As PCs became faster and more powerful, projecting energy consumption and cost using whole-building computer models has expanded. New building codes now allow use of such models to demonstrate the energy efficiency compliance of proposed building designs. Certification for LEED™ credits (, which may yield tax breaks and other benefits, also allows use of computer models to secure certification. Such models are also routinely used to quantify energy efficiency options under consideration in new or existing buildings. Once created, a computer model may be quite cost effective because only very small changes are needed to simulate each measure.

But what exactly is a computer model, and why is it any better than standard methods for calculating whole-building energy usage?

Simulating Energy Use and Cost

As buildings become complicated and their systems interact (e.g., lighting and HVAC) in complex ways, standard methods of calculating whole-building usage may introduce many simplifications, resulting in over or underestimated usage. (One of my first tasks in a new job in 1979 was to use a computer model to determine why a new building was using more than double the energy projected by its designers.) When purchasing energy for a new facility lacking any operating record upon which to base future consumption, such errors may become quite expensive.

When a building is a simple structure with only one or two types of systems (e.g., perimeter heating and window A/C units), manual or simplified computer calculation methods may yield acceptable results. But facilities utilizing variable-speed drives, temperature reset schedules, waste heat recovery, daylighting, etc., are best analyzed by a model that simulates simultaneous changes to building loads and operations on an hour-by-hour basis, while also adjusting equipment operations for such continuously changing loads. Doing so typically involves many thousands of calculations. A computer model is software which handles those tasks automatically and then presents its results in standard tables and charts.

How Do You Tell It What To Do?

Most computer models use extensive input forms in which data is entered describing characteristics of structures, HVAC (and other) systems, and a central plant to serve them. Data on energy pricing (e.g., from utility tariffs) may also be entered. Inside the model are complex calculation routines that simulate operation of mechanical and electrical systems using mathematical algorithms. The model then provides a variety of output reports that delineate loads, demands, and energy usage in zones and by mechanical/electrical systems.

One of the most commonly used models is DOE-2, developed in the '70s and updated numerous times. Many find its language and procedures somewhat cumbersome, however, and newer and simpler models (e.g., EnergyPlus) have been developed. A good descriptive listing of computer models (and other energy software) may be found at DOE's Building Energy Software Tools Directory at

Caveats With Computers

Producing a detailed computer model, especially for an existing building, may consume a significant amount of time and attention. If the goal is to determine the difference in incremental usage for single or multiple measures, the level of modeling detail may be less than needed to simulate all uses of energy in a building. On the other hand, using a computer model to project total building energy usage also involves simulation of operations that may be unrelated to space heating or cooling (e.g., exhaust fans in an unheated garage), and process-style loads that require detailed knowledge to quantify and schedule (e.g., sterilizers, distillation, incineration, sewage pumping).

Experience with modeling many buildings (both designed and existing) indicates that reconciling modeled results with actual usage can take almost as long as developing a base model, but the process often reveals a variety of savings opportunities that otherwise would not have been foreseen..

Computer models are also not cheap to produce. Depending on building size, complexity, and how the results will be portrayed, it may cost as much to model a building as it does to audit all of its energy systems (e.g., $.07 to $.15/ sq ft or more). It is therefore essential to specify the modeling task and its goals in some detail to avoid an expensive - and disappointing - result.

Before considering use of a computer model, a clear understanding of the limits and issues related to such simulations is essential. Jim Waltz (of Energy Resource Associates) has written a very good book on the subject titled Computerized Building Energy Simulation Handbook. It is available through ES