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CAIDI

The Customer Average Interruption Duration Index (CAIDI) is a key reliability metric employed by electric power utilities to measure the average duration of power outages experienced by customers during sustained interruptions.^[1] It quantifies the average time required to restore service to affected customers, providing insight into the efficiency of outage response and restoration efforts.^[2] CAIDI is particularly valuable for assessing system performance in the context of regulatory reporting and customer satisfaction, as it focuses on the impact of outage length rather than frequency.^[3] Defined in IEEE Standard 1366, CAIDI is calculated by dividing the total duration of customer interruptions (often in minutes) by the total number of customer interruptions over a specified period, such as a year.^[4] This formula yields a value typically expressed in minutes, allowing utilities to benchmark their restoration speed against industry standards or historical data.^[5] For instance, a lower CAIDI value indicates faster average restoration times, which can reflect effective maintenance, advanced grid technologies, or robust emergency protocols.^[6] In relation to other reliability indices, CAIDI is derived from the System Average Interruption Duration Index (SAIDI) and the System Average Interruption Frequency Index (SAIFI), specifically as CAIDI = SAIDI / SAIFI, emphasizing the duration aspect for customers who experience outages.^[7] It complements these metrics by highlighting areas for improvement in outage management.^[3] Regulatory bodies often mandate CAIDI reporting to ensure utilities maintain high service reliability.^[8]

Overview

Definition

The Customer Average Interruption Duration Index (CAIDI) is a key reliability metric used by electric power utilities to evaluate the performance of their distribution systems.^[9] It quantifies the average time required to restore service to customers who experience a sustained interruption, providing insight into the efficiency of outage response and repair processes.^[3] According to IEEE Standard 1366, CAIDI specifically applies to sustained interruptions, defined as those lasting more than five minutes, excluding brief momentary events that do not significantly impact customers.^[4] CAIDI is typically calculated and reported over a one-year period, reflecting the aggregate experience of affected customers rather than all customers in the system.^[9] It is expressed in units of time, most commonly minutes per customer interruption, though hours may be used depending on the scale of the utility.^[3] For instance, if the total duration of interruptions across all affected customers amounts to 500 customer-minutes from 50 separate customer interruptions, the CAIDI would be 10 minutes per interruption.^[9] This metric helps utilities prioritize improvements in restoration speed, contributing to broader assessments of system reliability.^[4]

Historical Development

The Customer Average Interruption Duration Index (CAIDI) originated in the late 20th century as part of efforts by the Institute of Electrical and Electronics Engineers (IEEE) to standardize reliability metrics for electric power distribution systems. It was first formalized within IEEE Std 1366-1998, a trial-use guide developed by the IEEE Power and Energy Society to define indices specifically tailored for distribution reliability assessment, drawing on earlier surveys of utility practices from the 1990s.^[10] This standard introduced CAIDI alongside related metrics like SAIDI and SAIFI to provide a consistent framework for measuring outage durations affecting customers.^[11] Key milestones in CAIDI's development occurred in the 1990s, with the IEEE's 1998 guide marking its initial codification, followed by reaffirmation in 2001 and full standard status in IEEE Std 1366-2003. Widespread adoption accelerated after 2000, driven by regulatory pressures in the United States, where bodies like the Federal Energy Regulatory Commission (FERC) and state public utility commissions began mandating reporting of IEEE-defined indices to enhance benchmarking and performance oversight.^[12] For instance, by the mid-2000s, multiple states had integrated these metrics into regulatory frameworks to evaluate utility reliability.^[12] CAIDI's evolution has centered on refining its scope to better reflect operational performance. Initially focused on sustained interruptions (lasting more than five minutes), subsequent revisions excluded momentary outages—handled separately via metrics like MAIFI—and introduced methods to isolate major events, such as the "2.5 beta" threshold for Major Event Days in IEEE Std 1366-2003, allowing utilities to report normalized indices excluding extreme weather impacts.^[12] Later updates, including the 2012 and 2022 editions, further emphasized statistical consistency in these exclusions to reduce year-to-year variability.^[13] While CAIDI remains primarily adopted in North America through IEEE standards, similar metrics have gained traction globally with variations in application. In Europe, equivalent indices like those derived from SAIDI and SAIFI are used by organizations such as Eurelectric for distribution reliability reporting, though ENTSO-E focuses more on transmission-level continuity under harmonized guidelines.^[14] In Australia, CAIDI is explicitly incorporated into regulatory frameworks by the Australian Energy Regulator, with thresholds adapted to local service standards, promoting its use alongside SAIDI and SAIFI for performance evaluation.^[15]

