A heat pumps ability to heat pool water depends on the outside air temperature, pool size, the current water temperature, the desired water temperature, and use of a solar blanket. Air temperature is important, because heat pumps heat the surrounding air to warm the pool water. Heat pumps operate best in temperatures that exceed 50°F. In temperatures below an average of 50°F, heat pumps cannot efficiently heat the outside air and therefore require more time to warm the pool.
Heat pumps are great for extending the swim season in the spring and fall and are up to 70% more efficient than gas and propane heaters, making them much more affordable to run.
Swimming pool heaters are sized according to their British Thermal Units (BTU) per hour. One BTU raises one pound of water by 1°F. One gallon of water is equal to 8.34 pounds of water, so 8.34 BTUs raises one gallon of water by 1°F. I typically suggest 85,000-145,000 BTU heaters depending on the size of the pool, based on 10,000 gallons to 20,000 gallons, but the larger the heat pump, the faster the pool will warm up. The greater the difference is between your current and desired water temperature, the longer you will need to run your heat pump.
Solar blankets reduce the required heating time. 75% of a swimming pool’s heat loss is because of evaporation. Adding a solar blanket will retain heat by minimizing evaporation. It acts as a barrier between the water and the air. It's easier to manage solar blankets when they are cut into 2'x2' squares, so they can be easily removed and stacked when not in use in a storage bin.
Overall, a heat pump usually requires between 24 and 72 hours to heat a swimming pool by 20°F depending on the factors mentioned above.