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Distribution System Reliability Metrics

Electric reliability is the ability of the distribution system to withstand events such as severe weather. Three metrics are commonly used to assess distribution system reliability:  SAIDI, SAIFI, and CAIDI.

These metrics are calculated in two ways:  1) “All Weather” which includes the entire year year and 2) “Excluding Major Event Days” which excludes events when the distribution system is experiencing major impacts. Excluding Major Event Days reduces the number of days used in the calculation and the resulting metric will be lower than the All Weather numbers.

Each year, the Institute of Electrical and Electronics Engineers surveys utilities across the country to gather electric reliability metrics.  IEEE uses the data provided to divide the survey respondents’ data into 4 groups (quartiles) to create benchmark electric reliability quartile data for utilities. (Benchmarking | IEEE PES Distribution Reliability Working Group)  Data in the 1st quartile indicates the highest electric reliability and data in the 4th quartile represents the lowest. While the utilities participating in IEEE’s annual benchmark study are kept anonymous, the benchmark data provides a useful comparison. 


Monthly Reliability Data

The rate-regulated electric utilities in Michigan submit monthly reliability and outage data every quarter. These reports show that data in tables and graphs.

The submitted monthly data can be requested by clicking here: Click here to request data.





SAIDI (System Average Interruption Duration Index) represents the total number of minutes of interruption the average customer experiences.  SAIDI is calculated by dividing the sum of all customer interruption minutes within the year by the number of customers served during the year. SAIDI minutes represent how long the average customer experiences an outage and lower SAIDI minutes equate to better electric reliability. For example, a SAIDI of 100 means that the average customer on the utility’s distribution system experiences a total of 100 minutes of outage.

SAIDI All Weather Data

All Weather

SAIDI Data Excluding Major Event Days

Excluding Major Event Days


SAIFI (System Average Interruption Frequency Index) represents the average number of times a customer experiences an outage during the year.  SAIFI is calculated by dividing the total number of customer interruptions by the total number of customers in the system. Lower SAIFI numbers represent less interruptions and better electric reliability.

SAIFI All Weather Data

All Weather

SAIFI Data Excluding Major Event Days

Excluding Major Event Days


CAIDI or Customer Average Interruption Duration Index is the average time required to restore service. It is calculated as total minutes of customer interruption divided by the total number of interruptions. The lower the number of minutes, the faster the utility restored service to customers.

CAIDI All Weather Data

All Weather

CAIDI Data Excluding Major Event Days

Excluding Major Event Days