Calculation and Components

Formula

The Customer Average Interruption Duration Index (CAIDI) is calculated using the formula

\text{CAIDI} = \frac{\sum_{i=1}^{n} (r_i \times N_i)}{\sum_{i=1}^{n} N_i}

where r_i is the duration of the i-th sustained interruption event (in minutes), N_i is the number of customers affected by that event, and the summation is over all n sustained interruption events in the reporting period.^[13] This represents the total customer-minutes of interruption divided by the total number of customer interruptions, providing the average restoration time per interruption experienced by customers.^[13] This formula derives from the ratio of the System Average Interruption Duration Index (SAIDI) to the System Average Interruption Frequency Index (SAIFI), as CAIDI = SAIDI / SAIFI, but it is typically computed directly from raw outage data to ensure accuracy in weighting by customer impact.^[13] The direct computation aggregates the customer-weighted durations without relying on system totals, avoiding potential division-by-zero issues if no interruptions occur.^[13] To compute CAIDI step by step: (1) For each sustained interruption event, multiply the outage duration r_i by the number of affected customers N_i to obtain customer-minutes of interruption, then sum these values across all events to get the total customer-minutes (\sum (r_i \times N_i)); (2) Sum the number of affected customers across all events to get the total number of customer interruptions (\sum N_i); (3) Divide the total customer-minutes by the total customer interruptions.^[13] This process uses \lambda = \sum N_i as the total customer interruptions and effectively computes a customer-weighted average duration r = \frac{\sum (r_i \times N_i)}{\sum N_i}, so CAIDI = \lambda \times r / \lambda = r.^[13] Only sustained interruptions are included, defined as those lasting longer than 5 minutes, excluding momentary interruptions (typically ≤5 minutes) which are addressed separately in the Momentary Average Interruption Frequency Index (MAIFI).^[13] Major events may be excluded per IEEE guidelines, which identify major event days (MEDs) using statistical thresholds—such as daily SAIDI exceeding a value derived from historical data (e.g., via the 2.5 beta method)—to separate anomalous weather-related impacts from normal performance.^[13] For example, consider a utility experiencing two sustained interruptions: one affecting 100 customers for 3 minutes and another affecting 50 customers for 10 minutes. The total customer-minutes is (3 × 100) + (10 × 50) = 800, and the total customer interruptions is 100 + 50 = 150, yielding CAIDI = 800 / 150 ≈ 5.33 minutes.^[13]

