The concept of a Home Energy Management System (HEMS) as a power-saving technology for home appliances has received much attention. Most of current HEMSs based on the Smart Energy Profile (SEP), KNX, or ECHONET standards are implemented as closed systems within a Home Area Network (HAN); therefore, cost is a serious problem in the large-scale deployment of HEMSs. One possible way to reduce the overhead is to implement a HEMS as a cloud-based Application Service Provider (ASP) service. In such setup, a HEMS server is located in the cloud, and it monitors and controls home appliance devices over the Internet. We propose here a novel HEMS architecture using a cloud-based ASP service. We describe an overall architecture and some potential interactive Web communication technologies that realize a HEMS over the cloud-based ASP services. We also present a mathematical analysis and experimental results for estimating protocol overheads and server loads. These show that the WebSocket protocol is the most suitable protocol, allowing accommodation of twice as many households as other protocols. Finally, we describe our prototype implementation of the proposed architecture with the WebSocket protocol.