This chapter outlines the data-protection issues involved in the handling of data, in particular data collected and processed within smart meters processes. Consequently, the proper handling and protection of personal data is an essential prerequisite for the realisation of smart cities. The process of decentralising energy systems can be an important factor facilitating the establishment of new forms of local cooperation based on the fulfilment of energy demand adapted to the needs of a specific community. Big data is created in smart cities because of the technology and applications used to collect and analyse personal information from inhabitants and residents, which is then shared across functional areas and used in the urban planning process. Data such as electricity consumption and reverse power flow values do not contain significant information on their own, as they are simply numerical values. Personal information, such as a name, is not limited to information that can be used to identify an individual.