State Commissions must ensure that the public have access to reliable service. There are metrics in place to measure reliability, particularly for electricity service.

How is Reliability for Electricity Service Measured?

Reliability Indices

System average interruption duration index (SAIDI)
The most often used performance measurement for a sustained interruption is the system average interruption duration index (SAIDI). This index measures the total duration of an interruption for the average customer during a given time period.

Customer average interruption duration index (CAIDI)
Once an outage occurs, the average time to restore service is found from the customer average interruption duration index (CAIDI).

System average interruption frequency index (SAIFI)
The system average interruption frequency index (SAIFI) is the average number of times that a system customer experiences an outage during the year (or time period under study).

Customer average interruption frequency index (CAIFI)
Similar to SAIFI is CAIFI, which is the customer average interruption frequency index. The CAIFI measures the average number of interruptions per customer interrupted per year. It is simply the number of interruptions that occurred divided by the number of customers affected by the interruptions.

Average service availability index (ASAI)
The average service availability index (ASAI) is the ratio of the total number of customer hours that service was available during a given time period to the total customer hours demanded. This is sometimes called the service reliability index.


Abdrabbi Bourezg, H. Meglouli, “Reliability assessment of power distribution systems using disjoint path-set algorithm,” J Ind Eng Int (2015) 11:45–57.