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

${\displaystyle {\mbox{SAIFI}}={\frac {\sum {\lambda _{i}N_{i}}}{\sum {N_{i}}}}}$

where ${\displaystyle \lambda _{i}}$ is the failure rate and ${\displaystyle N_{i}}$ is the number of customers for location ${\displaystyle i}$. In other words,

${\displaystyle {\mbox{SAIFI}}={\frac {\mbox{total number of customer interruptions}}{\mbox{total number of customers served}}}}$

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.