Tests: For the tests I used the water heater vs a regular bottle, each had one cup of water in it
The starting temperature of the water was 20C(68F)
The outside temperature was 10.5C
After half an hour the regular bottle was at 18C(64.4F), and the heater was at 26C(78.8F)
I would have done a longer test but the sun went down.
*There was a large amount of moisture on the walls of the heater wich reduces the amount of sunlight that can heat it. I poured some salt into it which will hopefully dry it out.
Step 1: Supplies
-A can of spraypaint
-Two clear 2 liter bottles
-A rubber hose and a small plastic tube(or a slightly smaller rubber hose that fits properly into the glass and plastic bottle)
-A vacuum pump
-A piece of plastic or cardboard