The System Average Interruption Frequency Index (SAIFI) is commonly used as a reliability index by electric power utilities.
This index measures the average number of times that a system customer experiences an outage during the year or during a given time period.
[1] SAIFI is normally calculated on either yearly or monthly basis.
Many factors cause power interruptions, including weather, vegetation patterns, and utility practices.
[2] SAIFI is measured in units of interruptions per customer.
According to IEEE Standard 1366-1998 the median value for North American utilities is approximately 1.10 interruptions per customer.