I. Introduction
ENERGY efficiency management in buildings has always been a focused topic all over the world. This can be mainly attributed to the large proportion of electric energy consumed by buildings. According to the U.S. Department of Energy, in 2010 the buildings consumed 74% of the countries' total electric energy (with 39% and 35% for residential and commercial buildings separately), and accounted for 82.2% (or $301.6 billion) of total U.S. electricity expenditures. And the upward trend would continue in the following twenty-five years by estimation [1]. In the United Kingdom, domestic energy consumption has reached 28.5% by the year 2009 [2]. Similar situation can also be observed in China: Residential buildings consumed 22–24% of the gross electric energy generation of the country in 2005 [3]. Meanwhile, efficiency of electric energy utilization is not so satisfactory and the potential for energy-saving is promising. However, it is necessary to first figure out how and where electric energy is consumed in buildings before taking energy-saving actions. The point of real-time energy monitoring is to learn current and historical power consumption, as well as to predict the future trend.