Grid reliability indices — SAIDI, SAIFI, CAIDI, and MAIFI — are the four numbers that should be driving your backup power investment decisions, but most commercial and industrial operators have never seen their own circuit-level data. Your utility publishes a reliability figure. That figure is a population-weighted average stripped of its worst days, calculated for a customer class dominated by residential meters. Your facility is not the average customer. Your loss function is not the average customer's loss function.

This is Energy Decision #32 in the complete C&I energy management series from Tactical Energy Group. If you are a plant manager, facility manager, operations executive, or finance leader at a manufacturer, hospital, cold storage operation, or large commercial building, and you are evaluating resilience capital or trying to determine whether your grid can support long-term growth — this post is built for you. By the end, you will know what each index actually measures, where the reporting gaps are, and how to translate these numbers directly into defensible capital decisions.

What Grid Reliability Indices Actually Are

Four indices define how utilities measure and report grid reliability. Each measures something different. All four are relevant to your facility, but for different decisions.

SAIDI — System Average Interruption Duration Index measures total outage duration per average customer per year, expressed in minutes. A SAIDI of 120 minutes means the average customer on that system loses two hours of power annually. Lower is better.

SAIFI — System Average Interruption Frequency Index measures how often sustained outages occur. A SAIFI of 1.5 means the average customer experiences one to two outages per year. Lower is better.

CAIDI — Customer Average Interruption Duration Index is SAIDI divided by SAIFI. If your utility reports SAIDI of 120 and SAIFI of 1.5, CAIDI is 80 minutes. That is how long the average outage lasts once it starts. CAIDI is a restoration speed metric — it tells you how quickly your utility gets power back after a fault, not how frequently faults happen or how long you lose per year in aggregate.

MAIFI — Momentary Average Interruption Frequency Index measures short-duration interruptions, typically those lasting under five minutes. These events do not count toward SAIDI or SAIFI in standard utility reporting, but they matter to your operation for reasons we will get to shortly.

For every index, a lower number is better. When your utility hands you a number, it is important to know which of these four you are looking at — and under what reporting conditions that number was calculated.

Why Utilities Report These Numbers — and Where the Reporting Breaks Down

Utilities report grid reliability indices to state regulators under frameworks that generally follow the IEEE 1366 standard, which defines how outages are counted, categorized, and excluded. The intent is valid: provide a consistent benchmark so regulators can compare utility performance over time and across systems.

The problem for your operation is not the indices themselves — it is two specific gaps in how they are reported.

Gap 1: Major Event Day exclusion. A Major Event Day, or MED, is a day when severe storms or natural disasters cause widespread outages outside normal operating conditions. IEEE 1366 defines a statistical threshold for what qualifies. Utilities report two sets of numbers: one with MEDs included, one without. The EIA reports 2024 national averages excluding MEDs at a SAIDI of 123.6 minutes, a SAIFI of 1.5, and a CAIDI of 131.6 minutes.

Here is why that distinction matters for your facility: the MED-excluded number is the number utilities typically lead with in public reporting, in presentations to regulators, and in conversations with customers. But Major Event Days are exactly the events most likely to cause extended outages at your facility — the kind that run six, twelve, or twenty-four hours and stop production for a shift or more. Consider one major regional utility that carries a five-year average SAIDI of approximately 5.1 hours per customer per year when MEDs are included. The gap between that utility's MED-included and MED-excluded SAIDI is the storm exposure that standard reporting obscures — and at many utilities, the MED-inclusive number is a multiple of the number they lead with.

When your utility presents a reliability number, your first question should always be: is that with or without Major Event Days?

Gap 2: MAIFI — the momentary interruption blind spot. Outages under five minutes do not count toward SAIDI or SAIFI. From a residential customer's perspective, a two-second power flicker is a minor annoyance — the TV blinks off and comes back. From your facility's perspective, a two-second interruption can stop a CNC machine, trip a variable frequency drive, or corrupt a PLC process the same as a three-hour outage in terms of production loss and restart cost.

