The Sidwell Friends School InvenTeam invented the Gray Water Electricity Generator (GWEG), a new device designed to capture energy from the downward flow of gray and rainwater in apartment buildings. In these buildings, turbines will be installed at the bottom of gray water pipes to harness the kinetic and potential energy of the downward flowing water and create a pressure head that will turn a generator. Although GWEG seems expensive at a first glance, the long-term benefits of the invention will make it a sustainable source of green energy, and complement other technologies such as solar, wind, and hydropower. GWEG, demonstrated in a three-story prototype, is the model for future implementations in apartment buildings with 15 stories or more. Based on our calculations, such versions will return approximately 7 kWh of energy to the grid per day.