Regional grid operators mostly expressed confidence Friday that they could overcome reliability challenges to the electricity system, as energy generation transitions away from around-the-clock coal and nuclear to natural gas and renewables.

The nation’s federally overseen grid operators, which run about 70 percent of the U.S. power supply, faced a Friday deadline to report to the Federal Energy Regulatory Commission on the state of the electricity system’s resilience, or its ability to bounce back from a major disruption such as this year’s bomb cyclone in the Northeast.