By the cooling load of the home only.
Rule of thumb is 500sf per ton. 1500sf = 3 ton
25 cubic feet
4-5 tons
2 ton
An average 3 ton heat pump on a 1200 to 1500 sq ft home in California will typically have a 1500Watt (1.5KW) to 5000 Watt (5 KW) auxiliary heat kit installed. This is not just for emergency heat in case of compressor failure or too low of an ambient temperature to operate a heat pump efficiently, but also to temper the air going into the home while a heat pump defrosts. Since the defrost mode is the AC mode with the outside fan motor of the air going indoors will be refrigerated, this is why the heat strip wil be used to temper the air to avoid having to bring the room back up to where it was when defrost began.
The size of a heat pump that you can use for a pool which is 18 feet wide by 33 feet long and has a depth of 4.5 feet can vary depending on specific brand of heat pump you wish to purchase. The minimum size of heat pump for an above ground pool of this size in 102,000 BTU.
it is a 12 seer 4 ton heat pump
2.0 ton or 2.5 ton
1500 sf
The rule of thumb is 2-1/2 tons per 1000 square feet.
Check your owners manual for details.