SAIFI

SAIFI, the System Average Interruption Frequency Index, is a reliability metric used in electric power systems to quantify the average number of interruptions experienced by an individual customer over a specified period, usually one year.

The index is calculated as the total number of customer interruptions during the period divided by the total number of customers served in that period. An outage that affects multiple customers contributes one interruption for each affected customer. The result is expressed in interruptions per customer per period.

SAIFI is a frequency measure and is typically used alongside SAIDI, which measures total outage duration per customer, and CAIDI, which is derived from SAIDI divided by SAIFI and represents average restoration time per interrupted customer. Utilities employ SAIFI for benchmarking, reliability trending, and planning, with reporting often available for the system as a whole or by feeder, region, or customer class.

Data quality and methodological choices influence SAIFI. Calculations rely on outage records from utility systems, and some programs separate or exclude planned maintenance outages from the metric. The chosen time period (annual, quarterly) and the scope (nationwide, regional, or service territory) affect comparisons. Internationally, standards such as IEEE 1366 provide guidance on reliability indices and reporting practices, contributing to consistency in how SAIFI is defined and used.