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

SAIDI = ∑ U i N i N T {\displaystyle {\mbox{SAIDI}}={\frac {\sum {U_{i}N_{i}}}{N_{T}}}}

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

SAIDI = sum of all customer interruption durations total number of customers served {\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.