Power grids have been blacked out with costs running into the billions of dollars. The effects are primarily in high latitude countries like Canada. Astronauts who are outside their spacecraft can be subject to dangerous proton fluxes. As society becomes increasingly reliant on sophisticated communication and power systems, it becomes increasingly vulnerable. They monitor data in real time, and provide warnings and alerts of events.
Warnings are typically about one hour in advance of the event.
Al Ronn discussed technological aspects of the problem. The impact on the chips in geostationary satellites can be severe. Solar cells can be burned out, through arcing. The energy dumped into the ionosphere generates heat which expands the atmosphere. This can result in increased drag on satellites, which can degrade orbits.