Solar Storms and the Power Grid: What a Geomagnetic Storm Can Do

Every so often the Sun hurls a cloud of magnetized plasma toward Earth, auroras appear far from the poles, and headlines ask whether the grid will survive. The honest answer is that severe geomagnetic storms are a real but manageable hazard. They have caused one famous large blackout, in Quebec in 1989, and the grid has since been tested by the strongest storm in two decades without major U.S. outages. Here is what a geomagnetic storm can and cannot do.

Key facts

• NOAA rates geomagnetic storms from G1 (minor) to G5 (extreme), tied to the Kp index; G5 conditions occur about 4 times per 11-year solar cycle on average (NOAA SWPC).
• At 2:44 a.m. on March 13, 1989, a geomagnetic storm collapsed Hydro-Québec's grid in less than a minute, and the province was dark for more than nine hours (Hydro-Québec).
• May 10–12, 2024 brought the first G5 storm since October 2003. NERC found the bulk power system "remained stable and connected" and "served all electrical load" (NERC).
• Since 2015, North American grid operators have been required to have geomagnetic disturbance (GMD) operating procedures.
• Coronal mass ejections typically take several days to reach Earth; the fastest have arrived in about 18 hours (NOAA SWPC).
• Our Grid Threat Monitor shows the live Kp index and G-scale level.

What a geomagnetic storm is

According to NOAA's Space Weather Prediction Center (SWPC), the largest storms come from coronal mass ejections (CMEs), huge eruptions of solar plasma carrying their own magnetic field. Weaker but longer-lasting storms can come from high-speed streams of solar wind. A storm is strongest when the incoming magnetic field points south, opposite Earth's, for many hours.

When that happens, currents in Earth's upper atmosphere and magnetosphere swing wildly. Those changing magnetic fields induce voltages in the ground, and in anything long and conductive sitting on it, including high-voltage transmission lines and pipelines.

The NOAA G-scale and the Kp index

The planetary Kp index runs from 0 to 9 and measures the largest magnetic disturbances in each three-hour window, based on ground magnetometers. Kp 5 or above counts as a storm, and each level maps to NOAA's G-scale:

Level	Kp	Effects on power systems (NOAA)	Average frequency per 11-year cycle
G1 Minor	5	Weak power grid fluctuations can occur	1,700 (900 days)
G2 Moderate	6	High-latitude systems may see voltage alarms; long storms may cause transformer damage	600 (360 days)
G3 Strong	7	Voltage corrections may be required; false alarms on some protection devices	200 (130 days)
G4 Severe	8, including 9-	Possible widespread voltage control problems; some protective systems will trip key assets	100 (60 days)
G5 Extreme	9	Widespread voltage and protection problems; some grid systems may see complete collapse or blackouts; transformers may be damaged	4 (4 days)

Source: NOAA Space Weather Scales.

The scale describes what can happen. Whether it does depends on latitude, local geology, grid design and how operators respond.

How geomagnetically induced currents affect transformers

Geomagnetic storms push slowly varying, nearly direct currents into power lines, which SWPC describes as quasi-DC currents with periods of 10 seconds to tens of minutes. These geomagnetically induced currents (GICs) flow along transmission lines and through grounded power transformers (NOAA SWPC).

Inside a transformer, that extra current can saturate the iron core. The consequences cascade:

• Heating. Stray currents can heat internal structures and, in severe cases, damage transformer parts.
• Distorted waveforms. Saturation produces harmonics, which can make protective relays trip equipment that is working fine.
• Reactive power demand. Saturated transformers soak up reactive power, the support that keeps voltage stable.

NERC warns that during a severe storm, rising reactive power demand plus the loss of voltage-support equipment can create a "perfect storm" leading to voltage collapse (NERC). NERC identifies Canada, the northern United States and areas with resistive or complex deep-earth geology as higher-risk regions.

Three storms that define the risk

September 1859: the Carrington Event

On September 1, 1859, astronomer Richard Carrington saw "a sudden flash of intense white light" from a group of sunspots. About 17 hours later, auroras lit the night sky as far south as Panama, magnetometer needles were pinned off-scale, and surges disrupted telegraph systems (NOAA NESDIS). Natural Resources Canada records that some telegraph operators received electric shocks (NRCan). There was no power grid to damage in 1859, which is why the event is the reference point for a modern worst case.

March 1989: the Hydro-Québec collapse

In the early hours of March 13, 1989, the storm induced harmonics in Hydro-Québec's La Grande network. Static var compensators, which hold up voltage, tripped one after another. The 735 kV network separated, 9,450 MW of generation was lost, and the rest of the 21,000 MW system collapsed within 25 seconds (NRCan). NOAA says six million people were left without power for nine hours (NOAA NESDIS).

Hydro-Québec then recalibrated its protection systems, added series compensation to its lines, set up real-time storm alerts and changed operating procedures. It says later intense storms caused no problems.

