Future Smart Grids have to meet high expectations from our societies on improved energy efficiency and sustainability. However, system uptake and acceptance will to a large extent depend on the perceived trust by different stakeholders. In the paper we address issues related to engineering of trustworthy systems. This type of Engineering will be grounded in reliable models of interoperability. A key issue is here interoperability of information exchange and sharing. Based on analysis of trust concerns, we propose a model of coordinating and monitoring by Service Level Agreements (SLAs) between stakeholders to ensure trustworthiness of system performance and behavior. We illustrate our methods on case related to trustworthy inclusion of DERs in Smart Grids.