To gauge the reliability of electric service, SNL Energy analyzed 991 utilities that filed reliability metrics with the U.S. Energy Information Administration in 2015. The weighted-average outage duration without major event days, or MED, of service disruptions for the electric utility sector increased by 7.24% and outage frequency increased by 2.43% compared to 2014.

Including MED, the weighted-average outage duration decreased by 3.47% though outage frequency increased by 1.56% year over year.

The metrics comprise two measurements: the system average interruption duration index, or SAIDI; and the system average interruption frequency index, or SAIFI. SAIDI is calculated as the sum of all customers interrupted multiplied by the number of minutes they experienced an interruption, divided by the total number of customers served by the utility. SAIFI is calculated as the sum of the total number of customers interrupted over the year divided by the total number of customers served. Each of these metrics is reported to the EIA, both including and excluding MED. Figures analyzed are from 2015, the latest year for which information is available.

The weighted-average SAIDI without MED was 105 minutes for the 70 utility segments reporting a 2015 customer count of more than 500,000. Out of these segments, 53% were at or below the average.

Among these 70 utility segments, East Kentucky Power Cooperative Inc. reported the lowest SAIDI without MED at 20 minutes, followed by Consolidated Edison Inc. subsidiary Consolidated Edison Co. of New York Inc., at 21 minutes.

American Electric Power Co. Inc. subsidiary Appalachian Power Co. reported the highest SAIDI without MED, at 312 minutes for its operating segment in Virginia. In late 2016, according to a press release, the company requested approval from the Virginia State Corporation Commission for an enhanced vegetation management program "to reduce the number of tree- and other vegetation-caused power outages" in the region.

Looking at states with utilities reporting more than 500,000 customers in 2015, state-level averages for SAIDI and SAIFI without MED showed a correlation between the two metrics. Six out of the 10 states with the lowest SAIDI without MED also had the lowest SAIFI without MED. Along those lines, six out of the 10 states with the highest SAIDI without MED also had the highest SAIFI without MED.

Some states, however, only have one utility with more than 500,000 customers, so correlations between the two reliability metrics don't always exist. Maine, for example, was the state with the lowest SAIDI without MED, but was the third-highest state in terms of SAIFI without MED. Central Maine Power Co., an Avangrid Inc. subsidiary, was the only utility in Maine with more than 500,000 reported customers in 2015. Aside from the company's annual $20 million vegetation management program, it also performed aerial inspections of its transmission lines in the spring and fall of 2015 to ensure reliable delivery of power throughout its service area.

Nevada appeared to be the most reliable state in 2015, as it reported the second-lowest SAIDI without MED, at 32 minutes, and the lowest SAIFI without MED, at 0.37. NV Energy Inc. subsidiary Nevada Power Co., which does business under the NV Energy name and is owned by Berkshire Hathaway Energy, was the only utility in Nevada with more than 500,000 reported customers in 2015.

Among the largest utility segments by customer count in 2015, Con Edison, with almost 3.4 million customers in New York City and Westchester County, N.Y., reported the lowest SAIDI without MED, at 21 minutes. The company also reported the lowest SAIFI without MED, at 0.11. On Jan. 30, 2015, Con Edison filed an electric distribution rate increase request seeking a revenue increase of $368 million to "support system reliability, storm hardening, better technology and online service enhancements." The request was approved June 17, 2015, without modification. Con Edison won the 2016 Outstanding System-­Wide Reliability Award for the ninth year in a row.

With about 1.5 million customers in 2015, AEP subsidiary AEP Ohio, known legally as Ohio Power Co., reported the highest SAIDI without MED at 202 minutes, well above the industry average of 110 minutes, and a SAIFI without MED of 1.32.

Among the largest 25 utility segments by customer count, PPL Corp. subsidiary PPL Electric Utilities Corp. in Pennsylvania reported the greatest improvement in terms of reliability, with an 18% year-over-year decrease in SAIDI without MED and a 16% decrease in SAIFI without MED. According to a May 5, 2015, press release, PPL Electric Utilities was able to prevent nearly 200,000 power outages in the first half of 2015 compared to the 2008-2011 period due to investments in its grid.

Did you appreciate this analysis? Click here to turn on alerts for future power Data Dispatches.