Spot electricity prices are posting record-low averages on grids covering the Northeast, mid-Atlantic and parts of the Midwest. While that’s good news for the more than 99 million Americans they serve, it’s not so good for generators that rely on the wholesale market for revenue. Daily demand touched all-time lows in some areas this month amid unusually mild weather. Access to the cheapest natural gas in the nation, as well as increased competition from hydro, wind and other renewables are additional factors.