I agree to an extent. The report is 30ish years old. Who knows how energy prices now compare and I presume the testing was done in Oregon- that's the state that people rust, not tan. So the measurable sunlight may be less than your area. Also, today's heaters are more efficient so that's a plus.
But for a project to do with your Grandson, it may be worth it's weight in gold!
BTW- I googled homemade solar water heater to find that report
Never stop learning (xcep fer speling en typeing)