California's solar power will plummet during Saturday's near-total solar eclipse. But your lights will stay on. | A New Shade of Green | Sherry Listgarten | Almanac Online |

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California's solar power will plummet during Saturday's near-total solar eclipse. But your lights will stay on.

Uploaded: Oct 8, 2023

On Saturday morning a solar eclipse lasting about three hours will obscure most of the sun across the full expanse of the state and beyond. The eclipse will eliminate much of our solar power. Grid operator CAISO notes in its messaging to market participants: “This is a unique event … during which approximately 9,700 MW of solar generation will rapidly go away and then return within the span of less than three hours.”

Solar power on the grid will plummet 9,687 MW (compared with a clear-sky October day) during the peak of the eclipse, and then rocket back up. Solar typically provides over half of the grid’s power on a clear October morning. Source: CAISO blog (2023)

Not only will much of the grid’s solar power vanish, but the load on the grid will go up at the very same time, as rooftop solar production drops and systems without battery storage turn to the grid for power.

Demand on the grid will increase by around 5,000 MW as rooftop solar production drops during the eclipse and systems without batteries turn to the grid for power. Source: CAISO blog (2023)

This sounds like a doomsday scenario, but most likely none of us will notice it. How can that be?

First of all, we’ve had some practice. A similar eclipse occurred on Monday August 21, 2017. It was also in the morning, also across much of California and the United States. CAISO planned for it for over a year, and the accurate forecasting allowed the state’s power market to function as expected. Supply came online as configured the day before, with few real-time adjustments needed. That kept energy prices stable, with CAISO reporting afterwards: “Energy prices in both the day-ahead and real-time markets remained stable during the day, with real-time prices going no higher than $30/MWh.”

The additional supply needed came from a mix of imports (making up about half of the shortfall), thermal power (gas), and hydropower (it was a good water year), as shown in this graph.

During the August 21, 2017 eclipse, additional resources were brought online when solar power disappeared. Source: CAISO report (2017)

This week’s eclipse is easier than the 2017 one in some ways but more difficult in others. On the plus side, it’s during a Saturday in October, when demand is typically lower than on a Monday in August. On the other hand, our reliance on solar power has increased substantially since 2017, both in CAISO and in the wider interconnected power market. Our greater reliance on grid solar means that we will see a bigger drop in our power supply. The larger amount of rooftop solar means that demand will go up more than it did in 2017. And our neighbors in the extended power market will start seeing those effects as well. (1)

Our reliance on both grid solar and rooftop (BTM: “Behind The Meter”) solar has grown substantially, increasing the effect of the eclipse on our power supply. WEIM: Western Energy Imbalance Market. Source: CAISO presentation (2023)

When you add up the impacts of both the reduced supply and the increased demand, we have a deficit of 14 GW over a typical October weekend morning.

This chart shows the combined effect of both the diminished solar supply and the increased load, causing a deficit of up to 14 GW compared to a clear-sky October day. Source: CAISO presentation (2023)

Fortunately we have access to plenty of extra energy, with imports, gas, and hydropower all likely to play key roles as they did in 2017. In addition, our recently-added five GW of battery storage on the grid will be instrumental in keeping the grid stable while the power supply changes quickly over the course of the eclipse.

The quick ramping down and then ramping up of solar seems to be the most challenging part of this. The grid operator estimates that we will need to ramp up non-solar energy at a rate of 120 MW per minute as the sun gets eclipsed. When the more powerful sun comes back midday, we will have to ramp down those same resources even faster, with an average rate of 190 MW going offline every minute. That is one large power plant every few minutes! That is faster than anything we would typically experience on a normal day. CAISO operators say that they can proactively slow the rate at which solar resources come back online in order to help manage this down-ramp.

