In 2015, compared to investor-owned utilities and rural electric cooperatives, municipal utility customers "experienced the lowest instances of power outages in both frequency and duration, averaging one outage and about two hours of interrupted service," the Energy Information Administration said Sept. 12 in the EIA's Today in Energy publication.
"Our data backs the results of this EIA analysis," said Alex Hofmann, director of energy and environmental services for the American Public Power Association. "Whether they are classified as rural or urban, municipal utilities prioritize service restoration and reliability."
Interruptions in electricity service "vary by frequency and duration throughout the country across the many electric distribution systems that serve roughly 145 million customers in the United States," the EIA noted in the Sept. 12 report, EIA Data Show Average Frequency and Duration of Electric Power Outages. While some customers have backup generators, most are simply without electricity when outages occur, the agency noted.
The reliability of electricity supply systems "is typically measured by considering the duration, frequency, and scale of service interruptions," and many of the standards in reporting these metrics were developed by the Institute of Electrical and Electronics Engineers, the EIA said. Most utilities use the IEEE standards when they report outage data, but some have developed other approaches, said the agency, which is part of the Department of Energy.
For utilities following the IEEE standard, aggregate yearly outage frequency and duration indices (SAIFI and SAIDI, respectively) are reported to the EIA and they include any interruption that lasts longer than five minutes.
When reporting to the EIA, utilities designate whether their reported indices exclude major events, which can represent outages caused by significant snowstorms, hurricanes, floods, or heat waves. Not all utilities are consistent in what they consider major events, though most use the IEEE definition, the EIA said. For instance, a significant snow storm "may be considered a major event by a utility in Virginia, but not by a utility in Maine," the agency said. "Major events are relatively rare, but they can result in longer outages."
With major events factored in, investor-owned utilities' customers "averaged slightly more than three hours without electric service, while co-op customers averaged nearly five hours without power," the EIA said. Co-op customers, on average, "experienced about twice as many outages as customers of investor-owned and municipal utilities."
The report noted that factors such as weather, population density, and tree density affect utilities' ability to maintain service.
"For example, co-ops are generally suppliers to rural homes with more powerline miles and trees per customer, increasing the likelihood that distribution lines will be affected by storms," the EIA said. "Municipalities are more likely to serve customers living in higher-density urban areas, which have fewer powerline miles per customer and, in some locations, underground distribution lines."