A solar eclipse sounds like a perfect excuse to look up. For the people who run Europe's power grid, it is a reminder that the sun does not always show up for work.
On Wednesday evening, August 12, the moon will slide in front of the sun, and solar power output will take a hit across the UK and western Europe. Thousands of tourists are expected in northern Spain to watch it, while grid operators are watching their screens instead.
The timing makes things a little trickier than usual. The region leans heavily on solar, and the eclipse lands right as the grid is already starting to shift from solar to other sources for the evening.
Eclipses are rare, but they are not surprises. Forecasters can calculate their timing hundreds of years ahead, which gives the people in charge of the grid time to prepare. This advance notice is crucial because solar capacity has expanded rapidly across Europe in recent years. As more solar panels are installed, even a short-lived dip in sunshine can create significant ripples in the power system, requiring grid operators to have backup plans ready.
The UK will feel it most directly around 8 p.m., when the moon covers 95% of the sun.
That should cut British solar output by just over 1 gigawatt.
Across Europe, the effect is bigger. Under clear skies, solar output could drop by roughly 9.7 gigawatts starting around 7:15 p.m., according to Entso-e, the group that represents European grid operators.
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The timing softens the blow. Because the sun will be close to the horizon, other generators will already be increasing output for evening demand, so the solar dip is not coming out of nowhere.
Natural gas, hydropower, and nuclear plants will cover the gap.
Ryan Carson, an energy forecaster at the UK's National Energy System Operator, said the eclipse's reach goes far beyond the UK. "The effects are quite far-reaching geographically, all the way across Europe," he said.
Carson said the team has had time to get ready. "We have been analyzing this since about one year ago," he said.
Spain is the one place getting a total eclipse, and its grid operators are not leaving anything to chance. Spanish solar plant operators say they are ready and are coordinating with Red Electrica, the national grid operator.
Acciona, whose solar installations sit directly in the eclipse's path, expects only a minor effect because the event happens at sunset, when solar production is already winding down. Still, the company has included the eclipse in its generation forecasts.
Manuel Calleja, Head of Centers at Acciona Energía, said the eclipse is built into the company's plans. "Everything is included in our models," he said.
"We include the eclipse in the forecast that we share with the TSO, so they know in advance how photovoltaic production is going to reduce," Calleja said. Photovoltaic is just a technical way of saying solar power.
The TSO, or transmission system operator, is the group that runs the high-voltage grid.
Calleja also said flexible sources like hydroelectric dams and gas-fired combined-cycle turbines can step in to cover temporary solar losses. Those plants would normally be starting up for Spain's evening peak anyway, which is part of why the impact should be manageable.
This eclipse should be a relatively minor event for the grid. The real challenge is coming next summer, when a second eclipse will arrive late in the morning, a time when solar power can be the largest source of electricity in many parts of Europe.
For investors and anyone who pays an electric bill, the eclipse is a small but useful preview. It shows what happens when a solar-heavy grid has to replace a big chunk of power in a hurry, and it hints at the balancing act that will only get more common as solar keeps growing.
The good news: this time, the lights should stay on. The next eclipse, next summer, will be the one worth watching.
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