New England solar growth leads to increasing duck curve

In a sign of increasing penetration of rooftop and customer-sited solar projects, New England's electricity grid is starting to experience the "duck curve" phenomenon, in which a mid-day drop in grid demand is followed by a steep evening increase -- a significant shift in the timing of consumer demand relative to historic patterns.

Regional grid operator ISO New England Inc. tracks demand for electricity across its six-state New England market. On a typical day, demand is lowest in the middle of the night, rises through the morning to reach a mid-day high, declines somewhat in mid-afternoon, and then rises in the evening to reach the daily peak in the evening. This aggregate pattern of consumer demand has historically been reliable.

But New England now has about 3,400 megawatts of solar power located in the region, most of which is generated locally or produced directly on site at retail customers’ homes or businesses. As a result, less electricity is used from the regional power system at midday when sunny conditions prevail.

With increasing solar penetration, this phenomenon can cause major shifts in the timing of demand for grid electricity, moving the time of daily low-demand from overnight hours to midafternoon. According to the grid operator, on seven days, afternoon demand for grid electricity has fallen below overnight low demand levels. This phenomenon first occurred on Saturday, April 21, 2018, which ISO-NE attributed to "the right combination of sunshine and mild weather led to light consumer demand on the high-voltage electric power system, coupled with record-high output from the more than 130,000 solar power installations in the region."

The following chart, prepared by ISO-NE, illustrates the effects of increasing quantities of behind-the-meter solar capacity on load shape.

Source: ISO-NE.

As ISO-NE observes, this "graph also demonstrates how the region will increasingly need power resources that are able to quickly ramp up their output to bridge the steepening slope between minimum and peak load hours."

Afternoon demand has fallen below overnight demand on six occasions since April 21, 2018 -- three of which occurred this month, on the weekend days February 15, 22, and 23, 2020. For the February 22-23 weekend, ISO-NE attributed the phenomenon to "mild temperatures and sunny skies".

ISO-NE expects at least 5,800 megawatts of behind-the-meter solar power in New England by 2027. As behind-the-meter solar power continues to grow in scale, ISO-NE forecasts "more occasions when demand may dip extremely low, particularly during the spring and fall".