BERLIN - The eclipse due to bring most of Europe into deep shadow on Friday morning will put an unprecedented strain on the electricity grid, which takes power from the world's biggest concentration of solar panels.

The moon will cross in front of the sun, blocking about 80 percent of its light across Europe from 8 to 11 a.m. London time. In Germany, the eclipse will briefly turn off thousands of panels, which provide about 40 percent of the nation's power on the most sunny days.

A drop of that magnitude will test the ability of utilities to keep the lights on as grid operators switch to usually idle natural-gas and coal plants to make up for the lost solar power. Success would inform nations from the United States to China working to integrate more renewables into their supplies, while failure would add to pressure for higher investment in grid-control technology -- and boost power prices in the process.

"Managing this event on the world's largest interconnected grid is an unprecedented challenge," said Konstantin Staschus, secretary-general of the European Network of Transmission System Operators for Electricity, or ENTSOE.

The darkest part of the eclipse will tear across the north Atlantic Ocean past southern Iceland, the Faroe Islands and reach Norway's Svalbard archipelago in the Arctic. Mainland Europe will see the sun partly obscured, with 87 percent cover in London, 83 percent in Denmark and about 25 percent in Turkey.

Previous eclipses such as one in 1999 passed without affecting power markets because photovoltaics only took off around 2004. Germany, which has about 38 gigawatts of Europe's 81 gigawatts of solar capacity, will see a 70 percent slump in PV generation about 10:40 a.m., according to the MeteoGroup forecaster. A gigawatt is about equal to the capacity of a nuclear reactor.

There'll be "good expected sunshine across the southern half of Germany,"  Stephen Davenport, of MeteoGroup's senior energy meteorologist, said Tuesday by email. "Assuming cloud does not increase too quickly, then there will be a surge of about 14 to 15 gigawatts to a peak of about 19 to 20 gigawatts once the event is over."

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Electricity markets will ripple from the effects of the eclipse all morning. Danske Commodities, a power broker, is increasing staff because consumption is hard to predict and may shift depending on "how many people go outside to look at it," said Bo Palmgren, the company's head of intraday trading.

"When you have plants ramping up and down, there is the potential for outages," Palmgren said Monday by phone from Aarhus, Denmark. "The hours before and after the eclipse will be interesting."

Swedish utility Vattenfall AB will seek to profit by selling output from power plants that normally aren't competitive, including gas- and oil-fired generators that "cost several hundred euros per megawatt-hour to operate," Hartmuth Fenn, head of intraday market access and dispatch, said by phone from Amsterdam on March 12.

Italy has the region's second-biggest solar market and will also be affected, although more of the sun will be visible at that latitude. Terna Rete Elettrica Nazionale SpA, the nation's grid operator, expects to lose about 7 gigawatts of the 19 gigawatts of available PV supplies.

Failures in one region would have an impact everywhere else because of interconnections between 34 national grids in Europe, according to RTE, the French network operator.

"If the loss of production is not immediately replaced by other forms of generation, it could pose a risk to the network and lead to power cuts," RTE said on its website on Monday.

European grid companies will organize for their control rooms to be in constant communication during the eclipse. They will use power reserves to balance their system and help others in the region. For this, they can rely on additional close to real-time data provided by the coordination initiatives like TSC in Munich and Coreso in Brussels, ENTSO-E said.

"Not all incidents can be ruled out even in normal operational times," ENTSOE's Staschus said. Transmission system operators are "taking the event seriously but are confident that they will be able to manage it."

Germany's four main grid companies have added staff and will stay in touch by live telephone conferences throughout the eclipse. They've also bought reserve power to prepare.

"It's not going to be a normal day," said Regina Koenig, a spokeswoman for Transnet BW, the German company operating the network in Baden-Wuerttemberg, the state where Daimler and Volkswagen's Porsche SE run their factories.

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Terna in Italy "has already been working for more than a year on the safe management of this natural phenomenon and on ensuring the same level of supply as on any other working day," said Antonio Carrano, head of the national control center.

The actual impact of the eclipse is hard to predict as it would decrease with greater cloud coverage, said Eleanor O'Neil, a meteorologist at WSI Corp.

"It can be tricky to calculate cloud amounts, even closer to the event," O'Neil said by email on March 13.

Weather forecasts for Friday show sunny skies in Berlin, Rome and Paris on the meteorological website Wetter.de. If that holds, prices for power reserve capacity "could rise extremely" in Germany, said Stephanie Moeller, a spokeswoman for RWE AG's energy trading unit RWE Supply & Trading GmbH.

The eclipse will help Germany test its ability to absorb variable power supplies from renewable generators, which in the case of solar switch off at night and with wind don't operate on calm days. By 2030, about half of Germany's power will come from renewables, said Agora Energiewende, a research group. Grid operators have yet to make their biggest planned investments to cope with unsteady supplies.

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"If today's inflexible power system succeeds in managing the solar eclipse, then the power system of 2030 will easily manage comparable situations," Patrick Graichen, head of Agora, said by email.

Utilities could prepare for that future by investing in energy storage, by encouraging customers to vary usage, and by offering pricing that better reflects actual supply and consumption, said Barry Fischer, a researcher and writer at Arlington, Virginia-based software company Opower Inc., which is helping utilities manage demand.

"The challenge posed by the eclipse resembles what is likely to be a more daily phenomenon, as Europe and other regions further expand their installed renewable capacity," Fischer said in a phone interview. The eclipse, he said, is "a window into the future of power systems."