Residential energy consumption is largely influenced by thermostat configuration and user interaction. Smart thermostats differ from other devices, offering new features and control. Little comparison or analysis of state-of-the-art systems is available, contributing to a gap in the literature. This thesis examines usability and user understanding related to smart thermostats, emphasizing comparison with literature. Two interview-based studies employing human-computer interaction methods are presented. Chapter two investigates usability, reporting quantitative metrics and user feedback. Smart thermostats demonstrated higher usability compared to programmable devices, while enabling the same or more functionality. Chapter three investigates users' understanding related to smart thermostat operation. Users demonstrated a relatively accurate conceptualization of their system and most employed temperature setbacks, suggesting that smart thermostats effectively communicate the function of the device and enable energy saving behaviour. Concrete objectives for future work are presented. This research brings the smart thermostat literature up-to-date, relative to manual and programmable thermostats.