Key Variables

The key variables in CAIDI calculations are the total customer interruption duration, denoted as ∑ U_i, and the total number of customer interruptions, denoted as N_c. The total customer interruption duration (∑ U_i) represents the aggregate over all sustained outage events of the product of each event's duration (in minutes) and the number of customers affected by that event.^[16] This metric captures the overall exposure of customers to outage time across the system. Similarly, the total number of customer interruptions (N_c) is the sum, across all sustained outage events, of the customers impacted per event, excluding those not affected.^[16] These variables are primarily sourced from utility operational systems and reporting mechanisms. Supervisory Control and Data Acquisition (SCADA) systems provide real-time fault detection and outage location data from substations and feeders.^[17] Outage Management Systems (OMS) integrate this with customer trouble calls, automated notifications from smart meters (via Advanced Metering Infrastructure or AMI), and automated meter reading (AMR) data to estimate affected customer counts and restoration times.^[17] Customer calls serve as a supplementary source, particularly for verifying outage extents in areas without advanced metering.^[18] Smart meter data enhances accuracy by enabling remote detection of outages without reliance on calls, reducing estimation errors in customer counts.^[17] Measurement of these variables involves specific challenges in defining and classifying outages. A sustained interruption, which contributes to both ∑ U_i and N_c, is defined by IEEE Standard 1366 as any interruption not classified as momentary, typically exceeding a threshold of 5 minutes.^[4] Handling partial outages—where only a subset of service is lost, such as voltage sags or single-phase faults—requires determining if the event qualifies as a full interruption; many utilities exclude partial power losses from sustained counts to focus on complete de-energizations. For islanded microgrids, interruptions may not be recorded if local generation maintains supply to customers during grid disconnection, thus excluding such periods from ∑ U_i and N_c to reflect continued service availability.^[19] Adjustments to these variables often exclude major events to isolate normal system performance. Major events are defined under IEEE Standard 1366 as interruptions or groups exceeding system design limits, occurring on major event days where daily SAIDI surpasses a historical threshold (TMED), calculated via methods like the 2.5β statistical approach or a fixed percentile of prior daily SAIDI values.^[4] Utilities may apply exclusions for events affecting more than 10% of customers, such as severe storms, following IEEE 1366 guidelines or state-specific rules aligned with NERC principles for equitable benchmarking.^[20] These exclusions prevent distortion of ∑ U_i and N_c by uncontrollable factors, ensuring CAIDI reflects operational reliability.

Relationships to Other Reliability Indices

Connection to SAIDI and SAIFI

The Customer Average Interruption Duration Index (CAIDI) is mathematically derived from the System Average Interruption Duration Index (SAIDI) and the System Average Interruption Frequency Index (SAIFI) according to IEEE Standard 1366, with the relationship expressed as CAIDI = SAIDI / SAIFI, where SAIDI represents the average total duration of interruptions per customer served and SAIFI denotes the average number of sustained interruptions per customer served.^[4] This formula isolates the average restoration time for affected customers per interruption event, providing a targeted measure of response efficiency that complements the broader impacts captured by SAIDI and the occurrence rate indicated by SAIFI.^[4] Conceptually, CAIDI serves as a diagnostic tool within the family of reliability indices, emphasizing the duration aspect of outages while SAIDI aggregates overall customer impact and SAIFI focuses on event frequency; together, they enable utilities to assess whether poor reliability stems from numerous brief interruptions or fewer prolonged ones.^[17] All three indices rely on shared input data—such as the number of customers interrupted (CI) and customer minutes interrupted (CMI)—but aggregate it differently: SAIFI uses CI relative to total customers served (CS), SAIDI incorporates CMI relative to CS, and CAIDI divides CMI by CI to highlight per-event duration.^[4] This interdependence allows CAIDI to reveal nuances, such as potential increases in average restoration time despite overall system improvements if mitigation strategies reduce the number of affected customers more than outage durations.^[17] For instance, in a system with 1,525 customers served experiencing a total of 3,075 customer interruptions and 242,975 customer minutes interrupted, SAIFI equals 2.02 interruptions per customer, SAIDI equals 159 minutes per customer, and CAIDI equals 79 minutes per interruption, demonstrating how the division yields the isolated restoration metric.^[4] Similarly, if SAIDI is 100 minutes and SAIFI is 1.0, CAIDI would be 100 minutes per interruption, illustrating the index's utility in quantifying event-specific durations.^[4] Extensions of CAIDI include variants adapted for specific contexts, such as those focusing on end-user perspectives in distribution systems with embedded generation, where indices like the Cost of Energy Not Supplied (CENS) incorporate economic impacts alongside duration to better reflect customer-end reliability.^[21] CAIDI, or Customer Average Interruption Duration Index, differs from other reliability metrics in its emphasis on the average time to restore service following a sustained interruption, providing insight into restoration efficiency rather than overall outage impact or occurrence rates. Unlike broader system-level measures, CAIDI specifically calculates the duration per affected customer per event, helping utilities assess response times without aggregating total exposure. In contrast to SAIDI (System Average Interruption Duration Index), which quantifies the total duration of sustained interruptions experienced by the average customer over a period—typically encompassing all outages' cumulative effect—CAIDI isolates the average duration of individual interruptions for those customers affected, ignoring unaffected ones. This makes CAIDI particularly useful for evaluating per-event restoration speed, whereas SAIDI reflects overall system reliability in terms of total downtime per customer. For instance, a system with frequent short outages might show a low CAIDI but high SAIDI if the total duration accumulates significantly.^[22] Compared to SAIFI (System Average Interruption Frequency Index), CAIDI addresses the duration aspect of outages, focusing on how long restoration takes once an interruption occurs, while SAIFI measures the frequency of sustained interruptions per average customer, counting events without regard to their length. Thus, a utility might achieve a low SAIFI through preventive measures but still have a high CAIDI if restoration processes are slow during the rare events that do happen. Notably, CAIDI can be derived as SAIDI divided by SAIFI, linking the metrics but highlighting their distinct emphases on resolution time versus occurrence rate.^[22] CAIDI also excludes momentary interruptions—defined as those lasting less than 5 minutes—focusing solely on sustained outages, in opposition to MAIFI (Momentary Average Interruption Frequency Index), which tracks the frequency of these brief events per customer. This distinction ensures CAIDI evaluates long-term service restoration, while MAIFI addresses transient reliability issues like flickers from reclosers, allowing utilities to prioritize different aspects of power quality.^[4] Relative to AIFI (Average Interruption Frequency Index), which counts the total number of sustained interruptions per customer without weighting by duration and often applies to specific districts or circuits, CAIDI is inherently duration-oriented, emphasizing average restoration time rather than mere counts. AIFI provides a localized frequency snapshot, but CAIDI's focus on time-to-restore enables targeted improvements in operational response.^[22] Overall, CAIDI's unique value lies in guiding utilities to enhance restoration efficiency, complementing frequency and total-duration metrics by revealing bottlenecks in outage resolution rather than outage prevention or aggregate impact.

