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Posted by Christian Cossette

Source: Alex McEachern, Power Standards Lab, email, web site

On Monday, August 21, the North American power grid will be the subject of a celestial experiment: now that we have lots of grid-connected solar power, what happens when the sun goes away?

You can watch this experiment unfold, live, on instruments at two free web sites: and (After the eclipse finishes, the data gathered by these instruments 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.

Minute by minute, grid operators carefully balance the output of thousands of grid-connected generators against the varying total load on the grid. This balancing act is eased by two characteristics of grid load called « 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 happen to be cycling on, others happen to be cycling off; and the grid-wide average, over the entire Western Interconnect, 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.

The eclipse will challenge, for the first time, both the diversity and predictability of a significant amount of solar power generation.

The eclipse hits large swaths of solar power generators simultaneously, reducing diversity; fortunately for the grid operators, we know the exact eclipse path and timing, so grid operators can plan for solar power losses.

But grid operators lack complete information about how much solar generation exists, and where it is located. The operators know exactly where the large solar arrays are, but there’s less information about where all the smaller rooftop solar arrays are, or how much power they can generate. And that’s what makes this experiment interesting.

You can see the balance between grid generation and grid load in the grid frequency, which is kept at 60 cycles per second, or 60 Hertz. Higher than 60.000 Hertz means there’s slightly more generation than load; lower, there’s slightly more load than generation. The grid operators dance with the frequency, adjusting the generator settings throughout the grid to maintain a steady flow of power.

Power Standards Lab is a private company in Alameda, California, that does research projects with the U.S. Department of Energy.

As a public service, PSL 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.

Readings from both instruments are available to the public at and

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

For more information…

Useful images, with captions – free use


Christian Cossette

About Christian Cossette

Christian Cossette est vice-président des ventes et du marketing de Power Survey. Il a plus de 20 ans d’expérience dans le domaine des hautes technologies pour de multiples secteurs notamment dans les secteurs des télécommunications, de l’électronique de puissance et de l’automatisation.

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