I started my new solar pool heater a few days ago. 250 meters/820 feet of 20mm/¾ inch PEM hose uncovered on a flat garage roof. Started filling the pool with 7°C/45°F tap water and I got 27°C/81°F after the heater with the water running at 6 liters/1,8 gallons per minute. The weather was a clear blue sky and an outside temp of 23°C/73°F. You can check out this pool heater www.youtube.com/@SolarProjectsSweden and yes, I jumped in for the premier bath of the season that same day with 26°C/79°F water in the pool. This will be hooked up to a 20v barrel pump that will run automatically on a solar panel when the sun is up and heats up the water. No sun - no pump - no heat. Since I'm located at 60° latitude in the northern hemisphere in Sweden, I'll guess it'll be some problems and math to solve along the way, but it'll be fun. I've spent 250 USD for the hose, 50 USD for the pump and 50 USD for the solar panel. If it works, I'll have a pool heater for 350 USD that doesn't require any manuel labour and it'll run for free.