Persistence in the Evaluation World

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Max Ma, ERS, for Zondits

In the utility M&V and evaluation world, persistence is generally defined as the change in expected savings relative to the baseline.1 Persistence is the little brother to the end-of–useful-life (EUL) metric. Frequently, persistence factors are built into the deemed values of EULs in quantifying lifetime savings.  Savings persistence accounts for equipment degradation as well as changes to facilities and operation. Recent field observations shed light on some examples and remind us of the evolving nature of persistence for traditional and nontraditional measures alike.

The persistence of many traditional measures, such as lighting and building insulation, is often neglected in M&V and evaluation efforts because it is deemed to have insignificant impact on savings values. For example, a Proctor Engineering study showed the technical degradation factor of 1.00 for both fluorescent T-8 lamps and building insulation throughout their EUL,2 meaning that the energy-efficient and baseline options would degrade at about the same rate and therefore no adjustments are necessary. However, the large-scale installation of LED lamps in

In the utility M&V and evaluation world, persistence is generally defined as the change in expected savings relative to the baseline.
recent efficiency projects has challenged that assumption. When installed and used as intended, LEDs maintain their light output more consistently than fluorescent lamps, leading to delamping when occupants realize that they did not need a one-for-one replacement after all. That results in a persistence factor greater than unity. On the other hand, some LEDs are inappropriately installed, if they are retrofitted to existing failing ballasts or if reflectors are not designed for narrow-angle lamps, for example. Some fixtures may then be reverted to the preexisting lighting technologies over the occupants’ complaints about flicker and glare, resulting in a persistence factor lower than unity.

The persistence of emerging efficiency measures is a more diverse topic still. Many efficiency opportunities recently added to utility program incentives, such as battery storage, ice storage, phase change materials, and demand management, take advantage of and are dependent on smart controls, which optimize the efficiency of the installed measures. The typical EUL quantification of these measures may reference the expected life of the controlled hardware, such as chillers, ice tanks, and other HVAC equipment. Unfortunately, the smart controls tend to have greater variability and uncertainty in persistence than their controlled equipment, due to any of the following:

  • Control strategies may change when there is a change in building ownership, management company, or controls contractor simply because the rationale for the optimized controls is not communicated to the new operator.
  • Operators, even within the same facility, may have constantly shifting goals even after the program M&V is complete for a newly installed measure. Some seek to minimize energy costs, some seek to maximize reliability, while some seek to minimize the maintenance workload.
  • Some controls deviate from the optimal after physical changes to the controlled equipment due to schedule, process, or load changes. In extreme cases, the original equipment manufacturer is no longer in business and no modifications can be made with the controls settings despite clear recommissioning opportunities.
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These in-field observations add to our statistical understanding of savings persistence and inform us of the need to perhaps allocate some resources to focused persistence studies on upcoming new measures, even when they may appear to be familiar at first glance. For certain measures, slight modifications in the M&V or evaluation stages to take into account savings persistence may prove cost-effective and informative to program administrators and utility customers alike.

  1. ERS, “Measure Life Study Prepared for Massachusetts Joint Utilities,” 2005
  2. Proctor Engineering, “Summary Report of Persistent Studies: Assessments of Technical Degradation Factors, Final Report,” 1999