Hi, All: My knowledge of physics is very rudimentary; please be patient. A friend of mine's father owns a small restaurant, asked me (as a math student) how he could save some money. All I could think of (not knowing much about either general economics nor restaurant business) was the use of electricity; specifically, the issue of turning lights/power on or off. AFAIK, it is the process of turning the power on that consumes most of the power, after which the power consumed (therefore the cost) decreases. So, if the power were to be repeatedly turned on and off, this would seem to be costly. Yet, turning it on just once and leaving it on for , say, 12 hrs at-a-time, may not be the best idea either. So I thought taking some data of the number of times the power is turned on , and compute the basic statistics; mean, sd, etc. Basically, there may be an optimal number of times to turn power on, given average use. Still: is there some physical law describing the use of power over time after a device is turned on, and how power use decreases over time, to be able to tell if, given the data, e.g., for the bathroom light, whether it is better to leave it on all day, or to turn it off , if the bathroom is used, say, 25 times a day on average? How about for heating/cooling? Thanks.