Residential consumers contribute to a substantial amount of electricity consumption. To ensure grid stability, a Home Energy Management System (EMS) that assists household inhabitants in operating home devices to achieve optimized energy usage is needed. Previous designs of Home EMS often have some issues. First, it places high implementation overhead on device vendors to manufacture devices compatible with the system. Second, it lacks the ability to scale when new device types and external variables are introduced. This paper proposes an architecture model and a functional framework for Home EMS that are simple and scalable. The framework defines functions of Home EMS components. A policy-based reasoning mechanism is proposed as part of the framework for the optimization of home energy usage. In addition, the inter-working of Home EMS components is presented. It relies on a semantic web language for data representation to provide meaningful information for reasoning and harmonization with existing standards.