By Ashwin Kota, HEXstream integrations and analytics lead 

Electricity is something we often take for granted. When we flip a switch, we expect the lights to turn on and the TV to hum to life. However, maintaining this consistent flow of power is no easy task. Utilities work hard to keep power reliable and avoid interrutions. 

But how do they measure how well they’re doing?

In this article we’ll take a look at four key metrics that utilities use to measure the reliability of electric systems: SAIDI, SAIFI, CAIDI and MAIFI. Let’s see what these metrics mean and why they matter to all of us.

Why Track Reliability Metrics?

Reliability in the electric system is all about the dependability of the power supply. Tracking reliability metrics helps utilities figure out where they need to make improvements. The goal is to keep power interruptions—also known as outages—as rare and short as possible. By studying reliability, utilities can see where they are doing well and where they need to make changes. This improves customer satisfaction, as fewer people experience the inconvenience of outages.

Understanding the Key Reliability Metrics

When it comes to reliability, there are four main metrics utilities look at. These metrics, often created using outage data, help utilities understand both the frequency and the duration of interruptions.

1. SAIDI (System Average Interruption Duration Index): SAIDI measures the average amount of time that customers experience outages in a given year. If the SAIDI score is 120 minutes,that means the average customer experiences a total of two hours of outages each year. Lower SAIDI values mean fewer or shorter outages, which is a good thing.
2. SAIFI (System Average Interruption Frequency Index): While SAIDI focuses on how long outages last, SAIFI looks at how often they happen. If the SAIFI score is 1.5, it means that, on average, customers experience about 1-2 outages per year. Like SAIDI, a lower SAIFI score is better, as it means fewer interruptions.
3. CAIDI (Customer Average Interruption Duration Index): CAIDI tells us how quickly the utility can restore power when an outage does happen. To get CAIDI, divide SAIDI by SAIFI; f SAIDI is 120 minutes and SAIFI is 1.5, CAIDI would be 80, meaning the average time to restore power is around 80 minutes. Utilities aim to keep CAIDI as low as possible so customers don’t stay without power for long.
4. MAIFI (Momentary Average Interruption Frequency Index): Unlike the other three metrics, MAIFI focuses on very brief interruptions (those lasting less than five minutes). These short interruptions are common but can still be annoying. MAIFI helps utilities track these momentary blips in service and figure out ways to reduce them.

The Role of Major Event Days (MEDs)

Not all outages are created equal. Some are caused by routine issues, like equipment failures, while others happen due to large-scale events, like major storms, earthquakes, or wildfires. These large, unpredictable events are called Major Event Days (MEDs). Since these events can cause big disruptions, utilities separate them from the usual daily data to get a clearer picture of regular performance.

Let’s break down how each metric is calculated with and without MEDs:

1. SAIDI
   • With MEDs: When we include MEDs, SAIDI will be higher because these big events cause longer outages. This version shows the impact of rare but high-impact events on the system.
   • Without MEDs: By removing MEDs from SAIDI, we get an average that shows how the system performs on a normal day. This helps utilities understand the typical reliability of their system without the influence of extreme events.
2. SAIFI
   • With MEDs: When calculated with MEDs, SAIFI shows how often outages happen, including during extreme events. This helps utilities plan for responses to emergencies.
   • Without MEDs: Removing MEDs from SAIFI shows how often interruptions occur in normal conditions. A lower SAIFI score here is a good sign of the system’s stability day-to-day.
3. CAIDI
   • With MEDs: Including MEDs in CAIDI reveals how quickly power is restored after any type of outage, including major events. This gives an idea of how well a utility can respond during emergencies.
   • Without MEDs: Without MEDs, CAIDI shows how efficiently outages are resolved in regular situations. A lower CAIDI here suggests a utility is well-prepared to handle everyday interruptions swiftly.
4. MAIFI
   • With MEDs: MEDs have less of an impact on MAIFI, as momentary outages are usually short and not caused by major events.
   • Without MEDs: Since MAIFI focuses on brief interruptions, it remains mostly stable regardless of MEDs. This score primarily helps track minor reliability issues that occur during normal operation.

The Difference Between With and Without MEDs is Important

By looking at metrics both with and without MEDs, utilities get a fuller picture of their grid’s reliability. Here’s why it matters:

• With MEDs: Including MEDs in calculations shows the utility’s ability to handle severe disruptions. It helps the utility understand the grid’s resilience and prepare for worst-case scenarios.
• Without MEDs: Calculations without MEDs provide a baseline, showing how well the system works under normal circumstances. This version of the metrics helps utilities improve routine reliability by targeting areas that need attention.

Together, these versions help utilities balance both daily reliability and readiness for rare events, enabling a more reliable and robust grid overall.

How Utilities Use Reliability Data

The information from these metrics doesn’t just sit on a shelf. Utilities use reliability data to make decisions about where and how to invest in the grid. Here are some examples:

• Targeting Problem Areas: By identifying the circuits with the worst SAIDI and SAIFI scores, utilities can focus their efforts on areas that need the most improvement. These “worst-performing circuits” often become the top priority for repairs and upgrades.
• Planning for Future Needs: In regions where outages are more common, utilities can strengthen infrastructure or install new technology to prevent future interruptions. This could include adding automated systems that make it easier to detect and respond to outages quickly.
• Tracking Improvement Over Time: By comparing metrics over the years, utilities can measure the effectiveness of their investments. A lower SAIDI or SAIFI over time shows that their reliability projects are working, which is a positive sign for both the utility and its customers.

Moving Toward a Reliable Future

Reliability isn’t a one-time fix; it requires constant attention and improvement. By using these metrics, utilities can understand the strengths and weaknesses of their systems. This knowledge helps them make informed decisions that lead to a better, more dependable power supply. For customers, it means fewer power interruptions and quicker service restoration when outages do happen.

In short, SAIDI, SAIFI, CAIDI and MAIFI are the cornerstones of electric-system reliability. By keeping an eye on these metrics, utilities can keep power flowing smoothly and ensure that we rarely have to think about what it takes to keep the lights on.