Applications in Power Systems

Utility Reporting and Benchmarking

Electric utilities in the United States annually calculate and report CAIDI as part of mandatory submissions to regulatory bodies, such as the U.S. Energy Information Administration (EIA) via Form EIA-861, which collects detailed reliability metrics including outage durations and customer impacts.^[9] These reports enable national aggregation of data, with utilities also incorporating CAIDI into their internal and public annual reliability reports to demonstrate service performance and compliance with industry standards.^[23] For instance, California utilities submit annual electric system reliability reports to the California Public Utilities Commission (CPUC), detailing CAIDI alongside other indices to track system-wide and divisional performance.^[24] Benchmarking against industry standards allows utilities to evaluate their CAIDI performance relative to peers and national norms. According to the EIA's Electric Power Annual 2023 (reflecting 2023 operations), the national average CAIDI excluding major event days stands at 118.5 minutes using any reporting method, providing a baseline for comparisons.^[9] The IEEE Distribution Reliability Working Group's 2022 benchmarking survey of 2021 data reports a median CAIDI of 121 minutes under IEEE methodology (excluding major events), with variations across utilities highlighting opportunities for improvement in restoration efficiency; more recent 2024 survey data for 2023 operations shows a median of 119 minutes.^[25]^[26] State-specific benchmarks, such as those in California, emphasize exclusion of major events with a restoration benchmark of 570 minutes for CAIDI during such periods, aiding utilities in contextualizing performance against regional averages like the Southwest's.^[22] Internally, utilities leverage CAIDI data to pinpoint areas of prolonged outages, such as rural overhead lines prone to vegetation interference or weather-related faults, guiding targeted infrastructure investments like automated fault isolation or enhanced tree-trimming programs.^[17] High-CAIDI zones, often identified through spatial analysis of outage patterns, inform decisions on upgrading urban versus rural networks to balance restoration times and reduce overall customer impact.^[27] These analyses support proactive capital allocation, ensuring investments yield measurable reductions in average interruption durations. Integration of Geographic Information Systems (GIS) and Outage Management Systems (OMS) enables real-time CAIDI tracking by combining spatial outage data with automated notification and restoration workflows.^[28] GIS provides locational accuracy for fault analysis, while OMS aggregates customer interruption data to compute preliminary CAIDI estimates during events, facilitating faster decision-making and post-event refinements.^[29] Such tools enhance accuracy in reliability reporting and support dynamic monitoring of CAIDI trends across feeders or districts. A representative case involves Habersham Electric Membership Corporation (EMC), which implemented smart grid upgrades including advanced metering infrastructure and distribution automation, achieving a CAIDI of 77.53 minutes in 2024—below the national average—through improved fault detection and isolation that expedited restorations.^[30] This initiative demonstrates how targeted smart grid enhancements can lower CAIDI by optimizing response times in high-risk areas.

