As a heat wave bakes the Northeast, demand is soaring for electricity that’s powering air conditioners, pressing New England’s “relatively small” electricity surplus, according to the region’s grid operator. ISO on Friday reported a capacity of 26,690 megawatts and an excess of 95 megawatts. The capacity is down slightly from Thursday and the surplus fell from 293 megawatts. Peak demand was forecast Friday at 23,750, up slightly from 23,318 megawatts Thursday. Though high, it’s not among the top 10 days when demand was greater on several July and August days in 2005, 2006, 2010, 2011 and 2013, according to ISO. ISO says if generator or transmission line outages were to occur, the grid operator could call on resources held in reserve, import emergency power from neighboring regions, ask businesses and residents to conserve electricity or put in place emergency procedures. Temperatures have topped 90 degrees in the Northeast since earlier this week and are expected to exceed the mid-90s Friday and into the weekend.