As part of our Shave the Peak program, we’ve been aggregating data released by ISO-New England, the non-profit Independent System Operator with a responsibility to maintain reliable electricity to the six New England states. Every day, we track various stats, including when and what the peak demand was, when and what the most expensive price of electricity was, how high CO₂ emissions reached, what the fuel mix looks like, etc.

In this blog, we are going to compare and contrast data from two days: A typical summer day and the summer peak day of this year (so far). For our typical summer day, we took data from July 2nd, 2025, a partly cloudy day with temperatures reaching a comfortable 83 degrees, with a peak load of 20,182 MW. Our peak summer day was June 24th, 2025, temperatures reaching a blistering 98 degrees (a record temperature for June), with a peak load of almost 26,000 MW. ISO-New England uses different methodologies to calculate peak demand depending on what it is being used for, accounting for the discrepancy we see in the fuel mix peak below. 

Fuel Mix Graphs

The two graphs below depict what the fuel mix was throughout the days we’re analyzing, or in other words, how much demand was met by each generation source.  

ISO-New England considers solar, wind, refuse, wood, and landfill gas as renewable sources of energy, their sum accounting for the light green section of the graphs. The yellow line indicates the actual system load, including estimated behind-the-meter solar (usually rooftop). In other words, the “duck curve” represents the impact of behind-the-meter solar generation on reducing demand for grid power.

Fuel Mix on Typical Summer Day (7/2/2025)

On a typical summer day, most demand is met with natural gas, which, as of current, is business as usual. The shape of the load is relatively linear throughout the day, with an uptick in demand between 4:00 PM and 7:00 PM. This occurrence is very normal, as most people are getting home from work within this time window, turning on their AC units, TVs, and other electrically run home appliances. The price of electricity peaked at 7:00 PM at a rate of $79.85 per megawatt hour (MWh), which is about $40 less than the average peak price this summer, according to our data collection.

Next, we’ll take a look at the fuel mix on June 24:

Fuel Mix on Summer Peak Day (6/24/2025)

To meet peak demand on a day like this, ISO-New England called upon “peaker plants”, a last resort resource of power generation. These plants typically run on dirty gas and oil, selling their power at extremely high rates. They only run for the hours of the highest demand, accounting for about 2-7% of the year's power generation. Between the hours of 6:00 PM and 8:00 PM on this day, electricity prices soared, reaching over $1,000 for that two-hour window.   

Note how the most polluting fuel sources (oil) are barely visible on a typical summer day but play a role in meeting peak demand. Oil usage during the peak on this day accounted for roughly the same amount of demand as renewable sources (as the sun is setting) and hydro combined. This reality highlights the importance of shaving the peak on days with extreme temperatures to reduce the use of polluting generation resources.  

CO₂ Emissions

Now that we’ve taken a look at what fuel sources met demand for both of these scenarios, let’s take a look at how that corresponds to total CO₂ emissions for each day. The two graphs below show the total CO₂ emission on July 2, 2025 (our typical day), and June 24, 2024 (our 2025 peak day so far). The red line is the total emissions, while the blue line indicates emissions from natural gas. The grey line you see spiking between 4:00 and 11:00 PM on June 24th is the emissions from burning oil. The peak emissions on this peak day clocked in at 152.09 metric tons of CO₂ at 8:21 PM. There is no visible grey line on July 2nd, affirming our narrative of dirty fuels coming into use on high-peaking days. On this typical summer day, the peak of emissions was 98.78 metric tons at 8:07 PM. That’s a difference of about 55 metric tons of CO₂ or about 55 percent.

Estimated Emissions on Typical Summer Day (7/2/2025)

Estimated Emissions on Summer Peak Day (6/24/2025)

Economists have attempted to estimate the social cost of carbon emissions based on factors like how temperature increases and sea level rise will impact agriculture, health, energy use, etc., and while there are many different estimates out there, the highest estimates from academic literature exceed $300 per metric ton of CO₂. If we use this estimate to put a monetary value on the loss to society, the emissions on our summer peaking day account for $16,500. Not only do peak days have a high energy cost for our electricity bills, but they also increase societal costs like health issues and food supply chains that will be impacted by the consequences of increased emissions. Particulate air pollution from burning fossil fuels for energy and petrochemicals has a global economic impact of $2.9 trillion a year ($8 billion per day) and claims almost 9 million lives per year.   

