The last time a "great geomagnetic storm" unleashed such energy on the earth (thereafter known as a CarringtonEvent for the British scientist who first detected it) was September 1, 1859.
The so-called CarringtonEvent of 1-2 September 1859 shorted telegraph wires, starting fires in North America and Europe, and caused bright aurorae (northern and southern lights) to be seen in Cuba and Hawaii.
And the effects would likely be much worse than the 1859 CarringtonEvent, a solar superstorm that wreaked havoc on telegraph lines and caused the aurora borealis to be visible as far south as Texas.