SAIFI

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

$$\mbox{SAIFI} = \frac{\sum{\lambda_i N_i}}{\sum{N_i}}$$

where $$\lambda_i$$ is the failure rate and $$N_i$$ is the number of customers for location $$i$$. In other words,

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

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.