Solar flares are classified according to their strength. The smallest ones are B-class, followed by C, M and X, the largest. A powerful X-class flare can create long lasting radiation storms, which can harm satellites, communications systems, and even ground-based technologies and power grids.
…. the United States, Europe, and other technologically advanced societies are currently not prepared to significantly mitigate the effects of, or effectively recover from, the potentially drastic consequences of a ‘big one’
–Steve Tracton, Washington Post