Light is used to measure the proverbial speed limit of the universe, it also enables photosynthesis in plants. Light is also used in fiber-optic cables that enable some of the fastest internet...
Depends on what house it is, whether you use air conditioning and put up Christmas lights, there are just too many variables for that vague of a question.
"There are numerous uses for a mag light. The most common use is for when it is dark and you need a portable light to help you see. A mag light is exactly like a flash light so whatever a flash light...