A recent analysis from Bloomberg Intelligence found that the US has spent nearly $1 trillion on disaster recovery and other climate-related needs over the 12 months ending May 1, 2025. Overall, increased climate costs from insurance premiums, power outages, disaster recovery, and uninsured damage are responsible for $7.7 trillion, or 36%, of US GDP growth since 2000. 

Enough Doom & Gloom

We don’t want to leave you on a sour note, but we do want to spark a change in behavior to keep our energy systems clean and better protect the environment. Consider signing up for our Shave the Peak program to receive text and email alerts on peak days with information on individual actions that can help reduce overall electricity usage during peak demand hours.  

Shave the Peak alerts often recommend saving energy during a specific timeframe, usually 4:00 PM to 8:00 PM, depending on the day’s specific conditions. The 6:00 PM to 8:00 PM period is typically the highest peak. If we see through our data monitoring that a peak day is likely to occur on a particular day or two, we’ll let participants know to be cognizant of their energy use for the upcoming days, rather than sending a notification for each day.  

Here are practical ways to "shave the peak" during those specific hours: 

• Pre-cooling works! Focus your AC usage on a couple of rooms, and get them nice and cool before 5 PM. Then, turn the AC up a few degrees, or turn it off if you can handle it.  
• Use fans wherever possible to compensate for AC, and draw the blinds to help keep rooms cool. If you're planning an outing to the pool or to the air-conditioned library or mall, do it during peak hours!  
• Don’t do the laundry. When you do, wash in cold water.  
• Don’t run the dishwasher.  
• If you have an electric car, schedule your charging session for 9:00 PM to 7:00 AM. Avoid peak times!  
• Sign up for Shave the Peak to get reminders to reduce your electricity use when it matters most. 

This week, from Tuesday, July 15th to Thursday, July 17th, peak load is projected to be over 22,000 MW for three days straight. If you’re signed up for our Shave the Peak program, you’ll receive a notification about this opportunity to conserve energy. Accuweather is projecting that these three days will be the only days above 90 degrees for the rest of the summer, but of course, that could change. We keep our notifications to days above 22,000 MW, but pollution levels are a lot higher on days with a peak load of 20,000 MW compared to a 15,000 MW peaking day. While there are benefits to Shaving the Peak on any day, we don’t want to wear out our welcome, so we keep alerts to extreme peaking days.   

Ask About Connected Solutions

Connected Solutions is a demand response program offered in several Northeast states, including Massachusetts, Rhode Island, and New York, that incentivizes homeowners and businesses to reduce energy consumption during peak demand periods. Participants can earn rewards by allowing their smart thermostats or battery storage systems to temporarily adjust energy usage during peak times, helping to stabilize the electric grid and reduce carbon emissions. For information, contact your local utility: Eversource, National Grid, Unitil, and Rhode Island Energy.  

The Future of Peak Shaving

Very briefly, but within a few years, peak shaving should evolve into something bigger and better. Utilities in Massachusetts and Rhode Island are beginning to install “smart meters” that will enable the implementation of more robust programs than what Connected Solutions is all about. We can expect to see time-varying rates and more load management programs to incentivize efficient energy use. The idea is to shift financial rewards away from peaker plants to participating consumers in ways that will benefit consumers by driving down the system-wide costs that we all pay for. We will keep you posted!  

In Conclusion

Collective action and the transition to more renewable sources of energy can keep the dirtiest resources out of the fuel mix. One might ask themselves if their individual action can truly make an impact on shaving the peak, but it is individuals that make up collective change. Turning your AC off during a peak demand event will make an impact in helping us Shave the Peak and safeguard the health and safety of each other and our environment.