In this paper, the uplink outage probability and energy efficiency of user terminals are investigated for heterogeneous networks (HetNets). In order to characterize the performance of users in small cells, the probability distribution function (PDF) of the distance between a user terminal and the access point (AP) in the typical small cell is derived in closed-form expressions. On this basis, the signal-to-interference ratio (SIR) of uplink terminals is analyzed to evaluate the performance with APs turning on in small cells. Simulation results show that user's density in small cells has a great impact on the uplink energy efficiency. These results provide some guidelines for developing new energy saving schemes in practical HetNet deployments.