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How Do I Make Energy Models of My Buildings? Part I

Fire up a simulation engine and see how your buildings perform

By Sara Fernández Cendón

As the green building movement evolves, it’s becoming more and more clear that the road between sustainable design intent and actual design performance is paved with data. The only way to consistently create buildings that consume less energy is to measure just how much energy each building creates or consumes. This demands energy modeling tools that can compare these rates to standard performance baselines. It’s a shift in green building practice that’s easily translated into talking points that extol the virtues of energy modeling without really explaining what that means or how to do it. It’s also a shift in practice that offers enormous power to architects by expanding the power of design to encompass the energy performance of every building they create.

Energy modeling is an issue that’s not helped by a fragmented (but occasionally overlapping) market of software options that require wildly different levels of technical expertise and data complexity for dramatically different audiences. But ultimately, the audience that matters most is the client, and energy modeling allows architects to make their case for high-performance, best-value design of buildings with untold detail and specificity.

What is energy modeling?

An important caveat for those in the energy modeling and building science community is that energy models do not predict actual building performance. Instead, building energy models are more analogous to the miles-per-gallon sticker prominently featured on every new car. A car’s estimated fuel economy, for example, isn’t an exact measurement of how much gas it will use per mile driven. The actual miles-per-gallon will vary depending on speed, air-conditioner use, and whether the car is driven in the city or on the highway, but the number is useful for car-shoppers because it allows for comparisons between models.

In the case of buildings, actual performance depends as much on how the building is used as it does on how it was designed. Poor performance might be the result of poor design, but it can also be the result of a mismatch between anticipated and actual use. If a building designed for nine-to-five occupancy is occupied around the clock, the building won’t perform as well as energy models might have suggested. For that reason, energy models are most useful as comparative tools that allow building designers to weigh options against one another; they provide a relative measure of what might be better or worse design alternatives.

While the value of energy modeling should be obvious to architects, designers often see energy performance as an issue to be resolved by engineers with individual building systems and technology—not as a holistic design issue. True, energy modeling is complex, and whole-building energy models are best created by modelers with specific expertise. But the modeling community agrees that the more architects understand about the connection between their choices and the energy profile of the resulting buildings, the better they’ll be able to design energy-efficient buildings. For that purpose, high-level comparative energy modeling performed during the conceptual design stages (Is option A or option B better for energy performance?) can give designers early feedback as they develop ideas. More sophisticated modeling carried out through the design process (and with heavier engineer involvement) can help them refine their choices to achieve the highest performance.

“To really use the power of the energy modeling tools, we need to do the modeling right at the beginning, during concept design,” says Lynn Bellenger, president of ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers). “Long before we decide what types of mechanical systems or what type of lighting will be used, we first want to look at the envelope, and how we orient the building, and the amount and type of glazing, to minimize how much energy that building will need for its entire life. Then come the rest of the decisions.”

Energy models run on engines

As evidenced by the DOE’s extensive Building Energy Software Tools Directory, the energy modeling tools currently available are many and varied. Uses range from modeling relative humidity to generating an overall home-efficiency score. To gain a broad sense of the available tools, one must first understand that there are two distinct components to any energy modeling software. First, there are the simulation engines—the “number crunchers.” Then there are the interfaces that connect users to the engines both through input and output mechanisms, which will be covered in an upcoming issue of AIArchitect.

Engines are the computation brains of energy modeling software. They run complex algorithms simulating real-world conditions that are the backbone of any energy modeling program’s key functions. Each engine type runs its calculations differently, meaning different energy modeling engines asked to calculate the same problem may give different results. When it comes to energy modeling tools, the engine requires a building’s architectural design and form to be broken down to its most basic, yet still representative geometry. User interfaces, then, make it easier to input a building’s geometry than manually entering coordinates to describe each unique feature of building’s form. This is similar to today’s 3D and 2D CAD tools that allow a building to be drawn in a much more visually understandable format than would ever be needed by an energy model’s computational engine. This allows for quick visual analysis by project teams, and faster data input into energy modeling software.

On the engine side, the most widely used program today is arguably DOE-2, which has been around for more than 30 years. DOE-2 is a whole-building energy analysis program that calculates energy performance and life-cycle cost of operation. Because it was developed with public funding by the DOE, the software is free and available for download. DOE-2 uses descriptions of the building layout, construction and operating schedules, mechanical systems, utility rates, and weather data to perform an hourly simulation of the building and to estimate utility bills.

A newer engine called EnergyPlus, also developed by DOE and also free to download, builds on DOE-2’s capabilities but is generally seen as more robust and versatile. As is true of most engines, working directly with DOE-2 and EnergyPlus allows for greater control and greater flexibility than working through user interfaces. User interfaces don’t allow as much flexibility, but they also don’t require as much technical expertise.

While some energy modelers work directly with engines or have built their own customized user interfaces, many work with market-ready energy modeling software that uses an engine and user interface. Today, one of the most commonly used whole-building energy modeling programs is eQUEST, a user interface for DOE-2.2.

Like its engine, eQUEST is free to download. Because it uses wizards (simple prompts that guide users through the process of entering information) for data input and because it generates graphical outputs, not just numbers, eQUEST is often considered to be among the more user-friendly energy modeling tools, in addition to being one of the most powerful. But because different energy modelers offer varying opinions on what tools work best, it’s always worthwhile to seek out expert opinions and do some research on your own. Most energy modelers agree that most of today’s energy modeling programs—like eQUEST, EnergyPlus, TRACE, or VisualDOE--can’t be used to their full capacity without a fairly sophisticated understanding of engineering.

And that’s why the AIA has convened an Energy Modeling Working Group (EMWG) that is currently preparing a practice guide that summarizes what an energy model is, how it works, and what skills an owner and architect might be looking for in hiring an energy modeling consultant or energy modeling staff. The guide will summarize the complexity of energy modeling and the various options available today in a way that’s useful both to sole practitioners and large-firm project managers and design directors.


*The listing of any company or product in this article should not be construed as an endorsement by the AIA. The AIA is not responsible for, and expressly disclaims all liability for, damages of any kind arising out of the use, reference to, reliance on, or performance of such listing or products. The AIA does not approve, sponsor, or endorse any product or material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

Recent Related:

With Energy Modeling, Virtual Models Lead to Real Sustainability

What is Next After Green

Understanding Differences in Environmentally Preferable Products

That Old Building May be the Greenest on the Block

Doer’s Profile: Jim Sealy, FAIA

Evidence-Based Design: The Deeper Meaning to Sustainability, Building Performance, and Everything Else


Visit the Committee on the Environment Web page on AIA KnowledgeNet.

Visit the Technology in Architectural Practice Web page on AIA KnowledgeNet.

Visit the AIA Sustainability Web page.

Visit the ICC’s International Green Construction Code Web page.

The DOE’s list of energy software tools is located here.

Do you know the Architect’s Knowledge Resource?

The AIA’s resource knowledge base can connect you to the AIA Best Practice Article “Energy Modeling and Daylighting Analysis.”

See what else the Architects Knowledge Resource has to offer for your practice.


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