In the course of doing some energy improvements (and considering various options), I used the rule of the thumb that 1W continuous costs $2/yr, which was more t

In the course of doing some energy improvements (and considering various options), I used the rule of the thumb that 1W continuous costs $2/yr, which was more than close enough mathematically, surely made the math easier to do in my head, and was in the units of the actual tradeoff that I was making (buy an efficiency improvement for $Y, save $Z/yr).