May 2024: the Gannon storm

On May 10–12, 2024, at least five CMEs struck Earth, producing conditions between G3 and G5 over three days. NERC named it the Gannon Storm in honor of space weather physicist Dr. Jenn Gannon. Its January 2026 review found:

• SWPC alerted grid operators by hotline about six hours before the storm began, and operators delayed maintenance and recalled lines and voltage-support equipment in higher-risk areas.
• System voltages stayed within operating limits. One northeastern U.S. nuclear unit reduced output because of high GIC levels, and some transformer temperature alarms, a 138 kV capacitor bank trip and a manageable 2% voltage drop on part of the Mid-Atlantic 500 kV system were reported.
• The storm produced the highest peak GICs of any event in NERC's database, which goes back to 2013. The top U.S. reading was 175.7 amps, on a monitor in Wisconsin.
• The grid "served all electrical load." NERC also cautioned that the Gannon storm was less intense than the March 1989 event.

The storm still had effects beyond the grid: NASA reported that some GPS-guided tractors veered off course or stopped working, delaying planting (NASA).

Year	Event	Grid impact
1859	Carrington Event	Telegraph disruption; no power grid existed
1989	March storm	Hydro-Québec collapse, more than nine hours
2003	October "Halloween" storms	Last G5 before 2024
2024	Gannon storm (G5)	Isolated equipment effects; all load served

How the grid is protected

Regulators turned the 1989 lesson into mandatory rules:

• FERC Order No. 779 (May 2013) directed NERC to write GMD standards in two stages, first for operating procedures and then for planning assessments (FERC Order No. 830).
• Operating procedures (EOP-010). Since 2015, reliability coordinators and transmission operators have been required to use GMD operating procedures. These were the playbooks used in May 2024.
• Planning assessments (TPL-007). FERC approved TPL-007-1 on September 22, 2016 in Order No. 830. It also ordered NERC to revise the benchmark event and collect GIC and magnetometer data.
• Today's standard, TPL-007-4, applies to facilities with large transformers whose high-voltage windings exceed 200 kV, and requires vulnerability and transformer thermal assessments against benchmark and supplemental storms (NERC). NERC says the benchmark is at least 2.5 times stronger than the March 1989 storm.

Watches, warnings and alerts: how much notice?

SWPC uses three product tiers, explained by the National Weather Service:

• Watch: the risk has risen but timing and strength are uncertain. Geomagnetic storm watches go out 1 to 3 days ahead.
• Warning: an event is imminent or likely, with lead times of minutes to a few hours.
• Alert: observed conditions have crossed a threshold, such as each G-level as it is reached.

Forecasters get the clearest read on a storm's strength when a CME passes monitoring satellites stationed about a million miles upstream, but NERC notes that can leave as little as 30 minutes of notice. That is why early hotline calls, like the one in May 2024, matter. You can follow the current Kp index and G-level on our Grid Threat Monitor.

Should you worry?

Not much, and certainly not every time auroras make the news. The National Weather Service says "the vast majority" of G5 storms "will not cause catastrophic damage to the electric grid" (NWS). May 2024 was a real-world test, and the grid passed.

The tail risk is a Carrington-class storm. A NOAA education page estimates storms of 1859's scale occur about once every 500 years, though estimates vary. In 2015, NOAA cited estimates that such a storm could leave 20 to 40 million Americans facing extended outages (NOAA NESDIS). Those figures are model results, not forecasts, and they were published before today's planning standard took effect. The realistic risk for most households is a regional outage, which is the same thing you already prepare for with storms.

Practical preparation

Ready.gov's space weather guidance is plain outage preparedness:

• Build a kit and a plan. Keep water, food, flashlights and spare batteries or power banks, and set a family communication plan. Our grid-down preparedness checklist covers it.
• Keep the car's gas tank at least half full, since gas stations need power to pump fuel.
• Know the manual release on your garage door opener.
• Plan for medicines. A closed refrigerator keeps medication cold for several hours; ask a pharmacist about specifics.
• During a storm, limit electricity use, follow Emergency Alert System instructions and disconnect appliances if officials tell you to.
• If the power goes out, follow food safety rules (how long food lasts) and run any generator safely outdoors (generator safety).

Wondering how solar storms compare with man-made pulses? See EMP and the power grid: facts vs. fiction.

The bottom line

Geomagnetic storms can stress the grid, and an extreme one could cause regional blackouts or damage transformers. But storms strong enough to do that are rare, forecasters usually give hours to days of notice, and since 1989 grid operators have adopted mandatory procedures and planning standards built around worse-than-1989 scenarios. The May 2024 G5 storm, the strongest in two decades, passed with all load served. Watch the forecasts, keep a basic outage kit, and enjoy the auroras.

PowerOutageTracker.com. "Solar Storms and the Power Grid: What a Geomagnetic Storm Can Do." Accessed November 2, 2026. https://poweroutagetracker.com/guides/solar-storms-and-the-power-grid