We have a hot tub that is hooked up 240V. We leave it off most of the time but when we want to use it, my wife is constantly leaving the cover open so that she can check the temperature because she doesn't like having to lift it to stick her hand in (she doesn't "trust" the thermostat setting). Even with the full 6500W of heating, there is a significant difference in the heating time if the cover is open. I keep telling her that by leaving the cover open, it's taking a lot longer to heat up and thus costing us a lot more (she complains about the cost too), but she can't wrap her head around thermal transfer rates etc. I've used the argument about how a covered pot boils faster, but it still doesn't register with her. We have similar arguments about heating the house, I have learned to just fight other battles that are more winnable...