Energy Savings Lifetimes and Persistence: Practices, Issues and Data

This technical brief explains the concepts of energy savings lifetimes and savings persistence and discusses how program administrators use these factors to calculate savings for efficiency measures, programs and portfolios. Savings lifetime is the length of time that one or more energy efficiency measures or activities save energy, and savings persistence is the change in savings throughout the functional life of a given efficiency measure or activity. Savings lifetimes are essential for assessing the lifecycle benefits and cost effectiveness of efficiency activities and for forecasting loads in resource planning. The brief also provides estimates of savings lifetimes derived from a national collection of costs and savings for electric efficiency programs and portfolios.

Lifetimes and persistence of energy savings are overlapping topics. However, the energy efficiency industry primarily has focused more on quantifying the lifetimes of savings and less on estimating savings persistence (or degradation) over the savings lifetime. The two issues are interrelated in practice because, where compelling data exist, savings persistence often is integrated into estimates of the lifetimes of energy savings for a given efficiency activity.

In the first section of the brief, we present information on common practices and issues associated with savings lifetimes as well as typical values for lifetimes for programs, market sectors and portfolios. We then cover similar topics about persistence. We also discuss opportunities for standardization and identify areas for further research. New research may be needed for certain measures before reaching conclusions about the significance of persistence in estimating lifetime efficiency benefits, including avoided generating capacity costs, delivery infrastructure costs, fuel use and emissions.