I am on a search for a new water trough heater & I would like a solar one if they exist. Has anyone ever heard of one? If not what brand electric heaters does everyone like?
Rubbermaid for in-tank heater that is installed at the drain plug works well.
You can build a box around your metal tank out of 2 x 4's and exterior plywood...big enough to easily fit around the tank and tall enough to put a top on it and not touch the tank/water. Paint your tank exterior flat black (stove paint works). Plywood three sides of the box....east/west/north sides. On southside put in plexiglass instead of plywood so the south side of the box will allow the sun to shine on the side of the tank. Insulate the dickens out of the other three sides and/or the tank (house insulation on inside of the box, spray on foam insulation on three sides of the tank). Put the box down over the tank (make it a couple inches taller than the tank is) and then make a top for the box, also out of 2 x 4 and plywood and foam insulate the inside of about 3/4 of it. Cut a drinking hole in the remaining uninsulated 1/4 of the top...you can split an old piece of hose and cover the edges of the hole so it isn't rough and horses won't be inclined to eat the wood. You can even use a float valve inside the box on the tank if you have a way to insulate your hose to it. The sun will warm the south side of the tank through the plexiglass/plastic and shining against the black side of the tank...the insulation and the top should keep it pretty clear of ice since there's only a small exposed surface area.
I don't like the ones that go in the drain plug. They invariably wind up leaking.
I like the submissible tank heaters. For some reason they can be hard to find at times though. I got a couple of floating heaters last time and more often than not they were OUT of the tank!! Busy ponies!!