This article is from The Spark, MIT Technology Review’s weekly climate newsletter. To receive it in your inbox every Wednesday, sign up here.

When I get home in the evening on a sweltering summer day, the first thing I do is beeline to my window air-conditioning units and crank them up.

People across the city, county, and even the state are probably doing the same thing. And like me, they might also be firing up the TV and an air fryer to start on dinner. This simple routine may not register in your mind as anything special, but it sure does register on the electrical grid.

These early evening hours in the summer are usually the time with the highest electricity demand. And a huge chunk of that power is going into cooling systems that keep us safe and comfortable. This is such a significant challenge for utilities and grid operators that some companies are trying to bring new cooling technologies to the market that can store up energy during other times to use during peak hours, as I covered in my latest story. 

Let’s dig into why that daily maximum is a crucial data point to consider as we plan to keep the lights (and AC) on while cleaning up our energy system. 

In some places where air-conditioning is common, like parts of the US, space cooling can represent more than 70% of peak residential electrical demand on hot days, according to data from the International Energy Agency. It’s no wonder that utilities sometimes send out notices begging customers to turn down their AC during heat waves. 

All that demand can add up—just look at data from the California Independent System Operator (CAISO), which oversees operation of electricity generation and transmission in the state. Take, for example, Monday, August 5. The minimum amount of power demand, at around four in the morning, was roughly 25,000 megawatts. The peak, at about six in the evening, was 42,000 megawatts. There’s a lot behind that huge difference between early morning and the evening peak, but a huge chunk of it comes down to air conditioners. 

These summer evenings often represent the highest loads the grid sees all year long, since cooling systems like my window air conditioners are such energy hogs. Winter days usually see less variation, and typically there are small peaks in both the morning and evening that can be attributed to heating systems. (See more about how this varies around the US in this piece from the Energy Information Agency.)

From a climate perspective, this early evening peak in the summer is inconveniently timed, since it hits right around when solar power is ramping down for the day. It’s an example of one of the perennial challenges of some renewable electricity sources: they might be available, but they’re not always available at the right times.

Grid operators often don’t have the luxury of choosing how they meet demand—they take what they can get, even if that means turning on fossil-fuel power plants to keep the lights on. So-called peaker plants are usually the ones tapped to meet the highest demand, and they’re typically more expensive and also less efficient than other power plants.  

Batteries are starting to come to the rescue, as I covered in this newsletter a few months ago. On April 16, CAISO data showed that energy storage systems were the single biggest power source on the grid starting just after 7 p.m. local time. But batteries are far from being able to solve peak demand—with higher summer grid loads, natural-gas plants are cranked up much higher in August than they were in April, so fossil fuels are powering summer evening routines in California.

We still need a whole lot more energy storage on the grid, and other sources of low-emissions electricity like geothermal, hydropower, and nuclear to help in these high-demand hours. But there’s also a growing interest in cooling systems that can act as their own batteries. 

A growing number of technologies do just this—the goal is to charge up the systems using electricity during times when demand is low, or when renewables are readily available. Then they can provide cooling during these peak-demand hours without adding stress to the grid. Check out my full story for more on how they work, and how far along they are. 

As the planet warms and more people install AC, we might be pushing the limits of what the grid can handle.  Even if generation capacity isn’t stretched thin, extreme heat and high loads can threaten transmission equipment. 

While asking people to bump up their thermostat can be a short-term fix on the hottest days, having technologies that allow us to be more flexible in how and when we use energy could be key to staying safe and comfortable even as the summer nights keep getting hotter. 

Now read the rest of The Spark

Related reading

Air-conditioning is something of an antihero for climate action, since it helps us adapt to a warming world but also contributes to that warming with sky-high energy demand, as I wrote about in a newsletter last year. 

Batteries could be key to meeting peak electricity demand—and they’re starting to make a dent, as I covered earlier this year. 

Another thing

A growing number of companies in China want to power fleets of bikes not with batteries, but with hydrogen. But reception has been mixed, with riders reporting trouble with range. Read more in the latest story from my colleague Zeyi Yang.

Part of the reason for the growing interest in hydrogen is concern over the safety of lithium-ion batteries. New York is trying to make e-bikes safer by deploying battery-swapping stations in the city. For all you need to know about the program, check out my May story on the topic.

Keeping up with climate  

A major renewable-energy company unveiled a first-of-its-kind robot to help install solar panels. The company claims Maximo can install panels twice as fast as humans, at half the cost. (New York Times)

The European Union got more electricity from solar and wind than fossil fuels in the first half of 2024. Reforms in permitting and Russia’s invasion of Ukraine are two factors pushing the rise of renewables. (Canary Media)

Stepping into the shade can make the temperature feel dozens of degrees cooler. Cities need to look beyond trees for shade. (The Atlantic)

Check out these interactive charts detailing how each US state gets its electricity, and how it’s changed in the last two decades. Some surprises for me included South Carolina and Iowa. (New York Times)

Electric-vehicle sales in Germany are continuing their slide, dropping by 37%. The ongoing slump comes after the country ended incentives last year that supported EVs. (Bloomberg)

Wildfire smoke can have negative health effects. Protect yourself by staying indoors on days when air quality is poor, wearing a mask, and—especially—avoiding outdoor exercise. (Wired)

→ I spoke about a new study that will follow survivors of last year’s Maui fire to track their health outcomes, along with other science news of the week, on the latest episode of Science Friday. (Science Friday)

A new bill snaking its way through the US Congress could make it easier to build renewable-energy projects—and some fossil-fuel projects too. Here’s why a growing cadre of energy experts is on board with these permitting reforms despite concessions for oil and gas. (Heatmap)

Kamala Harris tapped Tim Walz as her pick for vice president. The Minnesota governor brings some climate experience to the ticket, including a law that requires utilities to reach 100% renewable energy by 2040. (Grist)