Estimating Energy Use Early and Often

Sept. 28, 2010

Information provided by the California Energy Commission's Public Interest Energy Research (PIER) program.

Information provided by the California Energy Commission's Public Interest Energy Research (PIER) program.

The Problem

Building designers can use computer software, such as DOE-2 and EnergyPlus, to evaluate potential energy-saving measures. However, preparing the input in a way that accurately represents the proposed building and its energy-efficient features is a costly, labor-intensive process. As a result, energy analysis is often postponed to a later point in the design process, resulting in a greatly reduced number of cost-effective options for boosting efficiency than if it had been considered from the start.

The Solution

The Green Building Studio (GBS) is a web-based service that enables building design teams to integrate whole-building energy analysis into the early stages of the design process. Architects and engineers use their existing building information modeling (BIM) systems to communicate the project's building geometry to the GBS website, which conducts an energy analysis of the building design.

GBS enables the design team to look at the energy impact of early decisions, compare alternatives and share information more broadly than in conventional design practices.

Building An Energy Model

The heart of the system is an open data format called Green Building XML (gbXML), a simple means of sharing data between BIM and energy-simulation software. Designers construct their early building-design model from their own BIM system, and then they use gbXML and some simple user inputs to export a building energy model. Before a simulation, GBS will error-check the submitted gbXML file to ensure that all the building spaces are properly defined. Most commonly used BIM systems are compatible with gbXML, which is based on Extensible Markup Language (XML). Compatible systems include Autodesk Revit Building and Revit Systems; Autodesk Architectural Desktop (ADT) 2004, 2005, and 2006; Autodesk Building Systems (ABS); and Graphisoft ArchiCAD 9 and 10. In addition, major HVAC manufacturers accept gbXML input directly into their energy- and load-analysis packages. GBS uses the internal load information — such as lighting, equipment, number of people, and space type — at the room level from Revit Systems, ArchiCAD, ADT, or ABS.

Energy Analysis

The gbXML export from BIM feeds an energy model that is automatically developed through the use of regional building standards and codes, which enable the GBS system to make intelligent assumptions about items such as insulation and lighting levels. The resulting building description then goes through a DOE-2.2 hourly simulation using typical-year weather data for the building's location. The simulation generates estimates for annual energy consumption, costs, and a wide range of data on heating and cooling loads.

Comparing Alternatives

Designers can use a new Design Alternative feature to modify their GBS runs to compare the energy impact of design options such as window types, glazing types and orientation, lighting efficiency and controls, HVAC systems, wall insulation levels, building orientation, multilevel versus single-level designs, and other building geometry considerations that come up early in the design process. GBS offers five free simulations for each project. After the five free runs are used, each additional simulation requires a nominal fee based on the number of spaces being analyzed. In addition, GBS allows manufacturers to place their products in GBS so that users can apply the product to their design to evaluate its potential energy savings.

Information Sharing

The GBS system also facilitates direct sharing of detailed project and building data with engineering firms, which can then run the information through detailed design software, use it to specify equipment, and share it with other parties such as bid preparers and manufacturers of building products.

Saving Time On Plan Takeoff

GBS also makes it easy to go to the next step by creating detailed input files for more comprehensive energy analysis by DOE-2 and EnergyPlus. The files are created in minutes — a process that could take one to two weeks using plan takeoff or engineering services. Using this capability can reduce the cost of preliminary and downstream building design by $5,000 to $10,000 or more.

The GBS system also features a Product Advisor that enables the service to continue to be offered relatively inexpensively. Each building project that goes through GBS provides the design team with relevant product information as early in the process as the planning and schematic design stages. Using a web-style bidding and paid placement approach as well as a building relevance filter, the system sends highly targeted building component advertisements to the building designer's desktop in the GBS Product Advisor screen. These advertisements are linked to building product libraries online, providing targeted advertising placement for product manufacturers and project-appropriate information for the design team.

GBS is best suited for use in new construction and major renovation of commercial or residential buildings of any size, including single-family homes. It is currently enabled only for facilities located in the United States.

Editors Note: PIER supports public-interest energy research and development that helps improve the quality of life in California by bringing environmentally safe, affordable and reliable energy services and products to the marketplace. For more information, go to and