Wireless Sensor Networks (WSNs) consists of large number of multifunctional sensor nodes. Routing protocols developed for other ad-hoc networks cannot be applied directly in WSN because of the energy constraint of the sensor nodes. Sensor nodes are battery powered and deployed in harsh environments so it is not always possible to recharge or replace the batteries. So routing protocols developed for wireless sensor networks must be energy efficient so that the network lifetime can be prolonged. In this paper, a survey of cluster based energy efficient routing protocols has been done indicating their merits and demerits.