There is a tradeoff worth understanding here. Utilities install reclosers on feeders to convert what would have been sustained outages — outages that count toward SAIDI — into momentary interruptions that clear the fault and restore power quickly. That is a genuine grid reliability improvement on the SAIDI and SAIFI metrics. But the tradeoff is more MAIFI events. Your utility's SAIDI score can improve while the number of events that trip your sensitive equipment goes up. If your feeder has recently had reclosers installed, ask your utility whether MAIFI data is available for your circuit and evaluate your equipment's ride-through capability against it.

When Grid Reliability Indices Help You Make Decisions — and When They Hide Your Real Exposure

Grid reliability indices are genuinely useful tools when you know what they do and do not measure. They are damaging when you treat the headline number as a complete picture.

Where these indices help:

• Setting a baseline for capital justification. If your utility reports SAIDI of 180 minutes excluding MEDs while the national average is 123.6 and best-in-class utilities run under 60, you have a defensible data point that your service quality is below standard — and that the gap is not closing on its own. That benchmark gap belongs in your business case for backup power or on-site generation.
• Sizing backup generator runtime requirements. SAIDI tells you your annualized outage duration exposure. CAIDI gives you the average duration per event — a starting point for sizing, not a design target. Roughly half of sustained outages will exceed CAIDI, so backup autonomy should be sized to a higher percentile of the outage duration distribution, or to a worst-credible-event assumption tied to your MED-inclusive SAIDI data.
• Sizing UPS ride-through for sensitive equipment. MAIFI is your primary driver here. If your utility has reclosers on your feeder and MAIFI data is available, that data tells you how long and how often your sensitive equipment needs to ride through without a hard shutdown.
• Comparing utility performance over time. If you track your utility's SAIDI and SAIFI across a five-year rolling window — both MED-inclusive and MED-exclusive — you can identify whether reliability is trending up or down. A utility whose MED-exclusive SAIDI is rising year over year is signaling infrastructure deterioration that is unlikely to reverse without a major capital program.

Where these indices hide your real exposure:

• System averages conceal feeder-level performance. Your utility's reported SAIDI and SAIFI are system-wide population averages. Your service point sits on a specific feeder that may perform two or three times worse than the system average — or better. The headline number tells you nothing about your specific exposure unless you have circuit-level data.
• MED exclusion understates your worst-case production loss. The events that cause multi-shift production shutdowns are predominantly Major Event Days. Using MED-excluded SAIDI to size your backup autonomy or model your annual outage cost systematically underestimates the tail risk you are actually hedging against.
• MAIFI is invisible on standard reports. If your utility does not publish MAIFI data and you have not requested it specifically, you have no visibility into the event class that most affects VFDs, PLCs, and other sensitive equipment — and no basis for evaluating whether your current UPS and ride-through capabilities are adequate.

What Best-in-Class Performance Actually Looks Like

A SAIFI below 1.0 is generally considered strong utility performance. Best-in-class utilities reach SAIFI scores in the range of 0.5 to 0.8. United Power achieved a SAIFI of 0.65 in 2023 with SAIDI consistently under 60 minutes — less than half the national average of 123.6.

That benchmark is useful in two ways. First, it tells you what the electric utility industry is capable of delivering under normal operating conditions, which calibrates your expectations and helps you identify whether your utility is a laggard or a leader. Second, it gives you a realistic upper bound for the reliability improvement you could expect from utility-side investments — and therefore a floor for how much resilience work you still need to do on your side of the meter regardless of utility performance.

The capital justification math works like this. Start with your facility's hourly cost of an outage — lost production, spoilage, restart costs, labor, any downstream penalties. Multiply by your expected annual outage hours based on MED-inclusive SAIDI. That is your annualized outage cost exposure. The maximum justifiable capital investment in backup power or on-site generation is a function of how many years of avoided losses it takes to recover the investment at your cost of capital. The benchmark gap between your utility's actual performance and best-in-class performance tells you which assumptions are conservative versus aggressive when you build that model.

Vendor Pitches, Red Flags, and Questions That Smoke Out BS

If you are evaluating backup power, a battery energy storage system, or a microgrid, the vendor's proposal will include a reliability assumption. That assumption is where the math gets slippery.

