Radio Access Networks (RANs) are responsible for the majority of the energy consumption of cellular networks, and their energy consumption has been growing at an unsustainable rate. Virtualized RANs (vRANs) are an alternative to monolithic RANs, which can adapt to ever-evolving traffic patterns in an agile way. Additionally, virtualization allows multiple radio sites to share the same computing infrastructure, which can lower energy consumption. However, the transition to vRANs is a slow process and there are many open questions about how to best manage vRANs to meet the requirements of cellular operators. In particular, it is unclear whether vRANs can reduce energy consumption compared to monolithic RANs and under which conditions. Energy models are crucial to address this question and identify opportunities for reducing energy consumption in vRANs. In this paper, we present an overview of the energy-modelling approaches that can be applied to RANs and we review models of monolithic and virtualized RANs. Additionally, we characterize the energy consumption of RANs at the network, node, component, and functional levels. Lastly, we examine the techniques that can be used to improve the performance of vRANs and highlight the challenges, unanswered questions, and opportunities in this field, concerning energy consumption.