Solar Power in New England: Concentration and Impact

Solar power systems are rapidly being installed across the six states of New England and noticeably reducing the electricity drawn from the regional power system. But how much and when that electricity demand is reduced are critical questions when it comes to operating the grid and performing long-term system planning.

Solar Power Is Growing Quickly

State policies, state and federal support, tax credits, and falling technology costs are spurring remarkable growth in the installation of solar photovoltaic (PV) system in New England. The ISO’s 2018 PV Forecast anticipates almost 6,000 megawatts (MW) of nameplate PV capacity by 2027.

The region has about 2,400 MW of solar power. Of this, ISO system operators can only “see” about 100 MW in real time.

Projected Cumulative Growth in New England Solar Power

Regional Solar Power Reduces Demand from the Grid

The vast majority of New England’s solar power is in the form of small-scale (typically 5 MW or less) systems, including residential rooftop arrays. These systems are typically connected to local utilities or behind the meter (BTM) directly to retail customers—and not to the regional power system. As a consequence:

  • BTM PV reduces the amount of electricity being drawn from the grid. This is illustrated below by the region’s load profile (the amount of electricity drawn from the grid by hour) for May 23, 2015: a clear, relatively cool day near the summer solstice—ideal conditions for PV production.
  • The output from BTM PV can’t be monitored in real-time by ISO system operators. The challenge for the ISO, then, is to accurately predict the magnitude of the load-reduction caused by BTM PV in any given hour of any given day—and to quickly adjust to any load fluctuations in real time.


Output from Solar Power Systems Is Highly Weather-Dependent

ISO system operators rely on accurate forecasts of grid demand because it’s critical to keep the power supply in near-perfect balance with demand at all times. But anticipating how much solar power there’ll be at any moment is a complex problem. PV output depends on things like:

  • How high—or low—the sun is in the sky
  • The amount of cloud cover and haze
  • Temperature and humidity
  • Wind speeds
  • Snowfall

These changing weather conditions can lead to rapid and sizeable swings in electricity output from PV systems, which is why PV resources are called variable or intermittent. It’s also why as more variable resources are installed in New England, the region will rely more heavily on other power resources that can help balance the fluctuations of the combined load and behind-the-meter solar PV, such as efficient, fast-start natural gas power plants. New storage technologies may also one day help balance solar variability, as they become larger and more cost-effective.

The effects of cloud cover can be seen in the graph below of grid demand on March 24, a cloudy day, versus March 23, a sunny day.

Cloudy Day Vs Sunny Day

Solar Power’s Impact Varies by Season and Total Amount Installed

The amount of electricity New England uses varies greatly by season—so does PV’s impact. The representative load profiles below simulate the impact PV will have during each season as more and more is installed across the region. Compare:

  • Peak demand (the blue dots)—the day’s highest level of grid electricity use
  • How steep the climbs and drops become
  • Minimum demand (the green squares)—the day’s lowest level of grid electricity use

The ISO Is Developing a Better Picture of the Region’s Installed Solar Power

The ISO is studying other possible implications of PV displacing conventional power resources. Conventional resources have physical characteristics that are critical to helping regulate transmission line power flows, frequency, and voltage, which help maintain power grid reliability. Learn more in The Basics of Essential Reliability Services, a series of videos presented by the North American Electric Reliability Corporation (NERC).

The ISO is also actively pursuing new and innovative ways to accommodate the effects of large amounts of solar power. We developed the nation’s first long-term, multistate forecast of PV capacity. We’ve also prototyped a day-ahead forecast of regional behind-the-meter PV output based on forecasts of irradiance (the sun’s strength), which helps estimate how much electricity demand will be reduced by PV, and are working to upgrade this prototype into a production-grade forecasting system (similar to the ISO’s wind power forecast).

As part of this project, we’re collaborating with distribution utilities to understand and track the amount and location of PV capacity across New England. The heat maps below show the aggregated installed nameplate PV capacity by town within each state through December 31, 2017. Please note that the color scale varies by state in order to more effectively illustrate the distribution of PV capacity within each state.

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