When an electric power grid runs with more than 80% renewable energy, they worry that it could behave differently. No one had tried to restart a large power system like that after a blackout without bringing in outside power. The diesel generator arrived at NREL, but the agency decided to see if the model grid could be restarted without it. In today's blackouts, renewable power sources are customarily shut down and the system is restarted with outside power from traditional generators. Power pitfallsThe growth of renewable energy on the nation's three power grids is accelerating more quickly than many people think.