Understanding SAIDI and SAIFI: What Power Outage Metrics Mean for You
Every major electric utility in America reports two standardized metrics that tell you exactly how reliable their power delivery is. Here's what they mean and how to use them.
The Two Metrics That Define Utility Reliability
Electric utilities file annual reliability data with the U.S. Energy Information Administration (EIA) under Form 861. The two most important metrics are SAIDI and SAIFI, both developed by IEEE (Institute of Electrical and Electronics Engineers) as industry standards.
SAIDI: How Long Were You Without Power?
SAIDI stands for System Average Interruption Duration Index. It measures the total minutes per year that the average customer experiences power outages.
The formula: SAIDI = Total customer interruption minutes ÷ Total customers served
A SAIDI of 150 means the average customer loses power for 2.5 hours over the entire year, across all outages combined. A SAIDI of 600 means the average customer loses power for 10 hours per year.
Lower is better. The most reliable large utilities in the US have SAIDIs below 60 minutes per year.
SAIFI: How Many Times Did Your Power Go Out?
SAIFI stands for System Average Interruption Frequency Index. It measures how many times per year the average customer experiences a power interruption.
A SAIFI of 1.2 means the average customer has their power interrupted 1.2 times per year. A SAIFI of 3.0 means 3 outages per year.
Lower is better.
SAIDI Without Major Event Days (nMED)
Most utilities report two versions of SAIDI: with and without "major event days." Major event days are days when severe weather or other catastrophic events cause outages far beyond normal levels, think hurricanes, ice storms, or derecho wind events.
SAIDI with major event days reflects the full customer experience including weather-related outages.
SAIDI without major event days (nMED) reflects the utility's day-to-day infrastructure reliability, how well they maintain equipment, clear vegetation, and respond to routine faults.
Both are valuable. A utility with excellent nMED but high overall SAIDI may have great infrastructure but operate in a storm-prone region. A utility with high nMED has infrastructure problems regardless of weather.
How to Interpret the Numbers
Here's a rough guide to SAIDI benchmarks for US utilities:
- Under 60 minutes: Excellent, top tier national reliability
- 60–150 minutes: Above average, better than most
- 150–300 minutes: Average, typical for most US utilities
- 300–600 minutes: Below average, reliability issues present
- Over 600 minutes: Poor, significant infrastructure or operational problems
What Affects SAIDI and SAIFI?
Several factors influence how reliable a utility is:
- Geography: Utilities in hurricane-prone Florida, ice storm-prone Oklahoma, or wildfire-prone California naturally face more weather events
- Urban vs. rural: Rural utilities often have longer lines spanning more terrain, increasing exposure to outages
- Utility type: Cooperative utilities (member-owned, rural) often have higher SAIDIs than large investor-owned utilities serving dense urban areas
- Infrastructure age: Older equipment fails more frequently
- Vegetation management: Tree trimming programs significantly reduce outage frequency
- Underground vs. overhead lines: Underground lines are far more reliable but expensive
Using This Data for Real Decisions
SAIDI and SAIFI data can inform several practical decisions:
- Buying or renting a home: Check what utility serves the area and compare its reliability to the state average before committing
- Generator sizing: If your utility has a SAIDI over 300 minutes, a backup generator may be worth the investment
- Home business planning: High-SAIDI utilities mean more downtime risk for home offices and small businesses
- Medical equipment: Households with oxygen concentrators or other critical equipment need to know their outage risk
SAIDI and SAIFI: How Utilities Are Measured
Electric utility reliability is measured using two core metrics established by IEEE Standard 1366. Understanding these metrics helps you compare utilities on an apples-to-apples basis.
Core Metric Definitions
| Metric | Full Name | Unit | Lower = Better? |
|---|---|---|---|
| SAIDI | System Average Interruption Duration Index | Minutes/year | Yes |
| SAIFI | System Average Interruption Frequency Index | Events/year | Yes |
| CAIDI | Customer Average Interruption Duration Index | Minutes/event | Yes |
| MAIFI | Momentary Average Interruption Frequency Index | Events/year | Yes |
Worked Example: Reading SAIDI Numbers
If Utility A reports SAIDI = 90 minutes, it means the average customer experienced 90 minutes of total power interruptions that year. If Utility B reports SAIDI = 240, Utility B customers experienced roughly 2.7x more outage time.
For a residential customer paying $125.00 ... $185.00/month for electricity, a SAIDI of 240 minutes (4 hours of outages) represents a different reliability tier than 90 minutes (1.5 hours). Over a 10-year period, that difference compounds to 25 hours vs. 15 hours of cumulative outage time.
With vs. Without Major Event Days
Utilities report two versions of SAIDI: one including all outages, and one excluding "major event days" (MEDs) such as hurricanes, ice storms, and wildfires. The gap between these two numbers reveals how much extreme weather drives outages at that utility. A utility in Florida might have SAIDI = 300 with MEDs but SAIDI nMED = 90, showing that 70% of outage time is weather-related rather than infrastructure-related.
Frequently Asked Questions
What is a good SAIDI score?
A SAIDI below 60 minutes per year is considered excellent. The US national average is around 150-300 minutes depending on the year. Under 100 minutes is above average for most utilities.
What is SAIDI without major event days?
SAIDI without major event days (nMED) excludes outages from declared major events like hurricanes and ice storms. This metric better reflects day-to-day infrastructure reliability rather than weather vulnerability.
Data source: U.S. Energy Information Administration Form 861. Reliability metrics are self-reported by utilities annually.
