Do you now what's happening here?
"The secret" refers to what must be done if you want your solar panels to really heat the water. They must be kept clean or the sun won't be efficiently and freely absorbed to heat the water or make/generate electricity.
I'm no expert in the real technicalities in it, but I do know that when we have sandstorms after the last rain, we won't get as much hot water all summer as the years when the last rain is a clean one after the sandstorms.
When our elder son lived with us, it was no "challenge" for him to climb up on the roof and wipe the dust and grime off of our solar panels. That time is long over. He does do plenty of chores when paying one of his rare visits, but I haven't asked him to clean the solar panels. Maybe that's because I'm so phobic about heights and he likes to frighten me by jumping off into thick bushes. One of his old jobs was that of a plumber's assistant. The plumber had dreams of getting him fully trained and qualified and making a business together, but my son wasn't interested. At least the skills he learned comes in handy at times. And yes plumbers must climb on roofs, on anything where there are pipes and water.
For those with kids, who aren't phobic about heights, looking for summer jobs here in Israel this summer, cleaning solar panels would be a good way of making money.
ps I took this picture at Sha'ar Binyamin Shopping Center/Industrial Park.