It is very difficult to answer the question "What is Energy-modeling" in as few words as possible. Here is my latest attempt that will soon be edited and published in, Architectural Drafting and Design, published by Cengage, 7th Edition.

Definition of Energy Modeling

Energy modeling, short for Building energy modeling (often abbreviated as BEM), is the computer simulation of a building used to determine or estimate building energy usage. A virtual building is created in a software package, the building components are entered, and the building is simulated over the duration of 1 year using a weather file.

The components entered into the simulation are numerous and thus most energy models allow for some simplification. For instance, the lighting may be simplified into watts per square foot, as opposed to determining the exact wattage for each and every room. Other example inputs include: fan horsepowers, pump horsepowers, heating and air conditioning type and efficiency, receptacle power, people, and outdoor air requirements. Of these inputs, the heating and air conditioning inputs often require the most knowledge because of the plethora of options available on the market.

The reasons to perform an energy-model are numerous, and go beyond the scope of this text. The most common purpose of energy-modeling is to make accurate energy saving decisions on a building. To do this, a user creates the initial building in the simulation software under existing or standard conditions and then re-simulates the building using various energy conservation measures (ECM). The life cycle analysis is often considered for each ECM or a combination of ECM’s. These simulations vary from preliminary to detailed. It is widely understood that energy-modeling has the greatest impact when it is utilized early in the design process, and therefore preliminary models have the greatest impact on energy savings!

While detailed energy-models contain vast amounts of data input and require a skilled practitioner, a user with limited knowledge of building physics can perform a basic energy simulation. In the example shown Figures 1 and 2, a building is simulated using eQUEST (a free and well-respected energy-simulation package) in Orlando, FL with and without shading devices. The simulation shows a 10% savings of in energy and 11% savings in energy cost. Energy Cost savings in dollars often exceeds the savings in net energy usage because the ECM may substantially lower the electrical demand or kilowatt draw, which often incurs additional cost on the utility bill.
 
Thus, energy-modeling can be used to determine the best energy savings option, the best cost saving options, and the best carbon reduction options. Often, the best energy saving option will yield the best carbon reduction and cost savings, but in many scenarios the user must work with the owner or building manager to decide whether to pursue cost savings, energy savings, or carbon saving.

Given the energy, carbon, and cost saving potential, energy-modeling is a rapidly growing profession and software packages are continuously being developed and compared to empirical data, leading to more accurate simulations, which further enhances the energy-model.