How Weather Affects the Power Grid

Major Event Days: The Weather Signal in Reliability Data

Every utility's annual reliability numbers blend two fundamentally different experiences: normal day-to-day operations and catastrophic weather events. A utility in coastal Texas might have excellent baseline infrastructure but report terrible annual SAIDI because a single hurricane caused millions of customer-hours of outages in one week.

The IEEE Standard 1366 methodology addresses this by defining "major event days" - days when outages exceed a statistical threshold. EIA Form 861 data reports SAIDI both with and without these major event days, giving you two different views of the same utility. On PlainUtility, we show both metrics for every utility that reports them, check individual utility pages to see the spread.

Weather Events Ranked by Grid Impact

What it tells you: Different weather events affect the grid in different ways. Hurricanes and tropical storms cause the longest total outage durations because they affect large geographic areas and can damage transmission infrastructure that takes weeks to rebuild. Ice storms are particularly destructive because accumulated ice weight can snap poles and bring down miles of line simultaneously. Thunderstorms cause the most frequent individual outages but typically affect smaller areas with faster restoration.

What it doesn't tell you: SAIDI data does not break down outages by cause, you cannot determine from EIA data alone which outages were weather-related versus equipment failures, vegetation contact, or other causes. The major event day threshold is a statistical proxy that captures most severe weather impacts but may also include non-weather events and may miss moderate weather events that fall below the threshold.

How to use it: Compare SAIDI with and without major events on your utility's page. A large gap between the two numbers means weather dominates your outage experience. A small gap means infrastructure quality (good or bad) is the main driver. If weather is the dominant factor, personal preparedness (generators, battery backup) may be more impactful than switching providers.

Regional Weather Risk and Reliability

What it tells you: Geography strongly predicts reliability patterns. The Gulf Coast and Southeast face hurricane exposure. The Midwest and Northeast experience ice storms. The West Coast deals with wildfire-related shutoffs (Public Safety Power Shutoffs, or PSPS events). The Southwest experiences extreme heat that stresses equipment and triggers demand-side failures.

What it doesn't tell you: Regional averages mask significant variation within states. A utility in inland Florida may have very different major event exposure than a coastal utility in the same state. Mountain communities in the same state as urban valleys face different ice and wind risks.

How to use it: When comparing utilities across states, use nMED (no major event days) for the fairest infrastructure comparison. Use SAIDI with major events to understand your total expected outage experience in that location. The difference between the two tells you how much weather exposure to plan for.

What This Means for You

Step 1, Check both SAIDI metrics for your utility. Look at your utility's page on PlainUtility and compare SAIDI with vs without major events over the 2020-2023 period.

Step 2, Identify the weather signal. If the gap between with-MED and without-MED SAIDI is more than 100 minutes, weather is a dominant factor in your area. Personal preparedness matters more than utility choice.

Step 3, Track the nMED trend. Is your utility's baseline performance (nMED) improving, stable, or declining over 2020-2023? An improving nMED trend means the utility is investing in infrastructure regardless of weather.

Step 4, Plan for the worst case. The highest annual SAIDI across the 4-year period represents a reasonable worst-case scenario. Use this to plan backup power needs, especially for medical equipment, home offices, or food storage.

Weather Events and Grid Impact by Type

Major weather events are the single largest driver of electric utility outages in the United States. Understanding how different weather types affect the grid helps explain why SAIDI varies so dramatically between utilities in different regions.

Weather Impact on SAIDI by Event Type

Weather Event	Typical SAIDI Impact	Restoration Time
Hurricane (Category 3+)	500 ... 2,000 min	3 ... 14 days
Ice Storm	200 ... 800 min	2 ... 7 days
Severe Thunderstorm	30 ... 120 min	4 ... 24 hours
Wildfire (utility-affected)	100 ... 500 min	1 ... 5 days
Heat Wave (brownout/blackout)	10 ... 60 min	1 ... 8 hours

Worked Example: Hurricane Season SAIDI

Florida Power & Light (FPL) serves approximately 5.7 million customer accounts. During a typical hurricane season, FPL's SAIDI with major events can spike to 350 ... 500 minutes, while its nMED (non-major-event) SAIDI sits at roughly 55 ... 75 minutes. This means that roughly 80% ... 85% of FPL's total outage minutes are attributable to named storms rather than day-to-day infrastructure issues.

For a customer paying $145.00 ... $195.00/month for electricity, a multi-day hurricane outage can mean $200.00 ... $500.00 in spoiled food, hotel costs, or generator fuel, costs not captured in SAIDI metrics but directly experienced by ratepayers.

Frequently Asked Questions

What is a major event day in power reliability data?

A major event day (MED) is a day when the daily SAIDI exceeds a statistical threshold, typically 2.5 standard deviations above the utility's historical daily average. This classification, defined by IEEE Standard 1366, separates catastrophic weather events from normal operating conditions. The nMED (no Major Event Days) metric excludes these extreme days to show baseline infrastructure performance.

Which weather events cause the most power outages?

Hurricanes and tropical storms cause the longest outages, sometimes lasting weeks in affected areas. Ice storms are the second most damaging, as accumulated ice weight can bring down power lines and utility poles across entire regions. Thunderstorms with high winds cause the most frequent individual outages. Extreme heat causes grid stress and rolling blackouts but typically shorter individual outage durations.

Why does SAIDI vary so much between states?

State-level SAIDI differences reflect both weather exposure and infrastructure quality. States in hurricane zones (Florida, Louisiana, Texas) and ice storm corridors (the Midwest and Northeast) naturally experience more major event days. States with extensive underground distribution in urban cores (like Arizona, Nevada) tend to have lower baseline SAIDI. Compare utilities within the same state or climate zone for meaningful benchmarks.

Is the power grid becoming less reliable due to climate change?

The data shows increasing frequency of major event days in many regions, consistent with more severe weather patterns. However, many utilities are simultaneously investing in grid hardening, vegetation management, and underground conversion. The net effect varies by region, some utilities show improving nMED (baseline reliability) even as MED events become more frequent. PlainUtility tracks both metrics to show this distinction.

Regional Weather Exposure Patterns

A utility's weather exposure is largely determined by geography and cannot be fully mitigated by infrastructure investment. However, grid hardening programs can reduce both the frequency and duration of weather-related outages.

Utility SAIDI Comparison: Similar Size, Different Weather

Utility	SAIDI (with MED)	SAIDI (nMED)	Weather Exposure
典型 Northeast IOU	180 ... 280 min	80 ... 120 min	Winter storms, ice
典型 Gulf Coast IOU	300 ... 500 min	60 ... 90 min	Hurricanes, tropical storms
典型 Pacific NW PUD	90 ... 150 min	60 ... 80 min	Wind, moderate ice
典型 Midwest Co-op	200 ... 350 min	100 ... 160 min	Ice, thunderstorms, tornadoes

Source: EIA Form 861 reliability data. Ranges reflect 2020-2023 variation across similarly-sized utilities in each region.

Grid Hardening ROI

Utilities that invest in undergrounding power lines, vegetation management, and automated switching typically see 20% ... 40% reductions in nMED SAIDI. However, undergrounding costs $300,000 ... $2,500,000 per mile depending on terrain, making it cost-effective primarily in high-density areas. For rural cooperatives covering 3 ... 15 customers per mile of line, vegetation management programs at $5,000 ... $15,000 per mile remain the most cost-effective reliability improvement.