Regulatory and Performance Standards

In the United States, oversight of CAIDI primarily occurs at the state level through Public Utility Commissions (PUCs), which mandate reporting and set performance benchmarks for electric distribution utilities to ensure reliable service. The Federal Energy Regulatory Commission (FERC) provides high-level coordination for interstate reliability but delegates distribution-level metrics like CAIDI to state regulators, while the North American Electric Reliability Corporation (NERC) focuses on bulk power system standards that indirectly influence distribution practices. Utilities must submit annual CAIDI data to PUCs, often excluding major events, to demonstrate compliance with service quality rules.^[31]^[32] State-specific performance targets for CAIDI are established during rate case proceedings, with penalties applied for exceedances and incentives for superior performance. In New York, the Public Service Commission sets utility-specific CAIDI performance targets during rate cases, tailored to historical performance and infrastructure needs, and imposes financial penalties—totaling $28.9 million across five utilities in 2024—for failing customer service metrics including CAIDI; conversely, rebates or revenue adjustments are available for superior performance, encouraging faster restoration efforts.^[33]^[34] These mechanisms align utility investments with reliability goals, as seen in rate cases where targets are tailored to historical performance and infrastructure needs. Internationally, CAIDI aligns with guidelines from organizations like the International Electrotechnical Commission (IEC) and the International Council on Large Electric Systems (CIGRE), which promote standardized reliability indices for power systems. IEC standards reference CAIDI alongside SAIDI and SAIFI to evaluate interruption durations, informing global best practices for distribution networks. In the European Union, Directive 2019/944 emphasizes consumer protection through reliable supply, with regulators using CAIDI-like metrics in benchmarking reports to monitor distribution system performance and enforce minimum quality standards across member states.^[35]^[36] Compliance with CAIDI standards involves regular audits by PUCs or equivalent bodies, where utilities submit verified data for review, often including adjustments for uncontrollable factors like severe weather. Major event exemptions, defined per IEEE 1366 thresholds (e.g., days exceeding three times the annual SAIDI average), allow exclusion of outlier outages from performance calculations to focus on controllable reliability. Regulators may grant case-by-case adjustments during audits if utilities demonstrate proactive mitigation, ensuring fair evaluation.^[37] The emphasis on CAIDI reporting has driven significant investments in grid infrastructure, particularly following the 2003 Northeast blackout, which affected 50 million customers and prompted stricter U.S. reliability mandates under the Energy Policy Act of 2005. Enhanced reporting requirements post-blackout led to improved restoration protocols and resilience measures, reducing average interruption durations and bolstering overall system robustness against cascading failures.^[38]^[39]

