I had a solar heater for my pool. Electric or gas heaters generally cost $300-400 per month just for a swimming pool.
I am an engineer.
Three years ago I attended a week long training event on solar water heaters (for heating hot water for your house.) This was at the Midwest Renewable Energy Association. They showed a cost/benefit graph that showed a family of four would pay off their investment in 10 years. (Of course you needed a $2,500 federal tax credit and a $2,500 Wisconsin tax credit to make the numbers add up.) And did I fail to mention that to payback in 10 years also required no maintenance costs. Even though everything was only warrantied for five years and the water/antifreeze solution should be replaced every five years.
I drained my pool and painted the bottom black. It absorbed and retained heat very well and reduced the heating cost considerably. There were a couple of hot months in July and August when it got a bit warm, but it was offset by comfortable water temp in cooler months like Feb, March, April and October, November.