If Yellowstone erupts: ash, a dimmer sun, and your power

How likely, honestly

Very unlikely. USGS approximates the yearly chance of another giant, caldera-forming eruption at about 1 in 730,000. The buildup would be detectable weeks, months or even years ahead, through strong earthquake swarms and fast ground movement. The more likely events at Yellowstone are steam explosions and lava flows, which stay inside the park area (USGS).

So this page plans for something rare. It's here because people ask, and because the answer is useful for any big ash event.

What reaches you

Ash, thinning with distance. USGS modeling found decimeters to meters in the northern Rockies, centimeters to decimeters in the northern Midwest, and millimeters to centimeters on the East, West and Gulf Coasts (Mastin et al. 2014, USGS).
Power outages from the ash itself. Wet ash conducts electricity. Under 3 mm of it can short the insulators on power lines (USGS). After Mount St. Helens in 1980, the Northwest's main grid operator had 25 sustained outages in the first 10 days (USGS).
A dimmer sun for a year or two. Eruption haze cuts direct sunlight. After Pinatubo in 1991, California's big solar thermal plants ran about 30% below normal the next winter and spring (DOE OSTI). Global cooling from even a super-eruption is now thought to be no more than about 1.5 °C, against about half a degree after Pinatubo (NASA).
Weight on roofs. An inch of ash weighs 5–10 pounds per square foot dry and 10–15 wet. Collapse is rare and usually takes over 100 mm (about 4 inches) (USGS).

What it means for your power

The run above is a planning case, not a forecast: no modern grid has been through this. It assumes two weeks without the grid as the usual case and six months as the worst. It assumes three days of ash on your panels at the start, then 30% less sun all the way through, which is Pinatubo's loss in direct sunlight. Flat panels lose less than that, because they also use diffuse light, so it's a cautious number.

What helps:

Brush or rinse ash off the panels. Ash blocks the light.
More panels, not just more batteries. A dimmer sun for months is a panel problem: batteries cover the dark days, panels cover the dim months. The run's "what to add" shows the cheapest mix for your setup.
Off the grid tie. A system that runs your house on its own keeps working while ash shorts the lines. Grid-tied solar shuts off with the grid (more on that).
The basics: water and food for two weeks or more, and NIOSH-approved N95 respirators for anyone working outside or cleaning up ash (CDC).

Common questions

How likely is a Yellowstone super-eruption?

USGS approximates the yearly chance of another caldera-forming eruption at about 1 in 730,000. The buildup would be detectable weeks, months or years ahead through earthquake swarms and rapid ground deformation. The more likely events are steam explosions and lava flows, which stay local.

How much ash would reach my area?

USGS modeling found decimeters to meters in the northern Rockies, centimeters to decimeters in the northern Midwest, and millimeters to centimeters on the East, West and Gulf Coasts.

Would solar panels still work after a big eruption?

Yes, with less output. Ash has to be brushed or rinsed off, and a big eruption dims the sun for a while: after Pinatubo in 1991, California solar thermal plants ran about 30% below normal the next winter and spring. Flat panels lose less than that, because they also use diffuse light.

Would the power go out far from Yellowstone?

Likely, in places. Wet volcanic ash conducts electricity: under 3 mm can short power-line insulators. After Mount St. Helens in 1980, the Northwest's main grid operator had 25 sustained outages in the first 10 days.