Residential broadband gateways (comprising modem, router, and Wi-Fi access point), though individually consuming only 5-10 Watts of power, are significant contributors to overall network energy consumption due to large deployment numbers. Moreover, home gateways are typically always on, so as to provide continuous online presence to household devices for VoIP, smart metering, security surveillance, medical monitoring, etc. A natural solution for reducing the energy consumption of home gateways is to leverage the overlap of Wi-Fi networks common in urban environments and aggregate user traffic on to fewer gateways, thus putting the remaining to sleep. In this paper we propose, evaluate, and prototype an architecture that overcomes significant challenges in making this solution feasible at large-scale. We advocate a centralized approach, whereby a single authority co-ordinates the home gateways to maximize energy savings in a fair manner. Our solution can be implemented across heterogeneous ISPs, avoids client-side modifications (thus encompassing arbitrary user devices and operating systems), and permits explicit control of session migrations. We apply our solution to Wi-Fi traces collected in a building with 30 access points and 25,000 client connections, and evaluate via simulation the tradeoffs between energy savings, session disruptions, and fairness. We then prototype our system on commodity Wi-Fi access points, test it in a two-storey building emulating 6 residences, and demonstrate radio energy reduction of over 60 percent with little impact on user experience.