Energy efficiency of wireless cellular networks is a well-studied domain and many proposals have been done following different amelioration paths. In this paper, we analyze the efficient use of small cells in a renewable energy context and considering real-time price of grid energy. This is typically the case when the mobile network is powered by the Smart Grid, in addition to renewable energy when available. We study the use of an algorithm adapted for this context and analyze it in simulation. Our results show that a suitable number or density of small cells gives better cost and energy savings performance, if we take grid energy cost into account.