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
Step 2: Preparing Your Bottles
Take one of the pieces from the bottom and either cut or drill a hole in the bottom large enough to it over the mouth of your glass bottle. If you drill this, make sure you get it right the first time.
Put your bottle on a stick and spraypaint it black. Now wait for it to dry...
Put the bottom of your plastic bottle over the mouth of the glass bottle.