This kettle will heat 16 oz of water in full sun to a nice hot/warm temperature for some sun tea or washing hands, but you'll have to wait a few hours or less depending on outside temperatures. I've only recently made this during the winter and have achieved 95 degree water temps during 30 degree outside temps. Someone in a warm area of the world will have to make one and tell me how hot it can get during summer-like temperatures.
(Instructables member Kopomeroy built a few versions of this; they achieve 140-160 F during 80 F outside temps. See comments below.)
This takes three plastic bottles to build, one 2-liter, one 16 oz, and one 20 oz. Also needed are flat black spray paint, 5/8" (16mm) rubber heater hose, aluminum foil tape. The outer 2-liter bottle should preferably be clear.
After making this I realized a major problem when using a plastic bottle core; the breakdown of the plastic from heat and UV rays which could leach plastic chemicals into the water. This hot water might be useful just to wash your face or something but not to drink. An alternative to this design is to use a glass or aluminum bottle for the center bottle.
Step 1: Clean, Prep, and Paint
Peal off all the labels.
Give the 16-oz bottle a light spray with the flat black so it drys fast, a second coat may or may not be necessary.
Step 2: Cut Insulating Bottles
Step 3: Install Connection Hose
Step 4: Add Insulation
*Add about 2 tablespoons of dry rice inside as a the 2-liter before assembling. This will increase efficiency of the kettle by absorbing the reflective condensation that can form on the walls of the bottle.
Step 5: Add Reflector Foil Tape
Step 6: Performance Notes
I tried the plastic bottle version, the glass bottle version, and just a plastic bottle painted black. I laid them all out in the afternoon winter sun (outside temps were in the lower 30s Fahrenheit with full blue clear sky) and checked them after a two hours. I don't have a good digital thermometer currently, just a kitchen candy thermometer which is slow and not precise.
The glass and plastic kettles were about the same temperature, although the water from the glass kettle felt slightly warmer than from the plastic one. They measured about 95 degrees F if the candy thermometer worked right.
The black plastic bottle was just cold, just barely warmer than outside air temps.