Beating the heat: how ISO-NE prepares for the dog days of summer

Once the region rounds the bend on winter, ISO New England begins preparing to operate the region’s power grid during the hot, humid summer months. The days of highest electricity use occur during the summer in New England when warmer weather leads to increased use of energy-intensive air conditioning, creating complex challenges for the grid operator. With a toolbox of procedures, processes, and expertise in place to ensure reliability of the bulk power grid, the ISO is capable of dealing with the wide variety of situations that can arise when demand for power is peaking.

Thermal buildup
and electricity use

On an ordinary summer day, a hot afternoon will give way to a cooler evening and some of the heat absorbed by houses and other structures will dissipate. But when temperatures are high for several days in a row and don’t cool off that much at night, houses and other structures start to retain the heat in a phenomenon called thermal buildup. That’s when people’s tolerance for heat tends to wane. As air conditioning use rises, so does demand for electricity.

Factors that affect the grid

Temperature and dew point both play pivotal roles in determining demand on the system. The difference between a normal summer temperature of 90°F and heat wave conditions of 94°F can result in more than 2,000 additional megawatts (MW) of demand. While the colder winter weather also creates a rise in electricity usage, summer peak hours are different because they extend for longer periods, with the ramp beginning during midday and stretching into the evening. Instead of a two-to-three-hour peak that typically occurs during an early winter evening, hot summer weather usually creates numerous hours of consistently high demand.

A comparison of hourly load on summer and winter days reveals two very different peaks (pictured here are New England’s all-time peak load on August 2, 2006 and all-time winter peak load).

When this pattern of prolonged peaks extends over several days, the demand for electricity builds, and threatens to exceed available electric generating capacity and reserves. Summer weather-related issues can also cause a transmission line or a generator to suddenly go offline because of mechanical problems, reducing the amount of capacity and reserves available.

Because electricity cannot easily be stored in large quantities, the ISO must maintain a certain level of electric generation held in reserve for reliability purposes. ISO New England maintains both 10-minute and 30-minute reserves—named for the capability of generating resources to deliver electric energy within 10 or 30 minutes. The grid can enter a “capacity deficiency” when available resources are insufficient to meet anticipated demand plus the required level of reserves.

Pre-summer checklist

Each spring, maintenance is conducted on transmission and generation resources to prepare for the high loads that hotter weather brings. Heading into the summer months, ISO New England closely monitors weather forecasts, fuel source availability, and other factors that may affect the grid to get a sense of what conditions system operators will be facing. Read our 2018 outlook for a summary of grid conditions the ISO expects this summer.

Actions before and during a capacity deficiency

Short-term forecasts

As hot summer weather sets in, the grid operator keeps a watchful eye on changing conditions and weather-related issues affecting resources on the system. The ISO issues a Morning Report and a Seven-Day Forecast that list expected demand, capacity, and reserves on a daily and weekly basis, respectively. These reports give stakeholders and the public an understanding of the grid’s available resources and advance notice of any possible challenges on the horizon.


In the event of abnormal system conditions, the ISO issues Master/Local Control Center Procedure No. 2 (M/LCC2). This notification alerts applicable power system operations, maintenance, construction, and test personnel, as well as market participants, that an abnormal system condition has occurred or is expected to occur, and that they should cease any testing or maintenance on their resources that could affect reliability. While an M/LCC2 event is not an indication of a shortage of supply and reserves, many deficiencies are preceded by an M/LCC2 notification.


If grid reliability is jeopardized, the ISO has a series of long-established procedures it can employ. System operators can request emergency electricity imports from neighboring power grids if available, draw upon 30-minute reserves, and ask the public to conserve electricity voluntarily. These actions are among the measures in ISO Operating Procedure No. 4, Action During a Capacity Deficiency (OP 4). Implemented in any order, the actions can be applied New England-wide, by state, or targeted to specific areas in order to maintain system reliability and preserve New England’s 10-minute reserves by providing generation and load relief on the system. (See the Power System Alert Descriptions web page for an explanation of all the various alerts.)

How you can help

Most of the ISO’s procedures do not call for conservation by the public; however, it’s always a good idea to use electricity sensibly, in a manner consistent with your health and safety. New England’s businesses and residents can take steps to help keep demand in check, especially when consumer demand is expected to be high. Simple measures such as raising air conditioner settings a few degrees, installing energy-efficient light bulbs, and turning off unnecessary lights, TVs, and other home and office equipment can have an impact.

ISO New England posts information and frequent updates on power system conditions to its website. In the event of a capacity deficiency, the ISO provides OP-4 details in a number of places, including:

While life in New England involves a certain amount of unpredictability when it comes to the weather, the ISO is prepared to address the challenges of seasonal extremes. By closely monitoring grid conditions and communicating information about the impact of weather and fuel availability, the ISO can direct efforts to maintain efficient operation of the system.

Learn more

Inside ISO New England
summer, system operations