The energy consumption of software-controlled ICT systems ranging from mobile devices to data centers is increasingly gaining attention, however, energy optimization is still far from an established task in the software development process. Therefore, we have surveyed the available research on assessing the energy consumption of software systems, which showed a blatant lack of development tools, but several approaches exist for measuring the energy consumption. We group these approaches according to how measurement data is made available and compare several characteristics of the collected data. The survey shows that not only development tools for software energy optimization are still missing, but there is also a lack of fine-grained measurement approaches as well as approaches for general-purpose platforms.