Building systems especially large buildings can be quite complicated. Energy modeling takes into account of interactions between building components and systems and simulates the operation and performance of the entire building. In the energy efficiency business,  energy simulation tools enable us to identify and evaluate alternative energy conservation measures (ECM) and help us choose the most cost effective measures to improve energy efficiency without compromising comfort and energy codes and regulations. We use well recognized simulation tools such as EnergyPlus and eQuest to build base model and use energy consumption data or utility bills to calibrate the base model.