This paper presents an EE (energy efficiency) model and measuring approach for cloud computing. A mathematical expression of EE is first defined, as well as the measuring and calculation approaches, and the extreme conditions of EE are deduced. Then, to facilitate calculating EE, the mathematical expression between the computer power and the CPU working state is improved, thus EE can be calculated through CPU usage and CPU frequency, which simplifies the measurement of EE. In addition, the study designs and implements a series of experiments to verify the correctness of the proposed model. CPU-intensive, I/O intensive and interactive jobs are performed in both stand-alone environment and cloud environment to evaluate their EEs and to summarize their features as well as the optimization approaches. Both the theory and experiments have proved that the proposed EE model and measuring approach can accurately evaluate the EE of cloud systems and lay the foundation for EE optimization.