“A reliable grid is not measured by the absence of failure, but by how rarely the lights go out, how quickly they return, and how intelligently we learn from every interruption.” – MJ Martin
Electricity utilities need objective ways to answer two deceptively simple questions: How often does the power go out, and how long does it stay out?
Two of the most important industry metrics are SAIFI and SAIDI. They are standardized distribution-reliability indices addressed by IEEE 1366-2022, Guide for Electric Power Distribution Reliability Indices.
For a smart grid, these measurements are particularly important because they provide a quantitative way of determining whether investments in automation, communications, intelligent meters, sensors, switching and analytics are actually improving reliability.
SAIFI: How Often Does the Power Go Out?
SAIFI means System Average Interruption Frequency Index.
It measures the average number of sustained interruptions experienced by a customer during a defined period, normally one year.
The basic calculation is:
SAIFI = Total Customer Interruptions ÷ Total Customers Served
Consider a utility serving 100,000 customers. During the year, various outages collectively result in 200,000 customer interruptions.
SAIFI = 200,000 ÷ 100,000 = 2.0
The interpretation is straightforward:
The average customer experienced two sustained power interruptions during the year.
A lower SAIFI is generally better.
SAIFI primarily answers the reliability question: How frequently does the distribution system fail to maintain continuous service?
SAIDI: How Long Is the Power Off?
SAIDI means System Average Interruption Duration Index.
It measures the average total duration of sustained interruptions experienced by a customer during the reporting period. It is commonly expressed in minutes or hours per customer per year.
The calculation is:
SAIDI = Total Customer Interruption Duration ÷ Total Customers Served
Suppose our same 100,000-customer utility accumulates 30 million customer-minutes of interruption during the year.
SAIDI = 30,000,000 ÷ 100,000 = 300 minutes
Therefore, the average customer experienced:
300 minutes = 5 hours without electricity during the year.
Again, lower is generally better.
SAIDI answers a different question from SAIFI:
How much total outage time does the average customer experience?
SAIFI and SAIDI Tell Different Stories
The distinction is extremely important.
Imagine Utility A has:
SAIFI = 4.0
SAIDI = 120 minutes
Customers experience four outages annually, but total outage duration is only two hours.
Utility B has:
SAIFI = 1.0
SAIDI = 300 minutes
Customers experience only one outage, but that outage averages five hours.
Which utility is more reliable?
There is no completely satisfactory answer without considering both metrics. Utility A has a frequency problem. Utility B has a restoration-duration problem.
This is why SAIFI and SAIDI should normally be considered together.
There is also a closely related metric called CAIDI, Customer Average Interruption Duration Index:
CAIDI = SAIDI ÷ SAIFI
Using the first example:
300 minutes ÷ 2 interruptions = 150 minutes per interruption
CAIDI therefore tells us approximately how long an interruption lasts once a customer has been interrupted.
Why These Metrics Matter to a Smart Grid
This is where SAIFI and SAIDI become particularly interesting.
A traditional distribution network is largely reactive. A feeder trips, customers telephone the utility, dispatchers determine where the problem might be, crews patrol the circuit, locate the fault, isolate it and restore electricity.
A smart grid attempts to turn that process into something much closer to an automated nervous system.
AMI smart meters can provide outage or “last gasp” notifications. Fault indicators and line sensors can identify the affected section. SCADA and Distribution Management Systems can establish system conditions. Automated switches and reclosers can isolate a fault. FLISR, Fault Location, Isolation and Service Restoration, can then reconfigure healthy portions of the distribution system around the damaged section. IEEE describes FLISR as a central distribution-automation capability for locating faults, isolating affected sections and restoring unaffected customers through alternative supply paths.
The practical consequence can be dramatic.
Instead of 5,000 customers being without electricity for two hours while crews investigate, automation might isolate a damaged section containing 300 customers and restore the remaining 4,700 customers within minutes.
That directly attacks SAIDI because thousands of customer-minutes of interruption have been eliminated.
Depending upon the system configuration and how interruptions are classified, automation can also improve SAIFI by preventing faults or localized events from becoming sustained interruptions for larger customer populations.
SAIFI and SAIDI Become Smart-Grid KPIs
This makes these indices valuable when evaluating capital investments.
Suppose a utility spends $20 million installing feeder automation, communications, intelligent switches and an advanced distribution management system. Management should not simply report:
“We installed 400 intelligent switches.”
That measures equipment deployed, not value delivered.
A far more meaningful question is:
What happened to SAIFI and SAIDI?
If five-year SAIFI falls from 2.4 to 1.5 and SAIDI falls from 320 minutes to 170 minutes, the utility has evidence that customers are receiving a measurable reliability benefit.
Ontario uses reliability measures in electricity-distributor performance oversight. The Ontario Energy Board’s (OEB) utility scorecards allow customers and regulators to examine measures including how often power is interrupted and how long customers experience outages. The OEB explicitly uses performance reporting to encourage improvement, transparency and value for consumers.
One Important Caution
SAIFI and SAIDI cannot be compared casually between utilities.
Urban and rural networks have dramatically different characteristics. Underground and overhead systems behave differently. Forest exposure, ice storms, lightning, customer density, feeder length and accessibility can substantially affect reliability.
There is also the question of Major Event Days. IEEE methodology provides a statistical approach for identifying unusually severe events so utilities can examine normal system performance separately from extraordinary events.
That distinction matters enormously in Canada. A utility serving a dense downtown underground network should not necessarily be benchmarked directly against a utility operating thousands of kilometres of overhead distribution through forests and remote rural territory.
The Bigger Picture
Think of an electric distribution system like an airline.
SAIFI asks: How often are the flights cancelled?
SAIDI asks: How much total delay do passengers experience?
CAIDI asks: When something does go wrong, how long does it usually take us to recover?
A modern smart grid should improve all three dimensions.
And that provides perhaps the most important reason SAIFI and SAIDI matter. Smart-grid technology is not valuable merely because it is smart. It is valuable when intelligence creates a measurable improvement in customer service.
SAIFI and SAIDI convert something customers immediately understand, “How often did my electricity fail and how long was I in the dark?”, into engineering metrics that utilities, regulators and customers can track objectively.
For an electric utility, they are therefore much more than acronyms. They are two of the fundamental measures of whether the grid is doing its primary job: keeping the lights on.
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now Ontario Tech University] and on the Board of Advisers of five different Colleges in Ontario – Centennial College, Humber College, George Brown College, Durham College, Ryerson Polytechnic University [now Toronto Metropolitan University]. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees – in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and seven major certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 80 next generation MOOC (Massive Open Online Courses) [aka Micro Learning] continuous education programs in a wide variety of topics, including: Economics, Python Programming, Internet of Things, Cloud, Artificial Intelligence and Cognitive systems, Blockchain, Agile, Power BI, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.
Martin is a volunteer, a photographer, a learner, a technologist, a philosophizer, and a romantic optimist.