4.6 lbs/gallon x $1.70/lb = 7.82 $/gal . . . . or 2.5 times higher than I said.

I've worked on a few Raypaks, and didn't like them. But that was years ago, and they were millivolt heaters which are twitchy as all get out, no matter who makes them. I know nothing about the current models.

With respect to solar, it doesn't work if your yard is shaded. And yes, circulation is a problem. The units need a small separate pump OR they need to be on a bypass. Unfortunately few pool guys and hardly any homeowners know how to set up a bypass without restricting overall flow.