Study of ENERGY STAR Labeled Homes in Houston Finds Energy Use Projections from HERS Accurate

August 22nd, 2012 - Posted by RESNET under RESNET News

Over the years, there have been discussions over how accurate are home energy ratings in predicting the energy use of rated homes.  To enhance the discussion of the accuracy of home energy ratings’ energy use projections it would be good to review a study conducted and published by Advanced Energy on a large set of homes in Houston, Texas.  The authors of the study were Michael Blasnik of M. Blasnik & Associates and Shaun Hassel and Benjamin Hannas of Advanced Energy.

The objective of the U.S. Environmental Protection Agency supported “Houston Energy Efficiency Study” was to assess the actual energy use of groups of homes built to different energy efficiency specifications in Metropolitan Houston – typical non-program (baseline) homes, ENERGY STAR® homes labeled by a Home Energy Rating and guaranteed performance homes. More than 226,000 homes built from 2002 through 2007 by dozens of different production builders were included in this study. The large dataset also provided the opportunity to analyze how certain construction characteristics are related to actual energy usage.

Data collected for this project included billing data for all new homes built in the CenterPoint utility service territory from 2002 through 2007, information from property assessor databases of four counties, detailed building characteristics for tens of thousands of ENERGY STAR homes from CenterPoint’s ENERGY STAR Homes tracking database, and detailed data files from energy raters including the home energy rating software tool, REM/Rate, input files and building shell and duct leakage test data. The study did not involve any direct data collection in the field but instead relied upon existing data sources. This approach allowed the scope of the study to be much larger in terms of the number of homes analyzed but left some gaps in our understanding of some details, especially of baseline homes. The overall dataset includes hundreds of variables for 226,873 homes, including 114,035 potential baseline homes, 106,197 ENERGY STAR homes and 6,641 guaranteed performance homes.

Although consumption differences across groups of homes are smaller than advertised, ENERGY STAR homes perform very close to the predictions of the models on average, while baseline homes perform better than the reference homes defined by the HERS standard. ENERGY STAR uses a base case reference home defined as minimum local code specifications combined with the least efficient cooling, heating and hot water systems available, a leaky building envelope and a poor duct system. Using this yardstick to measure the performance of the ENERGY STAR houses in the study, they did quite well – showing a strong and fairly consistent relationship between actual and projected performance for both heating and cooling. Therefore the apparent lack of savings is attributable not to underperformance by the ENERGY STAR homes but to the fact that the baseline houses in Houston perform considerably better than the ENERGY STAR reference house.

The relationship between REM/Rate cooling load projections and actual electric usage was examined graphically and statistically for 10,258 homes with sufficient data. REM/Rate projected an average cooling load of 5,506 kWh/yr while the billing analysis estimated average cooling loads at 5,677 kWh/yr, about 3 percent higher – excellent overall agreement. Although the analysis found no systematic bias in the REM/rate cooling projections, there was a large amount of variability in the data. Findings revealed that the correlation was higher between house size and cooling load than between REM/Rate projected cooling load and actual usage. However, the study team feels confident in stating that when using current modeling software with energy-efficient new homes, there is a strong and fairly consistent relationship between actual and projected performance using REM/Rate for both heating and cooling. REM/Rate also estimated the average heating usage of program homes fairly well – only 4 percent lower than the measured loads.

To download the study click on Houston Energy Efficiency Study

  • Energy Saving Switch

    It’s interesting that the numbers match up.

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