SAIFI

The System Average Interruption Frequency Index (SAIFI) is commonly used as a reliability indicator by electric power utilities. SAIFI is the average number of interruptions that a customer would experience, and is calculated as

SAIFI = ∑ λ i N i N T

where λ i is the failure rate, N i is the number of customers for location i and N T is the total number of customers served. In other words,

SAIFI = total number of customer interruptions total number of customers served

SAIFI is measured in units of interruptions per customer. 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 1.10 interruptions per customer.