Expected Unserved Energy (EUE)

Expected unserved energy, or EUE, is a reliability metric that measures the expected amount of energy demand that cannot be served over a given period because available resources or network capability are insufficient. It is typically expressed in megawatt-hours and is one of the most informative adequacy metrics because it accounts for the size of shortage events, not just their likelihood.

EUE is especially useful in cost-benefit analysis because it can be linked to an assumed value of lost load or value of unserved energy. This makes it possible to compare the cost of reliability investments against the expected severity of shortage risk in energy terms.

Key Aspects of EUE:

• Energy-Weighted Reliability Measure: EUE reflects not only whether shortages happen, but also how much load is curtailed and for how long. A few large shortage events therefore matter more in EUE than in event-count based metrics.
• Complement to Probability Metrics: While LOLP and related measures indicate the chance of shortage, EUE gives a stronger sense of expected shortage magnitude. Using both types of metrics together provides a more complete adequacy picture.
• Planning and Economic Use: EUE is often used in resource-planning studies to compare portfolios and justify investments in generation, transmission, storage, or demand response. It is particularly useful where economic optimization of reliability is part of the planning framework.
• Dependence on Adequacy Assumptions: Like other probabilistic metrics, EUE depends on assumptions about outages, load uncertainty, weather, renewable availability, and transmission capability. Poor assumptions can make the result look more precise than it really is.
• Interpretation for Policy: A lower EUE generally means a more reliable system, but what level is acceptable depends on policy, customer sensitivity, and the estimated cost of unserved energy. The target is therefore partly technical and partly socio-economic.