The electricity meter is a very sensitive device that measures consumption in units of 1000 watts per hour and is expressed in kilowatt hours. In modern installations, it will be located in a weather-proof housing just outside the boundary, and will more than likely have a peep hole so that the meter reader can take readings without having to open it. Older installations have been mounted on the same board that houses the contact breakers and/or fuses inside the building. This device makes it possible for the electricity supply utility to charge for the amount of electricity used over a period of time. The more electricity used, the more units indicated by the meter. The rate per unit will be multiplied by the total number of units for the period measured, giving a total value for the amount consumed.