Limitations and Considerations

Measurement Constraints

The measurement of CAIDI relies heavily on accurate reporting of customer interruptions and durations, but data inaccuracies often arise from estimated customer counts during outages, particularly when outage management systems lack real-time visibility into affected populations. For instance, when direct data is unavailable, utilities may estimate the number of impacted customers using averages from broader areas like ZIP codes, which can introduce errors in the total customer-minutes interrupted (CMI) numerator.^[40] Additionally, underreporting is common in remote or rural areas without advanced metering infrastructure, where customer notifications or automated detection are limited, leading to incomplete outage logs and understated CAIDI values.^[17] Threshold definitions for sustained outages introduce further biases, as the cutoff for what constitutes a reportable event—typically five minutes per IEEE Standard 1366—can vary across utilities and jurisdictions, ranging from one to five minutes in practice. This inconsistency hampers comparability of CAIDI metrics between utilities, as shorter thresholds may classify more brief events as sustained, inflating durations. Further skewing the denominator of affected customers. As an average metric, CAIDI masks underlying disparities in outage experiences, such as prolonged interruptions in rural areas compared to shorter urban ones, by aggregating systemwide data without weighting for geographic or demographic differences. For example, system averages may obscure cases where a small fraction of circuits account for disproportionately long outages, failing to capture severity variations or economic impacts like lost productivity. This averaging effect can also mislead when reliability improvements reduce outage frequency more than duration, paradoxically increasing CAIDI despite overall gains.^[41]^[40] External factors like severe weather or cyber incidents can inflate CAIDI without reflecting core operational performance, as these events contribute to longer restoration times that are difficult to isolate. A single major storm, for instance, can dominate annual calculations by extending outage durations across large customer bases, prompting utilities to apply adjustments such as major event day exclusions under IEEE Standard 1366 to normalize values for regulatory comparison.^[22]

Modern Adaptations and Alternatives

In contemporary power systems, adaptations to the Customer Average Interruption Duration Index (CAIDI) increasingly integrate distributed energy resources (DER), such as solar microgrids, to mitigate outage durations. These resources provide localized backup power during grid disturbances, effectively reducing the average restoration time per affected customer by enabling faster recovery in isolated segments. Similarly, the incorporation of DER like distributed renewable energy resources (DRER) and smart devices (SD) in smart grids yields higher overall reliability, with CAIDI benefiting from reduced dependency on centralized restoration processes.^[42] Real-time CAIDI estimation via Internet of Things (IoT) sensors supports predictive restoration by enabling continuous monitoring of grid conditions and outage progression. IoT-enabled predictive maintenance uses real-time data from sensors to forecast equipment failures, allowing utilities to preemptively address issues that prolong interruptions and thus lower CAIDI values.^[43] This approach shifts from reactive to proactive management, with tools like AI analytics processing IoT inputs to optimize restoration times.^[44] Weighted variants of CAIDI address traditional limitations by incorporating factors such as customer impact to reflect disparities in outage experiences.^[45] Emerging equity-focused approaches in reliability assessments prioritize vulnerable customer segments, such as low-income or medically dependent households, to ensure fairer evaluations of service impacts.^[46] Alternatives to CAIDI include Customer Minutes of Interruption (CMI), which tracks total interruption minutes across all customers for more granular outage impact analysis rather than averages. CMI serves as a foundational component for calculating SAIDI and offers detailed visibility into cumulative effects, aiding in targeted improvements without the averaging that can mask disparities in CAIDI. AI-enhanced indices combine CAIDI with frequency metrics like SAIFI to create holistic scores, using machine learning to predict and optimize overall system performance. Post-2020 trends emphasize resilience in reliability metrics. Blockchain technology is emerging for transparent outage reporting, enabling secure, immutable logging of interruption data to verify CAIDI calculations and enhance stakeholder trust in utility performance.^[47]

References

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[PDF] Distributional Equity Analysis for Energy Efficiency and Other ...
Distributional equity analysis focuses on identifying and estimating the distribution of costs and benefits across different customer types. This analysis can ...
[48]
How software and AI bolster grid resilience - Utility Dive
Aug 26, 2024 · The enhanced program reduced customer interruptions by nearly 50 percent and performed significantly better across all reliability indices, ...
[49]
[PDF] Blockchain for Fault-Tolerant Grid Operations
This section identifies how utilities currently deal with fault conditions and presents insights in terms of what is needed to make the power distribution ...
[50]
[PDF] PG&E Wildfire Mitigation Plan R1 Volume 2 of 2
Jul 28, 2025 · Fuel density. Mass of fuel (vegetation) per area that could combust in a wildfire. ... (CAIDI). CAIDI measures the average duration of a ...