Energy Modeling

What is Energy Modeling?

Energy modeling, or simulation, is the practice of using computer-based programs to model the energy performance of an entire building or the systems within a building. This whole-building modeling provides valuable information about the building and system energy use as well as operating costs.

Whole-building simulation is typically performed for an entire year using typical meteorological year weather data. An important aspect of whole-building modeling is that it accounts for the interaction between different elements of the building, such as the impact of lighting on space conditioning loads or the impact of daylighting on electrical lighting loads. The impact of different building uses and occupancy patterns is also accounted for.The U.S. Green Building Council’s Leadership in Energy and Environmental Design Rating System (LEED™) requires energy modeling to assess the energy use of a building and to quantify the savings attributable to the proposed design. LEED requires energy modeling if any of the 10 points possible under Energy & Atmosphere Credit 1, for optimizing energy performance, are to be attained. In many cases, architects and building owners are inexperienced with energy modeling and don’t know how to harness this powerful tool to inform the design and decision-making process. Properly used, energy modeling can help optimize the building design and allow the design team to prioritize investment in the strategies that will have the greatest effect on the building’s energy use.

Why Use Energy Modeling?

Energy modeling can and should be used throughout the design process to optimize the building design for energy performance and reduced carbon emissions. Energy modeling has been applied to building design for decades, but with the advancement and proliferation of computers and the increased emphasis on building operating costs, the use of modeling has become more common and often required. It is a valuable tool to assist architects, engineers, and owners in assessing the impact of various design decisions. The size, complexity, and potential types of mechanical systems will affect which of the many modeling tools or software is most appropriate for the project, and the tool may change from one design phase to the next. It is beneficial to have someone in-house or as a consultant who is familiar with the alternative simulation tools and knows their capabilities in terms of such variables as the number of floors, the number of zones, the ability to define schedules, and the mechanical equipment options. The cost associated with energy modeling varies with the complexity of the building. A developer may find it difficult to absorb costs that will accrue savings to the future building owner in the form of reduced utility bills. However, this should be presented as a feature of the building and as a service that is worthy of a price premium. The building will also be more comfortable, and perform more reliably. In many instances, the cost of modeling can be recovered through the elimination of unnecessary factors of safety (frequently referred to as “Fudge Factors”), allowing credit for specification trade-offs, and/or qualifying for available incentives.

What Can Energy Modeling Do?

Reduce the overall energy use in your building.


The proper use of energy modeling throughout the building design process will inform the decision-making process on the energy use implications of various alternative design choices. This will result in a building that has been optimized to reduce energy loads and thus energy requirements.

Specify energy-efficient equipment and technologies.
Energy modeling provides the opportunity to evaluate the annual impact on energy use and operating costs of different types of equipment and systems with different efficiencies.
Use renewable strategies and purchase green power.
Many energy-modeling tools can simulate the performance of renewable energy strategies, such as waste heat recovery, solar thermal systems, and solar electric systems.
Using modeling to optimize the energy performance of the building will reduce the expense of purchasing green power, which is typically more per kilowatt-hour than the standard electricity price.
Using modeling can help reduce peak time energy demand and thus reduce peak power demand pressure on utilities and resulting carbon emissions.

When Should Energy Modeling Be Performed?

Energy modeling may be performed at any of the following times:

Conceptual Design
During conceptual design, a very simplified model that uses basic assumptions for many of the inputs can be used to examine large-scale impacts such as building configuration and orientation.
Schematic Design
At schematic design, a more detailed energy model is useful for identifying the primary energy uses. Understanding which loads (space heating, cooling, lighting, water heating, etc.) are dominant provides guidance on what aspects of the building design should be targeted for energy savings. The greater the energy use, the greater the opportunity for cost-effective energy savings. For instance, the designers could test alternative percentages of façade glass area on the principal orientations for energy and aesthetic implications.
Design Development
During design development, a detailed energy model should be used to conduct parametric analyses to evaluate alternative specifications and more fully understand the trade-offs between initial cost and life-cycle cost. The accuracy of the model is important at this stage; therefore, providing as much information as possible on the expected use and occupancy schedules of the building is important. The phrase “garbage in, garbage out” is particularly relevant to modeling at this stage. It is important to keep the modeler informed of all design and specification changes, and all model inputs and assumptions should be carefully reviewed. For example, using a default 8-to-5 Monday–Friday operating schedule for an office facility that operates 24/7 would significantly underestimate the building’s electricity use for lighting and equipment. Using the wrong glazing specifications could significantly affect the heating and cooling load predictions.
Construction Documents
At this phase, the energy modeling will be necessary to document compliance with codes such as the Energy Cost Budget method in the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 90.1 or the Total Building Performance section of the IECC. In many instances, energy modeling is also necessary to demonstrate qualification for specific certification programs, such as Energy Star and the U.S. Green Building Council’s Leadership in Energy and Environmental Design or incentives such as the federal tax credits.

Energy Modeling & Building Information Modeling:

There has been much discussion in recent years of integrating Building Information Modeling (BIM) and Energy Modeling. Most energy modeling is currently done independent of BIM because there are still significant limitations in BIM energy modeling capabilities. However, software developers are working to make it easier to derive the energy model from BIM.

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