I live on long island ny and have a short swimming season, I'm knocking ice off the pool one day and taking off the cover the next. I have never taken a reading but we start with 30,000 gal of COLD water. I tried the hose thing even built a solar panel and considered a solar fence everything worked well. The problem was that I couldn't heat enough water to make a real difference heck I could hardly melt the icebergs. I do the work on my pool at night last year the mosquitoes were killer so I had some bricks from a wall I just knocked down and also had about 20' of 1/2" copper pipe. So kill 2 bird's with 1 stone I built a little furnace/still , made a firebox piled bricks around it hung the copper coil from a piece of tin which was also the roof no mortar no nothing a real piece of sh*t. Used my return line to pump the water to the coil my filter running anyway might as well use it. It worked awesome got rid of the bugs all the scrape wood from around my house and was by far the best heater so far and it didn't cost a dime. It worked so well this year we upgraded my new one is sweet I think my pool will actually be warm before half the summers over. The new one cost $40 and wood is free so the price is right. I know you asked about solar but you might be better off with a little fire. If your interested let me know I will help anyway I can