In this letter, we investigate a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided integrated sensing and communication (ISAC) system. Our object is to maximize the energy efficiency (EE) of the ISAC system by jointly optimizing the beamforming at the base station (BS), the transmitting/reflecting coefficient matrices of the STAR-RIS, as well as the deployment location of the STAR-RIS. Since the formulated problem is non-convex, we propose an alternative optimization scheme by decomposing the original problem into three sub-problems, where the sub-problems are solved based on semidefinite relaxation (SDR) and fractional programming. Simulation results demonstrate that the STAR-RIS aided ISAC system achieves a higher EE than conventional RIS scheme, and the location optimization for STAR-RIS can significantly improve the system EE. Furthermore, the EE declines rapidly when the radar beampattern gain threshold increases to a larger value.