Building HVAC systems can be used to provide ancillary services to the electric power grid. A recent experimental study found that buildings consume more energy when providing ancillary services than during normal operation, even when the ancillary services signal is energy neutral. In modeling the building as a virtual battery, the study found that the round trip efficiency was only 0.5. The objective of this project is to study if the inefficiency can be explained by thermodynamics alone, or if the building control system has some impact on the efficiency results. To do this will require modeling, experimentation, and data analysis on real buildings. Assuming that the efficiency is impacted by the control design, we will develop controllers to improve efficiency. The results will help us benchmark buildings against other energy storage technologies, and eventually improve the environmental impact of power systems.