sity and resource consumption of the energy technology itself. The literature is replete with assessments of life-cycle energy usage from renewable and non-renewable sources of electricity. However, these assessments adopt a wide range of energy metrics, making internal comparisons problematic. Spitzley and Keoleian (2005) describe eight distinct energy metrics defined in the literature.

Energy metrics should therefore be used with cautions and caveats. No single metric defines the ideal energy generation technology without an accompanying statement of the core value for assessment. For example, a metric such as capacity factor will effectively measure for intermittence or dispatchability. A metric such as price per unit of energy produced measures economic value according to conventional accounting, financing, and cost-accounting assumptions.

This review focuses on two of the more commonly used energy metrics: (1) net energy ratio (NER), which quantifies how much net energy a technology produces over its life cycle, and (2) energy payback time, which defines how long it takes for a given energy technology to recoup the lifetime energy invested in its development once the technology starts generating electricity. These metrics offer insight into the overall energy and environmental performance of generation technologies, especially in making macro-level resource acquisition and development decisions.

Net Energy Ratio

The NER is defined as the ratio of useful energy output to the grid to the fossil-fuel energy consumed during the lifetime of the technology. As such, it is critical to assessing whether or not a renewable energy source reduces our use of fossil fuel.

Renewable energy sources generally have an NER value greater than one. For fossil-fuel energy technologies, the NER is commonly referred to as the lifecycle efficiency. However, there is some inconsistency in the literature on how the NER is defined when the energy technology itself is based on a fossil fuel. In these cases, some researchers include only indirect (external) energy inputs and not the (primary) energy inherent in the fuel (Meier, 2002; White, 2006; Denholm and Kulcinski, 2003). However, this interpretation of the ratio is not an accurate reflection of the total resource consumption of the energy technology in question. For example, the energy consumed by combusting coal in a coal-fired plant is not included in this alternate use of the term. In cases where the primary energy of the fuel is not included in the energy inputs, the NER is more accurately defined as an



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