When the temperature soars, so does the use of air conditioners—and demand for the electricity that powers them. Unusually high electricity use can strain power grids, sometimes forcing grid operators to implement rolling blackouts to avoid a system-wide collapse. Blackouts are not only unpleasant but also potentially life-threatening in extreme heat. The possibility of outages was a major concern during a record-breaking heat wave that smothered California, the most populous U.S. state, earlier this month.

A milder heat wave in August 2020 had caused California’s electricity demand to outstrip supply, and the state’s grid operator resorted to scheduled blackouts. About 800,000 homes and business had their power cut off for anywhere from 15 minutes to several hours. Just before California’s heat wave struck this September, temperatures and power demand were both projected to be higher than they were two years ago.

But impressively, California’s grid weathered the heat wave. Scientific American spoke with Michael Wara, policy director of the Sustainability Accelerator at Stanford University, about the strategic improvements and unconventional tactics that helped the grid hold up and how power systems can decarbonize and still stand up to climate extremes.

[An edited transcript of the interview follows.]

What happened in California during this heat wave?

All-time records were set all over the state. Demand for electricity that is needed to run air conditioners was going to be extremely high. Around 2, 2:30 P.M., the grid operator, CAISO (California Independent System Operator), said a level-three emergency would begin at 5:30 P.M. Level three means possible load shedding—it means [the potential for] rotating blackouts so that the grid itself doesn’t break.

What would it mean for the grid to break?

The thing to avoid is what happened in Texas in February of last year, where there was kind of a runaway grid failure. As the demand exceeds supply, there’ll be a voltage drop, and the power plants get out of sync with the grid. Turbines can go spinning out of sync, [causing damage]. Power plants will disconnect [to protect themselves and prevent this]. Demand and supply diverge even further, and then more power plants disconnect, et cetera, et cetera. And then you end up with a total blackout of the bulk power system, which can take days or weeks to fix. It’s something to avoid at all costs.

What happened after CAISO announced that a level-three emergency would go into effect?

We were all kind of bracing for these rotating outages to start. The expectation was they’d start around 6 P.M., which is when the sun starts to get lower on the horizon this time of year, and so the solar power plants start to produce less energy.

Instead what happened is that—in contrast to 2020—we had several thousand megawatts of battery storage that started providing energy and helped stabilize the situation. And then the grid operator called all of its demand response resources [large-scale consumers that the operator pays to reduce electricity usage] and reduced demand even further. It took about 1,000 megawatts or so off the demand at around 5 or 5:30 P.M.

And then something really interesting happened: CAISO sent a text to people in California and they said, look, you need to reduce your demand right now, or we’re going to have to start rotating outages. And basically, at the moment that that text was sent, demand fell in California by something like [3,000 megawatts.] It’s not totally clear yet if that was a result of that text. But it’s interesting to note that, simultaneous with the text, there was this large decrease in demand—and that allowed the system to ride out the evening hours.

Let’s assume for a moment that the drop in demand was a result of that text. A grassroots effort to save the grid is inspiring and impressive, but asking individuals to turn up their thermostats and turn off their lights is probably not a sustainable way to run the system, right?

We should not be leaving this to people. What if they’re not home? What if they don’t get the text? These heat waves are going to become more and more common. That is the future. Volunteerism is not going to work. If you ask people do this one time, they’re going to do it. But if you ask them to do it five times a summer, every summer? It’s not something you want to bank on.

What changed between the 2020 heat wave, which did result in rolling blackouts, and this heat wave, which did not?

The government really engaged in a thoughtful exercise in trying to figure out what the errors were, and we’ve seen substantial change since then. And boy, did that change pay off, because if we had been where we were in August 2020, we wouldn’t have had just a blackout [on one day]—we’d have had a blackout all weekend. We had system problems at 45 gigawatts peak demand in 2020. [On September 6 and 7] the grid was at something like 51 gigawatts and change. That’s more than a 10 percent increase in demand in two years. And we met that challenge and kept the lights on.

I think the real reason is that everyone in the California energy regulatory space and the companies were willing to have an honest conversation about what went wrong—and then fix it. And we were able to fix it because the clean technologies are so much less controversial to site than the dirty ones. The reality is: no one really cares if you site a utility-scale battery facility somewhere, and they love it if you’re pulling out a dirty power plant and replacing that with batteries. That’s what we have been doing for the past two years. I think there were 40 to 50 megawatts of batteries running in 2020. And [on September 6] we had close to 3,000 megawatts.

There was also a renewed push to get more demand response to be available during a crisis. Demand response is just programs that pay people [primarily large industrial and commercial consumers] during moments when the grid is stressed to stop consuming power. The last thing that they did—and this was controversial—was create a program during the summer months to basically waive environmental regulations during grid emergencies for large commercial diesel generators. Buildings can shift onto their diesel generators during a grid emergency, and that’s another way to reduce demand.

Burning fossil fuels was important for weathering this heat wave. How could a fully decarbonized grid respond to an event like this?

Those kinds of things are going to be less and less necessary if we maintain our current rate of battery deployment. If we can keep that up, it’s not going to be very many years until this is just a rearview mirror thing. And that’s going to be challenging because the batteries are getting a little bit more expensive because of supply chain issues and because of the growth in electric vehicles. But even if we only deploy 1,000 megawatts a year for five to six years, we’re well out of the woods and on our way to a very clean grid that relies on solar and wind.

How has California paved the way for renewables in the rest of the country?

I think the battery storage industry that California has created is going to make life much easier for everybody else as they walk down this path. It’s going to make it cheaper, more reliable, simpler to manage. I think California is doing what it should do with respect to climate change. We’re kind of like a 40-million-person, pilot-scale experiment in how to be a prosperous, energy-abundant place. We’re charting that path. It’s not always easy. It’s not always without some bumps in the road. But I think that because we’re going first, it’s going to be easier for other states and countries to follow.