The System Average Interruption Duration Index (SAIDI) is commonly used as a reliability index by electric power utilities. SAIDI is the average 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, and according to IEEE Standard 1366-1998 the median value for North American utilities is approximately 1.50 hours.