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August 20, 2017

Watch the North American power grids respond to the 2017 eclipse!

Source: Alex McEachern, Power Standards Lab, or
Unrestricted permission to copy and re-publish this text is hereby granted.
Credit “Alex McEachern, Power Standards Lab”

ON MONDAY, AUGUST 21, the North American power grid will be the subject of a celestial experiment: now that we have a lot of solar power connected, what happens when the sun goes away?

The 2017 eclipse sweeps directly across North America’s two major power grids. Live on-line meters show the precise grid frequency, an indication of stress.

You can watch this experiment unfold, live, on instruments at two free web sites:* and*. (After the eclipse finishes, the data gathered at these two sites will be shared with scientists around the world.)

The eclipse will pass through the two largest North American power grids, informally called the Western Interconnect and the Eastern Interconnect. The eclipse will miss the other two main grids: Texas and Quebec

Grid operators carefully balance the output of thousands of grid-connected generators against the total minute-by-minute load on the grid. Their balancing job is made easier by two characteristics of grid load: diversity and predictability. “Diversity”, in this context, means that all the loads on the grid never turn on and off at exactly the same time. For example, when some air conditioners are cycling on, some are cycling off; and the grid-wide average is far  steadier than any individual load. “Predictability” means that the grid operators know the general pattern of loads, and the general pattern of generators, including solar. For example, solar is predictably on during the day and off at night, and local weather forecasts can make fairly precise predictions about solar generation.

Live meters at* and* . Eclipse will show as a dip in solar irradiance, the meter on the left. Grid stress will show in frequency, the center meter.

The eclipse is challenging, for the first time, both the diversity and predictability of a significant amount of power generation. The eclipse hits large swaths of solar power generators simultaneously, reducing diversity; fortunately, the eclipse path and timing is highly predictable, so grid operators can plan for solar power losses. But what complicates this planning is that grid operators have incomplete information about how much solar generation exists, and where it is located. The operators know exactly where the large solar arrays are; but it’s far more difficult to know where all the smaller rooftop solar arrays are located. And that’s what makes this experiment interesting.

You can see the balance between grid generation and grid load in the grid frequency. If it’s higher than 60.000 Hz, there’s slightly more generation than load; if it’s lower, there’s slightly more load than generation. The grid operators dance with the frequency, adjusting the generator settings throughout the grid to maintain steady state.


Power Standards Lab, a private company in Alameda, California, that does research projects with the U.S. Department of Energy, has set up two public instruments that show the “solar irradiance”, which is the amount of sun power available right now, and the “Frequency”, which is the balance between grid generation and grid load. One of these instruments is on a rooftop in sunny Windsor, California, monitoring the Western Interconnect grid; the other is in a pasture in Zebulon, North Carolina, monitoring the Eastern Interconnect grid. Both are publicly available at* and*.

Researcher Alex McEachern installs a solar power sensor in Windsor, California for grid stress measurements during eclipse.
Researcher Alex McEachern installs a solar power sensor in Windsor, California for grid stress measurements during eclipse.












It’s an entirely different way to watch the eclipse unfold!


For more information…

(*) due to huge traffic to those links, you may experience some delay

Power Standards Lab
980 Atlantic Ave.
Alameda, California 94501
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