Cellular networks can be operated more energy-efficiently if operators agree to share base-stations during off-peak hours. We apply a micro-economic analysis for a single-cell two-operator scenario to investigate the conditions under which self-interested operators would agree to share resources in this manner. Our analysis yields a comprehensive treatment of the existence and number of Nash Equilibria. We consider the cases when the payment rates are exogenous, as well as when they can be set strategically by the operators. Through numerical solutions we examine the quality of the best and worst Nash Equilibria in comparison with the globally optimized solution. Our results show that there is often a sensitive dependence on key parameters such as energy price, capacity, load, revenues, penalties and payments.