What are SAIDI and SAIFI?
SAIDI and SAIFI are the two most widely used measures of how reliable an electricity network is. They are used by distributors and regulators in New Zealand, Australia and internationally.
- SAIDI (System Average Interruption Duration Index) is the average total time, in minutes, that each customer on the network was without power over a period, usually a year.
- SAIFI (System Average Interruption Frequency Index) is the average number of interruptions each customer experienced over the same period.
Together they show both how often customers lose power and how long they are off for.
How they are calculated
The calculations are simple in principle:
SAIDI = total customer minutes interrupted / total customers servedSAIFI = total customer interruptions / total customers served
For example, if a network serves 100,000 customers and an outage cuts power to 2,000 customers for 90 minutes, that event adds 180,000 customer minutes. Divided across the whole customer base, it adds 1.8 minutes to SAIDI and 0.02 to SAIFI.
A related measure, CAIDI (Customer Average Interruption Duration Index), is SAIDI divided by SAIFI. It shows the average length of an interruption for customers who had one.
Most networks only count sustained interruptions above a minimum length in these figures, with very short interruptions (momentary interruptions) tracked separately. The threshold depends on the reporting rules that apply.
Planned, unplanned and major events
Reliability is usually reported separately for planned and unplanned outages, because they mean different things. Planned SAIDI reflects how much scheduled maintenance and upgrade work a network is doing; unplanned SAIDI reflects the resilience of its assets and how quickly it restores faults.
Major storms can produce a single day with more outage minutes than the rest of the year combined. Regulators commonly use methods to identify and normalise these major events, so one extreme storm doesn’t distort the measure of underlying performance. The exact method depends on the regulatory regime.
How SAIDI and SAIFI are used
- Regulation. In New Zealand, the Commerce Commission uses reliability measures in the quality standards it sets for regulated electricity distribution businesses. In Australia, the Australian Energy Regulator uses them in service performance incentives for DNSPs. The detail of the targets and incentives varies by network and regulatory period.
- Asset management. Networks use SAIDI and SAIFI by feeder or area to decide where to invest in vegetation management, automation or undergrounding.
- Public reporting. Reliability figures are disclosed so customers and stakeholders can see how networks perform over time.
Where the contact centre fits
The raw data for SAIDI and SAIFI comes from outage records in the outage management system: when each interruption started, when supply was restored and how many customers were affected.
The contact centre influences those numbers in practical ways. On low-voltage parts of the network, a customer call to the fault line may be the first sign of a fault. Fast answering, accurate location details and good fault triage help the control room find faults sooner and send field crews to the right place, which shortens outages. Accurate logging of when customers reported loss of supply also supports accurate reliability reporting.