The chair of the Public Utilities Regulatory Authority says that Eversource Energy badly underestimated the threat of Tropical Storm Isaias by preparing for between 125,000 and 380,000 outages, while more than 800,000 of its customers lost power at the peak and nearly 600,000 still were in the dark Wednesday night. PURA is opening an investigation of Eversource and the state’s other power company, United Illuminating, at the request of Gov. Ned Lamont, said Marissa P. Gillett, the chair of the regulatory authority. Eversource, the state’s largest provider of electricity, already is the target of consumer ire over a rate hike and sharply higher electric bills while people are home during the pandemic, and because of past responses to weather events.