Photographer: Tomohiro Ohsumi/Bloomberg
Quicktake Q&A

What Comes of Solar Power When the Sun's Eclipsed

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The last time a total eclipse cast a shadow across the entirety of the U.S., from West Coast to East, was 99 years ago. Back then, the modern power grid was just getting started, and harnessing the sun’s energy on a widespread scale was little more than a notion in the minds of scientists. On Aug. 21, when the moon will completely obscure the sun across a swath of the U.S., the rare daytime darkness will affect a real -- though still small -- segment of the energy supply.

1. How much will the eclipse affect solar power?

Enough to notice. It will reduce the sunlight that reaches 1,900 solar-power plants in the U.S., according to the U.S. Energy Information Administration. More than 12,000 megawatts of solar generation may be forced off line during the roughly four-hour event, equivalent to the power of about 12 nuclear reactors. Though few power plants are within the narrow band of land that will see a total eclipse, some reduction in sunlight will be experienced everywhere in the U.S.

2. How is this different from when the sun goes down each day?

At night, when solar panels produce no energy, most people are sleeping and demand for power is low. This eclipse, by contrast, will coincide with the ramping up of the American workday, with the so-called path of totality starting at 9:05 a.m. local time near Lincoln City, Oregon, and leaving the U.S. at 4:09 p.m. East Coast time.

3. Doesn’t the sun go in and out on a typical day?

Sure. And where it’s overcast along the eclipse’s path on Aug. 21, giant solar farms will already be operating at reduced capacity, and homes and businesses equipped with rooftop panels will be relying more on the power grid for electricity. Still, it’s the very rare cloudy day when the sun gets completely blacked out, even for a short time.

4. How dependent is the U.S. on solar power?

It’s still a very small part of the U.S. electricity mix, lagging behind even other renewable sources of energy. Solar power accounted for 2.6 percent of U.S. power generation in May, compared with 6.9 percent for wind power and 10 percent for hydroelectric sources. By contrast, traditional gas-fired plants produced 30 percent.

5. Where will the eclipse have its biggest effect on power?

California is by far the leading U.S. state when it comes to generating solar energy, and the moon will block roughly 70 percent of its sunlight during peak eclipse around 10:20 a.m. local time. North Carolina, the No. 2 state in solar-power capacity, will see its sunlight reduced by more than 90 percent, and the impact there might be greater, since the eclipse will happen in mid-afternoon, the time of day when electricity demand is at its highest.

6. What will compensate for the reduction in solar power?

Power grid operators and utilities say they’ll tap natural-gas-fired power plants, keep more electricity supplies in reserve and encourage conservation to make up for the loss of solar energy. (The California Public Utilities Commission urged companies and residents to pledge to "do one small thing to reduce energy usage" during the eclipse.) Utilities will be using new technologies such as smart thermostats to control customer demand. The eclipse is happening at a good time for California, where hydropower supplies are plentiful because of melting snow.

7. Might there be service disruptions?

That’s highly unlikely, says the North American Electric Reliability Council Corp., noting that transmission operators have had months to prepare.

8. Will there be any change in power prices?

It’s possible that the sudden ramp-down in the solar power supply will cause wholesale electricity prices to spike. California’s grid operator expects to see solar generation fall off at a rate of 70 megawatts a minute over an 82-minute period. Taking that power offline so quickly threatens to create bottlenecks along the grid. Keeping a healthy margin of reserves on hand will help transmission operators avoid these choke points. In fact, prices may end up falling as people venture outside to watch the eclipse and the moon’s shade curbs the demand for air conditioning. Prices may also slump when the sun reemerges and panels start producing electricity again.

The Reference Shelf

  • A QuickTake explaining how solar energy went from marginal power supply to major source of energy.
  • A NASA website about the eclipse explains why it’s rare and where to watch it.
  • During this rare eclipse, natural gas will catch a glimpse of its future role in the U.S. power mix -- as solar energy’s backup.
  • A QuickTake on why the world needs a better battery if solar and wind farms are going to take off.
  • Bloomberg Television recorded the March 2015 eclipse, during which the moon blocked about 80 percent of sunlight in Europe.
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