This paper discusses the development of modeling approach for reactive energy management. The objective of reactive energy management is to adopt the long-term anticipated performance to actual situation, preserving the occupant's comfort and energy expenses. It performs continuous performance monitoring of whole building, taking into account unplanned events and system failures. Reactive energy management mechanism uses fast dynamics models with short-time simulation, able to manage the real-time observations from sensor data. This work emphasizes the development of low-order resistance-capacitance (R-C) thermal model with their equivalent state-space representation. At the end, a case study demonstrates the significance of fast dynamics reactive models to detect the discrepancies from reality. (C) 2015 Published by Elsevier Ltd.