A 1972 Solar Storm Set Off 4,000 Sea Mines

Faraday cages work because charges rearrange to counter electric fields and currents are induced to counter magnetic field changes. In the real world, metals have electrical resistance so these effects can be slowed down in a sense, so some fields get through. Thicker metal is like having multiple layers and hence multiple tries, so the result is fields decay exponentially with regards to depth into the metal. Mesh size comes into play too as it affects the resistivity, and small waves can fit through the mesh holes.

At high frequencies, a metal foil or braid is enough. At lower frequencies you need metal plating. And at the very low end, where it is basically like turning on a static magnet or carrying a box from one place to another, the induced currents that resist change of magnetic fields inside the box will decay quickly and the field soaks into the box so that the box makes very little difference (unless made of a magnetic material).

Thank you for taking the time to explain, PlasmaGryphon! I didn't know about the thickness of the metal in a faraday cage - I only thought that the size of the mesh mattered.

Pretty much any consumer/household/commercial (and industrial equipment smaller than a substation) electronics would be fine in a geomagnetic storm, as a geomagnetic storm is very different from something like an EMP that many picture or treat it as online. Induced voltage is essentially change in magnetic field times area of receiving circuit, divided by time that change happens over. An EMP happens quickly, so you're dividing by a small number, and can get a large voltage even with small area. A geomagnetic storm is very slow, and a rather small magnetic field change too, so you can only get a large voltage when dealing with a large amount of area. (I think I once worked it out as being comparable to having electronics in your pocket while walking past a fridge magnet on a table....)

So this only becomes relevant for systems many kilometers in size, they will experience an induced (relatively) DC voltage and current. A lot of systems already have specific circuit breakers installed to deal with this situation, and there is the possibility of other systems tripping before permanent damage happens even without the DC breakers. A breaker tripping from a geomagnetic storm is what caused the 1989 blackout in Quebec, and they had things working again less than a day later. There is concern over how many systems have proper protections in place and how many damaged systems can be replaced on a short timescale with given reserves of spares, but still a lot of systems have been protected for some time.

Also, very little of the induced voltages and currents would get through the transformer to your household grid connection, so really most of the risk to your personal goods would be the same as any other blackout or brownout: a slight chance of damage when power cuts.

(And to reply to Alex in the same message, a Faraday cage that can stop very slow effects would be impractically massive. The slower the changing fields are, the thicker the piece of metal you need to stop it. As the geomagnetic storms change fields over the course of hours, this looks like essentially DC compared to faraday cages that deal with stuff changing in a millionth of a second. You can somewhat block static magnetic fields with special alloys like mu metal, but it would still be pointless for anything small enough to fit inside.)

The above is specific to things near the ground, as there are extra problems for stuff in/near space, and temporary complications with ground to space communication. So in a really bad situation satellite communication systems and GPS could go down, plus there might be temporary loss of radio reflections off the ionosphere (like AM stations, line of site stuff like cell phones would be unaffected).

The solution would be to Faraday cage everything!