Solar-generated attacks on the unprotected U.S. electric grid cost $10 billion a year, and they are escalating, making the system vulnerable to a catastrophic failure that could cut off electricity for months, federal officials and energy experts warned Wednesday.

In what was the most blunt assessment of security for the grid that feeds and fuels virtually every element of life in the country, the officials said the system is an easy target for solar, nuclear, and even small arms attacks from terrorists.