth. Multiply those three values together, then multiply the number by 7.5 to find out how many gallons your pool holds.
Pool heaters use the British Thermal Unit (BTU) to describe how many pounds of water they can heat. For every one BTU, the heater can warm one pound of water one degree.
Now that you know how many gallons of water your pool holds, you can multiply that number by 8.3 to find how many pounds of water you need to heat. That total number will show you how many BTUs you need in a pool heater. (6)
Many solar panels will advertise how many BTUs of energy they can produce per square foot per hour. Add up how many square feet of paneling you are considering buying and make sure they can heat your entire pool throughout the day.