BESTEST-EX: Building Energy Simulation Test for Existing Homes

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 account for about 21% of national total energy consumption. 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.


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.

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 included audit software developers, researchers, representatives of federal and state government agencies, and other stakeholders.


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 requirement—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 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.


The following documents are available as PDFs.

Overview and Methodology

Overview of BESTEST-EX

Building Energy Simulation Test for Existing Homes (BESTEST-EX) Methodology

Technical Highlight: NREL Improves Building Energy Simulation Programs Through Diagnostic Testing

Test Suite Documents

Building Energy Simulation Test for Existing Homes (BESTEST-EX): Instructions for Implementing the Test Procedure, Calibration Test Reference Results, and Example Acceptance-Range Criteria

Building Energy Simulation Test for Existing Homes (BESTEST-EX)

Example Procedures for Developing Acceptance Range Criteria for BESTEST-EX

Additional Background Materials

What Did They Do in IEA 34/43? Or How To Diagnose and Repair Bugs in 500,000 Lines of Code

Model Validation and Testing: The Methodological Foundation of ASHRAE Standard 140

A Comparative Validation Based Certification Test for Home Rating System Software

Assessing and Improving the Accuracy of Energy Analysis for Residential Buildings

ANSI/ASHRAE Standard 140

Deep In Data: Empirical Data Based Software Accuracy Testing Using the Building America Field Data Repository

Assessment of the U.S. Department of Energy's Home Energy Scoring Tool