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
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.
- Yeddanapudi, Sree. "Distribution System Reliability Evaluation". Iowa State University. Retrieved 18 June 2011.