Red flags to watch for:

• The vendor uses the utility's MED-excluded SAIDI to model your annual outage risk. This systematically understates your exposure and makes the investment look less justifiable than it actually is — convenient if the vendor is trying to simplify the pitch but damaging if you rely on it to make a capital decision.
• The vendor sizes backup autonomy to CAIDI rather than to a higher-percentile outage duration. CAIDI is the average restoration time. Sizing to the average means your backup runs out during roughly half of all outages that actually occur.
• The vendor has no circuit-level data. If the proposal uses system-wide reliability figures and no one has requested or reviewed your feeder's specific history, the reliability assumption in the model is a guess.
• MAIFI is not addressed. For any facility with sensitive equipment — CNC, VFDs, PLCs, medical devices, temperature-controlled storage — a backup power proposal that does not address momentary interruptions is incomplete.

Questions to ask in every vendor or utility meeting on this topic:

• What SAIDI figure are you using in this model — and is that with or without Major Event Days included?
• What is the basis for the backup autonomy duration you specified? Which percentile of the outage duration distribution does that cover?
• Do you have circuit-level reliability data for our specific feeder, or is this modeled from system-wide averages?
• Has anyone reviewed MAIFI data for our feeder and evaluated our equipment's ride-through capability against it?
• If our utility has installed reclosers on our circuit, how does that affect the MAIFI assumption in this proposal?

How to Request Circuit-Level Reliability Data From Your Utility

You are entitled to request circuit-level reliability data from your utility. Most utilities maintain this data internally as part of their outage management systems. Getting it may require a formal written request to your account representative or to the utility's commercial and industrial services team — it is often not published and may require some persistence to obtain.

Request specifically:

• SAIDI and SAIFI for your feeder, reported separately from system-wide averages, for the most recent five years.
• Both MED-included and MED-excluded figures.
• MAIFI data for your feeder if available.
• Recloser operation counts on your feeder if reclosers are installed.

Once you have circuit-level data, compare it to the system-wide reported averages. If your feeder performs materially worse than the system average, that gap is the number that belongs in your capital request — not the headline figure your utility publishes.

If your utility cannot or will not provide circuit-level data, that is itself a data point. It means your reliability investment decisions are being made without the information that most directly describes your exposure.

What You Can Do This Week

You do not need to wait for a vendor proposal or a capital planning cycle to start closing the information gap. Here are four concrete actions you can take this week:

1. Ask your utility for MED-inclusive SAIDI. Call or email your account representative and request both MED-included and MED-excluded SAIDI for the most recent five years. If they cannot provide it, escalate the request in writing.
2. Request circuit-level reliability data. Ask for SAIDI, SAIFI, and MAIFI specific to your service point's feeder — not system-wide averages.
3. Calculate your facility's hourly outage cost. Pull your fully loaded production cost per hour, include spoilage risk, restart costs, and any downstream penalties. Multiply by your MED-inclusive annualized outage hours. This is the exposure number your capital request needs to defend against.
4. Evaluate your sensitive equipment's ride-through capability. Identify which equipment — VFDs, PLCs, CNC machines, temperature-controlled storage — is vulnerable to momentary interruptions under five minutes. Ask your maintenance team whether current UPS coverage addresses those assets.
5. Compare your utility's reported SAIDI to the 2024 national average of 123.6 minutes (MED-excluded) and to best-in-class performance under 60 minutes. If your utility's number is above 123.6 MED-excluded, that context belongs in every resilience conversation you have internally.

The Bottom Line on Grid Reliability Indices

Grid reliability indices — SAIDI, SAIFI, CAIDI, and MAIFI — are the right starting point for any resilience capital decision. But the number your utility reports in its annual performance summary is not your number. It is a population-weighted average that excludes the worst days, aggregates across your entire service territory, and treats your facility the same as a residential meter.

Your actual exposure is determined by three things the standard report does not show you: your feeder's specific performance history, your utility's MED-inclusive outage record, and the MAIFI data that captures the momentary interruptions your sensitive equipment cannot tolerate.

