Unexpectedly mild temperatures in Boston on Saturday left New England’s power grid with a large excess of power, sending electricity prices plummeting to roughly -$150 a megawatt-hour and forcing generators to pay consumers to take their power.

Temperatures in Boston climbed to a high of just 71 degrees Fahrenheit on Saturday, according to the National Weather Service, roughly 20 degrees below the day’s original forecast, which projected temperatures reaching into the mid-90s, in keeping with the rest of the region.

But cloud cover and a late-in-the-day incoming breeze from the Atlantic protected Boston from feeling the effects of the blistering heat wave that scorched much of the Northeast.

According to data from grid operator ISO New England, the lower-than-expected temperatures caused spot power to drop to negative levels around 10:30 a.m., where they remained until almost 2:00 p.m.

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“Weather forecasts had projected temperatures in the mid-90s for the Boston area, and those temperatures did not materialize,” Matthew Kakley, a spokesman for ISO New England, told Bloomberg News in an interview. “This led to electricity demand coming in well under forecast.”

That prompted New England’s control room operators to issue an unprecedented “minimum generation” emergency alert to generators, Kakely added, asking them to dial back their supply to keep pace with demand.