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

where is the failure rate and is the number of customers for location . In other words,

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.

• Layton, Lee (2004). "Electric System Reliability Indices" (PDF). www.egr.unlv.edu.