If you leave a 60 watt light on for 24 hours, and if your electricity costs about 15 cents per kilowatt/hour, then that light will cost less than 25 cents. If you accidentally leave it on for a whole month, the cost will still only be under $8.00. But there is a hidden cost. The power to run that light comes from a generating station that is most likely either a fossil fuel plant that pollutes the air, or is nuclear, which is risky. If you and thousands of other people turned off unnecessary lights, we may be able to use fewer nuclear or fossil-fuel generating stations.