The single most important concept here: the gap between the number your utility reports and the number that reflects your actual exposure is the gap your capital investment needs to close. Close the information gap first. Request circuit-level data, get both MED sets, and calculate your facility's actual annualized outage cost before you evaluate any proposal.

If you are looking for a deeper framework on what to do once you have these numbers, the post on C&I backup power strategy for diesel, natural gas, and battery systems walks through how to match backup architecture to your specific risk profile. And if your resilience concerns extend to full grid independence, the C&I microgrid framework for industrial facilities covers the business case requirements in detail.

Frequently Asked Questions: Grid Reliability Indices (SAIDI, SAIFI, CAIDI, MAIFI)

Q: What does SAIDI mean for my facility?

A: SAIDI — System Average Interruption Duration Index — measures how many total minutes per year the average customer on your utility's system loses power. A SAIDI of 123.6 minutes means the average customer experiences roughly two hours of outages annually. Your facility's actual exposure may be higher or lower depending on which feeder you sit on and whether you are looking at MED-included or MED-excluded figures — the system average tells you nothing about your specific circuit.

Q: Why does my utility report two SAIDI numbers — one with Major Event Days and one without?

A: IEEE 1366 allows utilities to exclude Major Event Days — days when severe weather or disasters cause abnormally widespread outages — from their standard SAIDI calculation. Utilities report both figures, but the MED-excluded number is the one they typically lead with because it is lower. For your facility, the MED-inclusive figure is the more relevant number for business continuity planning, because Major Event Days are exactly the events most likely to cause extended multi-shift production shutdowns.

Q: What is the national average SAIDI in 2024?

A: The EIA reports 2024 national average SAIDI at 123.6 minutes excluding Major Event Days, with a national average SAIFI of 1.5 and CAIDI of 131.6 minutes. Best-in-class utilities achieve SAIDI consistently under 60 minutes with SAIFI in the 0.5 to 0.8 range. If your utility is above 123.6 MED-excluded, it is performing below the national average and that gap belongs in your resilience capital justification.

Q: What is MAIFI and why doesn't it show up on my utility's standard reliability report?

A: MAIFI — Momentary Average Interruption Frequency Index — measures interruptions that last under five minutes. IEEE 1366 excludes momentary interruptions from SAIDI and SAIFI calculations, so they do not appear on standard reliability reports. But a two-second power flicker can trip a variable frequency drive, stop a CNC machine, or corrupt a PLC process the same as a multi-hour outage in terms of production loss. If your feeder has reclosers installed, MAIFI events may be increasing even as your utility's SAIDI score improves — request MAIFI data specifically and evaluate your equipment's ride-through capability against it.

Q: How do I use SAIDI and CAIDI to size a backup generator or UPS?

A: SAIDI gives you your annualized outage duration exposure — use the MED-inclusive figure for conservative sizing. CAIDI gives you the average outage duration once an event starts, which is a starting point for generator runtime sizing, not a design target. Because roughly half of sustained outages will exceed CAIDI, backup autonomy should be sized to a higher percentile of the outage duration distribution or to a worst-credible-event assumption derived from your MED-inclusive SAIDI data. UPS sizing for sensitive equipment should be driven by MAIFI data, which tells you how long and how often momentary ride-through capability is required.

Q: How do I request circuit-level reliability data from my utility?

A: Contact your utility account representative in writing and request SAIDI, SAIFI, and MAIFI data specific to your service point's feeder — not system-wide averages — for the most recent five years, with both MED-included and MED-excluded figures. Most utilities maintain this data internally through their outage management systems; it is not typically published and may require a formal request or some persistence to obtain. If your circuit performs materially worse than the system-wide average, that feeder-level figure is the number that belongs in your capital request.

If you are an Indiana C&I operator spending five figures or more on electricity each month and you are actively evaluating a resilience project, get your free TEG Energy Decision Blueprint — built specifically for Indiana C&I operators who are in energy improvement mode. We pull your data, review your rate and operational realities, and give you our full take in writing with no obligation.

Watch this episode of The TEG Podcast on YouTube: Grid Reliability Indices (SAIDI, SAIFI, CAIDI, MAIFI) Explained for C&I Operators