How the Lights Stay On

Chapter 6: The Sun Doesn’t Ask Permission

Imagine a spring afternoon in California. The sky is clear. The temperature is in the seventies. People are at work or school. Air conditioners are not running because it is not yet hot enough. Lights are off in most homes.

And across the deserts and rooftops of the state, several million solar panels are producing electricity at full output.

There is so much solar electricity flowing onto the grid right now that the operators at California’s grid control center — CAISO, the California Independent System Operator — have a problem most engineers in the twentieth century would have considered absurd. There is more electricity than anyone in the state can use.

So the operators do something genuinely strange. They start paying their neighbors — Arizona, Nevada, sometimes states even farther away — to take the excess electricity off their hands. Sometimes the wholesale price of electricity, the price that big buyers pay, drops below zero. A generator selling electricity at minus twenty dollars per megawatt-hour is actually paying customers to take it.

When even paying neighbors is not enough, the operators tell solar farms to curtail — to disconnect from the grid and stop producing. Beautifully working solar panels, sitting in the sun on a clear day, are deliberately turned off. The electricity they could have made is simply not made. Free, clean energy is allowed to bounce off the panels and warm the desert air instead of becoming usable power.

In 2024, California curtailed 3.4 million megawatt-hours of solar and wind energy this way — a 29 percent increase from the year before. Solar accounted for 93 percent of it. That is enough electricity to power roughly 300,000 American homes for a year, simply not generated.

If this sounds wasteful and strange to you, you are paying attention. The reason it happens is not a flaw of the engineers. It is a flaw of the timing — and the timing is one of the deepest, most genuine challenges of the energy transition.

For the entire history of electricity, generation followed demand.

If people wanted more electricity at six in the evening — and they did, predictably, every day, because that is when they came home and turned on their lights and started cooking — then the grid operators ramped up the power plants to meet that demand. Coal plants, gas plants, hydroelectric dams, nuclear reactors. These were all what engineers call dispatchable sources. You could tell them when to produce more electricity and they would produce more electricity. The operator was in charge of the timing. The plants did what they were asked.

Demand goes up: ramp generators up. Demand goes down: ramp generators down.

The system was simple enough that it could be managed by humans in control rooms with telephones, which is exactly how it was managed for most of the twentieth century.

Solar and wind do not work this way.

Solar panels generate electricity when the sun is shining. The brighter, the more. They produce nothing at night, less in the morning and evening, less on cloudy days, almost nothing in a storm. They do not care whether the grid needs the electricity right now.

Wind turbines generate electricity when the wind is blowing. They do not care about your dinner.

This is what engineers mean when they call these sources intermittent or variable. The sun does not ask permission. The wind does not check the schedule.

If you want a grid that runs on solar and wind, you have to flip the entire hundred-year-old logic of electricity inside out. Instead of generation following demand, demand has to follow generation — or, more precisely, you have to find ways to store generation until the time when demand needs it.

This is a much harder problem than it sounds.

To see exactly how hard, take a look at something the people who run California’s grid call the duck curve.

The duck curve is a chart. It shows electricity demand on the California grid over a single day, from midnight to midnight, after you subtract the electricity being produced by solar panels. This is called the net load — the amount of electricity the grid operators have to produce from other sources to meet what people are actually using.

In 2014, before California had much solar, this chart looked like a smooth, gentle hump. Demand was lowest in the early morning hours. It rose through the morning, plateaued in the afternoon, peaked around six or seven in the evening when people came home, and fell back down at night. A normal-looking curve.

Today, the chart looks like a duck.

Here is why. In the middle of the day, when solar panels are producing maximum output, the net load — the demand the grid still has to meet from other sources — drops dramatically. The chart sags downward into a deep belly. Sometimes the belly goes below the morning floor. There is less demand for non-solar electricity at noon than there is at four in the morning. (In CAISO today, solar supplies almost half of all electricity demand between 8 AM and 4 PM. The midday belly is that big.)

Then the sun starts to set. Solar production falls off a cliff. People come home from work and turn on their air conditioners, ovens, lights, televisions. The net load shoots upward, almost vertically. The chart rises into a sharp peak. That is the head of the duck.

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