The nature of wireless sensor networks necessitates specific design requirements, of which energy efficiency is paramount. In this paper we present SEER (Simple Energy Efficient Routing), a novel routing protocol for wireless sensor networks intended to optimise network lifetime. SEER uses a flat network structure for scalability and source initiated communication, along with event-driven reporting to reduce the number of message transmissions. Computational efficiency is achieved by using a relatively simple method for routing path selection. Routing decisions are based on the distance to the base station as well as on remaining battery energy levels of nodes on the path towards the base station. SEER minimizes the number of messages that are sent through the network and thus reduces the overall energy consumption. Simulation results show that SEER achieves significant energy savings for a set of specific conditions.