Since the power consumption of different electrical appliances in a household can be recorded by individual smart meters, it becomes possible to start considering in more details the interactions of the residents with those devices throughout the day. Appliances usages should not be considered as independent events, but rather as enablers for activities. In this work, we propose an automated method for determining when an electrical device is triggered solely from its power trace. Knowing when an appliance is powered on is required for identifying recurrent patterns that could later be understood as activities. Leveraging activity knowledge over time will allow us to design personalized energy efficient measures. We envision the design of future ambient intelligence systems, where the smart home can optimize the energy consumption in regards to the lifestyles of its residents.