Energy is a significant bottleneck of smartphone operation. Today's smartphone batteries can normally support less than two day's continuous daily use. It is therefore important to find out where the energy goes inside a smartphone. In this paper, we present a hardware based method for Android smartphones. We conduct a comprehensive power evaluation under a predefined set of test cases, and identify a number of primary power-hungry modules, such as the screen display, GPS and WiFi modules. Finally, an energy model for these modules is established.