A Summer Peaking System

New England typically reaches its highest demand for electricity during the summer when warmer weather leads to increased use of energy-intensive air conditioning.

  • Peak demand on a normal summer day has typically ranged from 17,500 MW to 22,000 MW. Summer peaks on the hottest and most humid days have averaged roughly 25,600 MW since 2000. Read about how the ISO prepares for summer operations.
  • In winter, peak demand on a normal day has typically ranged from approximately 18,000 MW to 19,700 MW. Winter peaks on the coldest days have averaged roughly 21,000 MW since 2000. Read about how it can be more challenging to meet the winter peak than summer peak.
  • Typical spring peak demand ranges from 15,000 MW to 16,900 MW; typical fall peak demand ranges from 15,900 MW to 17,300 MW. The peaks can be much higher if summer-like weather creeps into the late spring or early fall periods.

Until 1989, New England was a winter-peaking system, and in the early 1990s, the region had nearly twin winter and summer peaks. Growing use of air conditioning and a decline in electric heating contributed to this dramatic change.

New England’s Electricity Use

The region’s millions of households and businesses create the demand for electricity on the power grid, which must be produced the instant it is needed because electricity cannot easily be stored in large quantities. (Learn how ISO New England runs the power grid.)

Top 10 Demand Days

Peak demand is the highest amount of electricity used in a single hour, and the ISO must ensure that the region has sufficient power resources to meet the peak. The chart below shows the days with the highest peak demand recorded in New England since the ISO began managing the power grid in 1997. The highest peaks typically occur during the work week in summer. Note that as demand for electricity from the regional grid declines, these records are harder to break. Nevertheless, high spikes in demand still occur, and New England’s power system must remain prepared to meet those peaks even if they aren’t historically high.

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Data for the top ten demand days comes from the Daily Summary of Hourly Data Report, available on the Zonal Information page. The daily peak load for the regional system is included in the “System_Peak” column on the “ISONE CA” worksheet. The “DT” (date type) column represents the day of the week, where Monday is 1 and Sunday is 7.

Despite significant declines in grid energy use on an annual basis, hot, humid summer days can still cause high spikes in electricity demand, as shown in this graph.

Fast Stats
  • 7.2 million retail electricity customers; population 14.8 million
  • 123,307 gigawatt-hours (GWh) total annual energy served in 2018 (subject to adjustments)
  • 136,355 GWh all-time highest total annual energy served, set in 2005
  • 28,130 megawatts (MW) all-time summer peak demand, set on August 2, 2006
  • 22,818 MW all-time winter peak demand, set on January 15, 2004
  • -0.4% average annual growth in regional electricity demand forecasted through 2028, after factoring in energy efficiency (EE) and distributed generation (DG)
  • -0.4% average annual growth in summer peak demand forecasted through 2028 under typical weather conditions after subtracting EE and DG; -0.3% under extreme summer weather
  • -0.6% average annual growth in winter peak demand forecasted through 2028 under both typical and extreme weather conditions after subtracting EE and DG
Peak demand vs. annual energy use

Energy Efficiency and Solar Power Have Reduced Growth in Annual and Peak Demand

Since 2005, total annual demand for electricity from the region’s power system has been declining. Several factors have played a role, though weather is typically the biggest determinant of electricity demand in any given year:

  • The Great Recession during the late 2000s and slow recovery dampened demand for electricity.
  • The New England states began investing heavily in energy-efficiency (EE) programs, which reduce demand from the grid and help flatten growth in peak demand.
  • The region has seen rapid growth in solar photovoltaic (PV) systems connected to local utilities or installed “behind the meter” directly at retail customer sites, which serve to reduce demand from the regional grid and flatten growth in peak demand. See how hourly load varies by season and with the impact of solar power.

10-Year Forecast: Electricity Demand Continues to Decline

The ISO forecasts that EE measures and behind-the-meter solar photovoltaic (PV) resources will continue to noticeably reduce demand for electricity from the regional high-voltage power system, as illustrated in the graphs that follow.

  • New England’s annual electricity use is expected to decline by an average of -0.4% annually over the next decade, from 125,823 gigawatt-hours (GWh) in 2019 to 121,366 GWh in 2028.
  • Summer peak demand under typical weather conditions (the “50/50” forecast) is also expected to decline at an annual average rate of -0.41%, from 25,323 MW in 2019 to 24,408 MW in 2028.
  • Summer peak demand under extreme summer weather (the “90/10” forecast), such as an extended heat wave, pushes the expected peak demand up to 27,212 MW in 2019 to 26,576 MW in 2028—a slight decline at an average rate of -0.26% per year.
  • Winter peak demand under typical weather conditions (the “50/50” forecast) is expected to decline an average of-0.6% annually, from 20,476 MW in 2019 to 19,368 MW in 2028.
  • Winter peak demand under extreme weather (the “90/10” forecast) is also forecasted to decline an average of -0.6% annually, from 21,173 MW in 2019 to 20,130 MW in 2028.

Without the demand-reducing effects of EE and PV, New England’s overall electricity use would be expected to grow an average of 1.14% annually over the next decade, from 145,610 GWh in 2019 to 161,312 GWh in 2028. In summer, peak demand under typical weather would rise at an average annual rate of 0.7%, from 28,943 MW in 2019 to 30,832 MW in 2028, and peak demand under extreme summer weather would also rise at an average annual rate of 0.76%, from 30,832 MW in 2019 to 32,999 MW in 2028. In winter, typical peak demand would rise by an average 0.6% annually, from 23,144 MW this year to 24,376 MW in 2028, while peak demand under extreme cold would rise by an average 0.6% annually as well, from 23,841 MW this year to 25,138 MW in 2028.

Forecasted Effects of EE and PV on Electricity Demand in New England

Demand Type


Forecast Type



Average Annual Rate of Change







(Minus EE and PV)









(the "50/50" forecast)





(Minus EE and PV)




Extended heat wave of (the "90/10" forecast)





(Minus EE and PV)
27,212 26,576 -0.26%


(the "50/50 forecast)





(Minus EE and PV)
20,476 19,368 -0.6%

(the "90/10" forecast)





Net (Minus EE and PV) 21,173 20,130 -0.6%

See Key Stats—Resource Mix for details on the growth in EE, PV, and other clean-energy resources.

annual energy use and summer peak demand

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Copyright ©2019 ISO New England Inc.