I've gone through 3 solar water heaters that introduced me to this specific solar method, which is to use a glass vacuum tube to keep the heat in from a parabolic dish along its axis minimizing heat loss. I had been making tea with these as heaters even in winter. I got the larger one that I made the coffee with in 2018 good trade off of heating and portability. The company is "GoSun", and they have pretty good products in general, including a larger cooker and a hybrid of electric and solar.