Security in Wireless Sensor Networks (WSNs) is an omnipresent topic. In many application scenarios, like the surveillance of critical areas or infrastructures, security mecha- nisms have to be used to build reliable and secure applications. Up to now, most of the used cryptographic algorithms have been implemented in software despite the resource constraints in terms of processing power, memory and energy. In the past few years, the usage of special hardware accelerated security modules has been proposed as a viable alternative to software implementations. However, the energy-efficiency has not yet been evaluated in-depth. In this paper, we analyze the VaultIC420 security module and present an evaluation of its energy-efficiency. We compare the performance and energy-efficiency of the hardware module to common software implementations like TinyECC. For the energy measurements, we use IRIS sensor nodes in the SANDbed testbed at the Karlsruhe Institute of Technology. Our evaluation shows, that the VaultIC420 can save up to 76% of energy using different MAC layer protocols. It also shows, that the current draw of the VaultIC420 requires a duty-cycling mechanisms to achieve any savings compared to the software implementation.