Base stations have been identified to be the most power consuming part in current mobile networks. Since their power profile only depends to a small fraction on the actual traffic load, putting some base stations in sleep mode has been identified as a solution to scale the network's power consumption with the actual load. However, the traditional cellular approach limits the achievable energy efficiency because the requirements on system availability impose continuous full coverage of the service area. This paper presents a new system architecture that overcomes this limitation based on the new paradigm of “cell on-demand”, which is currently studied in the Beyond Cellular Green Generation (BCG ² ) project of the GreenTouch consortium. We first outline the key characteristics of the new system architecture and the main technical challenges. Then, we present an analytical study for estimating the achievable energy-efficiency gains. We show that - depending on the daily load profile - we can improve current network energy efficiency by more than 50 times.