The System Average Interruption Duration Index (SAIDI)^[1] is commonly used as a reliability index by electric power utilities. SAIDI is the average cumulative outage duration for each customer served, and is calculated as:

${\displaystyle {\mbox{SAIDI}}={\frac {\sum {U_{i}N_{i}}}{N_{T}}}}$

where ${\displaystyle N_{i}}$ is the number of customers and ${\displaystyle U_{i}}$ is the annual outage time for location ${\displaystyle i}$, and ${\displaystyle N_{T}}$ is the total number of customers served. In other words,

${\displaystyle {\mbox{SAIDI}}={\frac {\mbox{sum of all customer interruption durations}}{\mbox{total number of customers served}}}}$

SAIDI is measured in units of time, often minutes or hours; it is usually measured over the course of a year. According to IEEE Standard 1366-1998, the median value for North American utilities is approximately 1.50 hours. According to the U.S. Energy Information Administration Annual Electric Power Industry Report, it is 2.0 hours, rising to the range of 3.5 to 8 hours, when "major events" are included.^[2]

The following is a table of SAIDI for different countries, calculated using the methodology in the World Bank's Doing Business 2016-2020 studies:

• EIA Reliability Metrics video by the U.S. Energy Information Administration on Youtube