Boiling water

According to the web, and informally confirmed by my personal observation: an uncovered pot of water takes 50% longer to boil than a covered one. In other words, if it takes $1.00 of energy* to boil a covered pot of water, you'll pay another fifty cents for the luxury of leaving the cover off (in addition to having to wait 50 percent longer).

And this should be obvious, but if you boil twice as much water as you need, such as two cups of water for one cup of tea, then you're paying twice the going rate for boiling a cup of water for absolutely no reason at all.

*Today it costs less than that. I'm using a round dollar for the sake of simplicity.