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The Building Energy Simulation Test for Existing Homes (BESTEST-EX) is a method for testing home energy audit software and associated calibration methods. BESTEST-EX is one of the tools supporting energy analysis software development, created by NREL on behalf of the U.S. Department of Energy (DOE).


Residential buildings in the United States accounted for about 22% of national annual primary energy consumption in 2008, according to DOE's Buildings Energy Data Book. Reducing the energy use of existing homes offers significant energy-saving opportunities, which can be identified through building simulation software tools that calculate optimal packages of retrofit measures. To support the larger effort of assessing and improving the accuracy of energy analysis for residential buildings, the NREL Buildings Research team developed BESTEST-EX, a method for testing the accuracy of building energy audit software and calibration procedures.


Building Physics Cases
Utility Bill Calibration Cases

BESTEST-EX is a test procedure that allows software developers to evaluate their audit tools' performance in modeling energy use and savings in existing homes when utility bills are available for model calibration. BESTEST-EX includes two types of test cases: building physics and utility bill calibration. Learn more about these test cases by selecting the appropriate button on the right.

Ideally, the BESTEST-EX calibration tests are run such that a program is tested against itself, as described in the report, Building Energy Simulation Test for Existing Homes (BESTEST-EX): Instructions for Implementing the Test Procedure, Calibration Test Reference Results, and Example Acceptance-Range Criteria, (see Appendix B, pp. 22-24 "Performing Calibration Tests Without Using Reference Programs"). Alternatively, BESTEST-EX allows comparison of the software being tested with state-of-the-art simulation tools such as EnergyPlus, SUNREL, and DOE2-1E; this comparison method is similar to energy analysis tests previously developed by NREL, such as HERS BESTEST and other BESTEST suites included in ANSI/ASHRAE Standard 140. The current Standard 140 test procedures are referenced by a number of certifying agencies and energy efficiency standards, including ANSI/ASHRAE Standard 90.1, the International Energy Conservation Code (IECC), the Internal Revenue Service for qualifying software used for calculating commercial building tax deductions, Residential Energy Services Network (RESNET), and others. When referencing Standard 140, these codes and certifying agencies provide certification requirements not included in the Standard 140 method of test.

To help formulate and vet the test procedures, NREL formed the Residential Software Accuracy Working Group. Participants include audit software developers, researchers, representatives of federal and state government agencies, and other stakeholders. Please contact the NREL research team if you would like more information about the Working Group.


Through development of BESTEST-EX, NREL researchers aim to increase the quality and accuracy of energy analysis tools for the building retrofit market. The BESTEST-EX methodology:

  • Tests software predictions of retrofit energy savings in existing homes
  • Provides a method of testing that other certifying agencies and energy efficiency standards can cite in their certification requirements — by itself the BESTEST-EX test method is not a certification test, and cannot be "passed" or "failed"
  • Quantifies impacts of uncertainties in input audit data and occupant behavior.

Intended Users

BESTEST-EX is targeted primarily to developers of energy audit tools who want to evaluate their software's ability to predict energy use and savings. In addition, retrofit building managers and utility program managers may find the BESTEST-EX methodology useful in comparing their building simulation tools to "state-of-the-art" analysis software.

Targeted Measures

BESTEST-EX currently addresses retrofit measures related to the building envelope such as air tightness, attic and wall insulation, windows, thermostat control, reflective (cool) roofs, and external shading. Additional retrofit measures are being considered for future versions of BESTEST-EX.

Development Status

The first generation of the software test suite is complete.

BESTEST-EX is planned to be applied within an ANSI/RESNET Standard Method of Test (SMOT) for the Evaluation of Building Energy Analysis Model Calibration Methods. When completed, the ANSI/RESNET SMOT will specify test procedures for evaluating calibration methods used in conjunction with predicting building energy use and savings, when applying existing building energy consumption data, and pre- and post-retrofit building description data.

As a follow-on project to BESTEST-EX, the NREL research team, in collaboration with the Residential Software Accuracy Working Group, has begun developing empirical data-based software accuracy tests. The test suite will apply existing building description and utility bill data from the Building America Field Data Repository, which has been translated into Home Performance Extensible Markup Language (HPXML) to facilitate data accessibility.


The following documents are available as Adobe Acrobat PDFs.

Overview and Methodology

Test Suite Documents

Additional Background Materials

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