Software testing plays an important role in building quality software and improving maintainability. However, there are no research studies to analyze its impact on energy efficiency. In this paper, we conduct a preliminary study on the impact of unit tests and code coverage on the energy consumption of software. Our empirical study analyzes the energy consumption of multiple JSON libraries and the relation of their energy efficiency to test metrics. Although our study has limitations in the size of the data set, we found that there are hints for a positive correlation between line coverage and energy consumption.