Memorandum to: Scott Dimetrosky, eeb evaluation Consultant From

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Gas Takeback Factor

The gas takeback factor is a commonly used to describe the amount of additional natural gas usage that will result from an efficient lighting retrofit. It describes the proportion of lighting savings that is negated by the increase in heating requirements. The gas takeback factor is essentially the same as the heating IE factor, except that it is unitless and applies only to gas homes. Because it equates electricity and gas, it is best viewed as a way to contextualize interactive effects rather than measure them. The gas takeback factor is calculated in the following manner:

As the above equation demonstrates, lighting savings are converted from kWh to ccf for the purposes of calculating this factor.

Table details the results of the gas takeback factor analysis. There is no factor for homes heated with something other than gas. Among gas-heated homes, the average gas takeback factor is 0.56, meaning that 56% of the lighting savings are negated by the increase in gas use that results from the retrofit. The factor ranges from 0.352 to 0.881 for individual homes.

Table : Gas Takeback Factor


Gas Takeback Factor

Number of homes








a Weighted with heating fuel proportional weight—

see Section .

Comparison of Results

Lighting interactive effects studies have been conducted in a number of other states in recent years, including New York, California, Minnesota, Maryland, Vermont, and a consortium of states in the Northwest, as well as a national study in Canada. Other jurisdictions have used a variety of methodologies for calculating IE factors. The majority, like this study, have used building energy simulation software, but at least one—the consortium of states in the Northwest—used a spreadsheet approach.

New York. The “New York Standard Approach for Estimating Savings from Energy Efficiency Programs” 15 notes that DOE-2 single-family prototype models were used to calculate interactive effects factors for seven regions of the state, within five HVAC configuration categories.

California. The California Energy Commission and California Public Utilities Commission (CPUC) relied on DOE-2 prototype models in developing IE factors. Documentation detailing the results of this modeling is accessible on the Database for Energy Efficiency Resources (DEER) website.16

Canada. The Canadian Centre for Housing Technology sponsored a 2005 study that made use of the Centre’s testing facility and HOT200017 energy modeling software to calculate interactive effects. The study simulated energy use for 11 cities in nine of the 13 Canadian provinces.

Minnesota. The Minnesota Technical Reference Manual notes that DOE-2/Equest building simulation was used to calculate interactive effects factors. The prototype models used to calculate Minnesota’s interactive effects factors were based on the California DEER prototypes and altered to take local building codes and construction practices into account.

Northwest states. The Regional Technical Forum (RTF) of the Northwest Power and Conservation Council used a spreadsheet approach to arrive at a single electric IE factor for all residential buildings. The spreadsheet is available for download from the RTF website.18

Maryland. A study conducted by Lisa Gartland of Opinion Dynamics Corp. in 2011 used an unspecified building energy modeling software to analyze interactive effects in retail and office buildings in Maryland. This report is not available publicly, but a draft version is referenced in a 2012 CPUC study which includes a literature review.19

Vermont. The Efficiency Vermont Technical Reference Manual, which delineates the IE factors in use in the state, makes no mention of the methods used to calculate them. The manual indicates that the residential electric IE factor is 1.0—Vermont does not adjust lighting savings in residential buildings to account for interactive effects. The factor of 1.03 given in Table is applicable to lighting upgrades in commercial buildings.

Electric Energy IE Factor Comparison

Table compares electric IE factors from other studies with the factors from this analysis. Average electric energy IE factors range from 1.03 to 1.22 among the other jurisdictions.

Table : Comparison of Electric Energy IE Factor Results


Average IE Factor

Connecticut overall


Excluding homes heated primarily by electricity


Excluding homes heated by electricity in any amount


New Yorka






Minnesotab, 22


Northwest States23


Maryland (commercial buildings)


Vermont (commercial buildings)24


a New York factor is for gas-heated sites with cooling equipment, which is the

category most directly comparable to the overall Connecticut sample.

b Minnesota factor is for single-family homes with known cooling configurations.
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