Reliability Indices Calculator

CAIDI is calculated as SAIDI divided by SAIFI, so it measures average outage length per interruption rather than how often or how broadly outages hit customers. If a change in outage data increases both total customer-hours and total interruption events by roughly the same proportion, SAIDI and SAIFI both shift but their ratio — and therefore CAIDI — stays close to where it was, because CAIDI is only capturing outage severity per event, not frequency or reach.

The industry-standard Momentary Average Interruption Frequency Index counts brief, non-sustained interruptions (typically under 5 minutes) using separate momentary-outage data that this calculator doesn't collect as an input. What this calculator labels Maifi is simply Major Event Days divided by Total Customers Served — a rough severity signal for how storm-heavy the period was, not a count of momentary blips. Don't cite this calculator's Maifi value as MAIFI in any regulatory or reliability report.

ASAI converts average outage duration into a percentage of uptime by computing 1 minus (SAIDI divided by 8,760), where 8,760 is the total number of hours in a standard 365-day year. So a SAIDI of 87.6 customer-hours per customer works out to exactly 1% of the year without power, giving an ASAI of 99%. Because the formula assumes a fixed 8,760-hour year, it doesn't automatically correct for leap years unless you adjust the input duration accordingly.

That depends on which version of the indices you're trying to reproduce. Real-world utility SAIDI/SAIFI reporting under IEEE 1366 typically strips out major event days and reports them separately, since a few severe storms can dwarf routine performance and make the standard indices meaningless for year-over-year comparison. This calculator does not perform that exclusion for you — it computes SAIFI, SAIDI, CAIDI, and ASAI directly from whatever totals you enter, so if you want a major-event-excluded ('IEEE 1366 normal') result, you need to remove those events from your Total Customer-Hours and Customer Interruptions inputs before entering them.