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Under perfect conditions, how much do you expect to save with this blanket? It seems to me that these things floating on the water surface are just another thing that could get dirty and slimely if not cared for correctly.
Also, how much do you really save if, after having this wet sheet of plastic hang out in the winter air while you are in the tub, then you place the cold (maybe frozen) blanket on the water? Kinda like tossing a few ice cubes into your spa before closing it up? No? It's not goning to make a big difference, but the 2 or 3 pennies you saved, just went out the window when you put that cold blanket on the water.