System Average Interruption Duration Index (SAIDI)

SAIDI measures the total duration of sustained interruptions the average customer experiences in a calendar year. It is calculated as the sum of all customer interruption durations divided by the total number of customers served.

• Formula: SAIDI = Σ (Customer Interruption Durations) / Total Customers Served
• Unit: Typically expressed in minutes per year or hours per year
• Threshold: Only counts interruptions lasting longer than 5 minutes; momentary outages are excluded and tracked via MAIFI

Regulatory reporting often distinguishes between normal operations and catastrophic events. A Major Event Day (MED) is a day where daily SAIDI exceeds a statistically derived threshold, typically calculated using 2.5 standard deviations above the historical daily average.

• Purpose: Prevents hurricanes, ice storms, and wildfires from skewing baseline reliability trends
• IEEE 1366: The governing standard for MED identification methodology
• Impact: Utilities report both total SAIDI and SAIDI excluding MEDs to provide a normalized view of day-to-day grid performance

While often reported together, SAIDI and SAIFI measure fundamentally different aspects of reliability. SAIFI (System Average Interruption Frequency Index) counts how often the average customer loses power, while SAIDI measures how long they were out.

• SAIFI: Σ (Customers Interrupted) / Total Customers Served — unit is interruptions per year
• SAIDI: Σ (Customer Minutes Interrupted) / Total Customers Served — unit is minutes per year
• CAIDI: Customer Average Interruption Duration Index — SAIDI divided by SAIFI, giving the average restoration time per outage
• Relationship: A utility can have excellent SAIFI but poor SAIDI if restoration times are slow

Public utility commissions use SAIDI as a key performance indicator in performance-based regulation frameworks. Utilities that exceed target thresholds may earn financial incentives, while persistent underperformance triggers penalties.

• UK RIIO Model: Ofgem sets explicit SAIDI targets for network operators with revenue adjustments tied to performance
• US State PUCs: States like California and New York incorporate SAIDI into rate cases and grid modernization approvals
• IEEE 1366 Benchmarks: Provides percentile rankings (e.g., 25th, 50th, 75th) for peer utility comparison by region and density class

Dynamic feeder reconfiguration and self-healing grid automation directly reduce SAIDI by minimizing the number of customers affected by a fault and accelerating service restoration.

• Fault Isolation: Automated sectionalizing switches isolate the faulted segment in seconds, preventing upstream customers from experiencing sustained interruptions
• Service Restoration: Tie switches transfer de-energized customers to healthy adjacent feeders, reducing customer minutes interrupted
• CVR Integration: Conservation Voltage Reduction, when coordinated with topology changes, maintains voltage compliance during reconfiguration without extending outage duration

Accurate SAIDI calculation depends on granular outage data from multiple operational technology systems. Modern utilities integrate these sources to automate reliability reporting.

• OMS: Outage Management System logs customer outage start and restoration timestamps
• AMI: Advanced Metering Infrastructure provides last-gasp and restoration pings for precise duration tracking
• SCADA: Supervisory Control and Data Acquisition confirms device operations and fault clearing times
• GIS: Geographic Information System maps customer counts to feeder segments for accurate normalization

Measures how often the average customer experiences a sustained interruption. While SAIDI tracks duration, SAIFI tracks frequency.

• Formula: (Total Number of Customer Interruptions) / (Total Number of Customers Served)
• Unit: Interruptions per customer per year
• Relationship: A utility can have a low SAIFI but a high SAIDI if outages are rare but take a long time to repair.
• Benchmarking: Often analyzed alongside SAIDI to distinguish between persistent fault-prone circuits and circuits with slow restoration times.

Represents the average time required to restore service to the average customer per sustained interruption. CAIDI is a direct measure of operational response speed.

• Formula: SAIDI / SAIFI
• Unit: Minutes or hours per interruption
• Operational Insight: A rising CAIDI indicates deteriorating crew response times, difficult fault location, or complex Distribution Automation failures.
• Limitation: CAIDI can be misleading if a utility has many short outages, as the denominator (SAIFI) increases, artificially lowering the index.

Tracks the average number of momentary interruptions (less than 5 minutes) per customer. These short-duration events are not captured by SAIDI or SAIFI but are critical for sensitive digital equipment.

• Cause: Typically caused by recloser operations clearing temporary faults like tree branches contacting lines.
• Relevance: Essential for benchmarking performance for customers with semiconductor manufacturing or data centers.
• Exclusion: MAIFI events do not count toward regulatory SAIDI penalties, creating a potential perverse incentive to avoid sustained lockouts.

A metric identifying the proportion of customers who experience a sustained number of interruptions above a defined threshold, often used to flag chronic reliability problems.

• Focus: Shifts from system averages to the specific experience of the worst-served customers.
• Regulatory Use: Regulators use CEMI to ensure utilities are not hiding pockets of poor performance behind a healthy system-wide SAIDI.
• Thresholds: Commonly defined as customers experiencing more than 3 or 5 sustained interruptions in a year.

Represents the fraction of time that a customer has power available during a defined period, often expressed as a percentage of 'nines' of reliability.

• Formula: (Customer Hours of Available Service) / (Customer Hours Demanded)
• Relation to SAIDI: ASAI = 1 - (SAIDI / 8,760 hours per year)
• High-Reliability Target: A SAIDI of 52.56 minutes corresponds to an ASAI of 99.99% ('four nines').
• Contractual Metric: Often used in service level agreements for critical infrastructure and microgrids.

A statistical filtering methodology defined in IEEE 1366 to separate normal day reliability performance from catastrophic weather events for fair year-over-year benchmarking.

• Threshold: A day where daily SAIDI exceeds a calculated threshold using a 2.5 beta method on the log-normal distribution of historical daily SAIDI values.
• Purpose: Excluding MEDs prevents a single hurricane or ice storm from skewing a utility's 5-year reliability trend.
• Reporting: Utilities must report both total SAIDI and SAIDI with MEDs excluded to regulators.