Resource Adequacy Basics

An important aspect of grid reliability is resource adequacy—a power system's ability to supply enough electricity, at the right locations, to keep the lights on year-round during all hours. 

This means system planners must ensure the mix of resources can meet demand during hot summer afternoons and cold winter nights.

Measuring Resource Adequacy

Resource adequacy is measured by the probability of an outage due to insufficient capacity. It is measured at the system level to capture the overall impact of outages of individual components including generators and transmission.

Several metrics are used for resource adequacy. For example, a resource adequacy standard might be less than 1 day in 10 years of outages caused by a lack of generation. Once the target or metric is established, power system planners perform grid simulations of many possible power plant outages under different system conditions to ensure the system can achieve the resource adequacy standard.

Planning Resource Adequacy With a Changing Grid Mix

Renewable energy can help maintain or enhance the resource adequacy of the U.S. power grid. A critical factor in maintaining resource adequacy under a changing grid mix is accurately assessing renewable energy potential and future demand for electricity, particularly when there will likely be stress on the power system, like a hot summer afternoon.

How much capacity a generator can reliably contribute to resource adequacy during periods of high system stress is known as its capacity credit. The capacity credit of solar photovoltaics and wind have traditionally been based on their historical performance during high-risk or high-stress periods, but that approach is not exact enough for planning.

More recently, utilities and system planners have started transitioning to what's called probabilistic reliability-based methods, which use thousands of detailed computer simulations of different conditions to precisely quantify a resource's contribution to resource adequacy.

Increasingly, resource adequacy also accounts for the role of storage, changes in demand patterns, and impact of transmission outages and interregional coordination—which is important to deliver generation from many resources to load sites and enable access to a greater diversity of variable renewable resources and load across neighboring regions.

Resource Adequacy and Extreme Weather Events

Extreme weather events pose significant uncertainty to planning resource adequacy. These events can increase demand on the grid such as extremely hot days when lots of air conditioners are running. The changing duration, magnitude, and frequency of extreme weather events make planning challenging, but NREL is coming up with new ways to incorporate weather data into power sector modeling. Several organizations also monitor resource adequacy and the potential impact of extreme weather events, including the North American Electric Reliability Corporation that publishes seasonal assessments of resource adequacy projections.

Planning for extreme events on the power system also involves resilience—another key aspect of grid reliability.

Additional Resources

Causes of Three Recent Major Blackouts and What Is Being Done in Response, NREL Fact Sheet (2024)

Maintaining a Reliable Future Grid With More Wind and Solar, NREL Fact Sheet (2024)

An Introduction to Grid Services: Concepts, Technical Requirements, and Provision from Wind, NREL Technical Report (2021)

The Evolving Role of Extreme Weather Events in the U.S. Power System With High Levels of Variable Renewable Energy, NREL Technical Report (2021)