Occasionally, a large solar storm can rain energy down on the earth, overpowering electrical grids. About once a century, a giant pulse can knock out worldwide power systems for months or even years. It's been 90 years since the last super storm, but scientists say we are on the verge of another period of high solar activity. Significant storms have hit earth several times over the last 150 years, most notably in 1859 and 1921. Those occurred before the development of the modern power grid; recovering from a storm that size today would cost up to $2 trillion a year for several years. Storms don't have to be big to do damage. [A] storm in 2003 caused a blackout in Sweden and fried 14 high-voltage transformers in South Africa. The storm was relatively weak, but by damaging transformers it put parts of the country off-line for months. That's because high-voltage transformers ... are the most sensitive part of a grid; a strong electromagnetic pulse can easily fuse their copper wiring, damaging them beyond repair. Even worse, transformers are hard to replace. They weigh up to 100 tons, so they can't be easily moved from the factories in Europe and Asia where most of them are made; right now, there's already a three-year waiting list for new ones.