The y-axis in this chart measures how quickly we ramp up or down certain sources of energy. While the ramp-up rate during the eclipse is something we might normally handle on an October evening, the ramp-down is much faster than we would typically see. Source: CAISO presentation (2023)

The grid operators seem pretty confident about managing the eclipse, saying “We do not expect any impacts to businesses or residents.” CAISO's Senior Energy Meteorologist Jessica Stewart and Lead Forecast Modeler John Rudolph took the time to jointly write a great blog post about their approach. For the grid operators, this is just another exercise in planning, with the key being to get accurate forecasts and good market bids a day ahead. With good forecasts, they can keep costs and resource use down. But it will still be interesting to look at the CAISO dashboard to see how things go. How are we backfilling the supply? How useful are batteries? What does the ramp back look like?

I love that we are using cleaner energy every year and getting more comfortable managing events like this. Texas will be seeing the same thing but without access to imports, so it will be interesting to see how they manage it in comparison. And there’s more to look forward to -- another total eclipse is coming to the US on April 24, 2024!

Notes and References
0. One more thing happening on Saturday, October 14 is the Electric Home Tour, with 40 homes throughout San Mateo and Santa Clara counties. It’s free, but you need to register. Check it out if you’d like to see some of these efficient electric heaters, stoves, driers, or solar roofs in action and/or talk with the owners about their experience. The tour consists mainly of single-family homes, but there is also one retrofit townhouse, one duplex, one apartment complex, and a few ADUs, so hopefully you can find a home like yours to visit if you are interested.

1. The Western Energy Imbalance Market is much bigger than it was in 2017, with 22 members now compared with 4-5 in 2017. So these numbers reflect not only a growth in solar but also a growth in the market itself.

2. The eclipse leads to cooler temperatures and sometimes less wind, so those are additional effects that the operators account for.

3. The path of Saturday’s eclipse is shown below.

Current Climate Data
Global impacts (August 2023), US impacts (August 2023), CO2 metric, Climate dashboard

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Posted by Eddie, a resident of Fairmeadow,
on Oct 9, 2023 at 3:02 pm

Eddie is a registered user.

I don't understand why this is such a big deal.
If I look at house's solar production, there are days (granted only a few) where the production has been cut by ~80-90% due to unusually heavy cloud cover, wild fire smoke (remember the orange sun in the summer of 2020), ...
And these were clearly unplanned (or at least less planned than an eclipse).


Posted by Sherry Listgarten, a Almanac Online blogger,
on Oct 9, 2023 at 5:13 pm

Sherry Listgarten is a registered user.

@Eddie, one thing to keep in mind is that California is a big state with a big expanse of desert that has much of the solar. It is very unusual for it to be very cloudy or even smoky over much of the solar. Here is a graph of the supply today; you can see the clouds here barely put a dent in the solar.

Here is the "orange sky" day (September 9, 2020).

I think the main thing that makes this eclipse less of a big deal is that it's not happening on a hot summer afternoon when we are at peak demand and would have less capacity and imports to spare. Hopefully if/when that does happen, we'll have bidirectional EV charging that we can call on to power the grid for a few hours.

Posted by Eric Muller, a resident of Los Altos,
on Oct 10, 2023 at 11:14 am

Eric Muller is a registered user.

What's the big deal? So there is a decrease in utility-scale solar production of 5,068 MW in 1 hour 25 minutes and an increase of 10,800 MW in 1 hour 35 minutes.

Those two things occurred on October 4: the first starting at 4:30pm, the second starting at 7:00am. And October 4 is not special, this happen every day. And the "additional load" from BTM solar was similarly affected. As far as CAISO is concerned, October 14 is just running a funny clock.

Also, if you have the capacity, the difference between adding 10 and 20 GW of natural gas is "just" to start n or 2*n units. So the steepness does not sound that much of a problem (the stability of the transmission grid may be, though)

Seems to me that the largest effect is a financial one: additional costs for running natural gas plants for a few hours (and of course a consequent environmental cost).

Of course, I don't know that much about electric grids, but the story as presented does not make sense. What did I miss?


I would prefer to characterize the effect on BTM solar as a "loss of production" rather than an "increase in load". For one thing, that's a more accurate description of the physical situation. But more importantly, utility-scale solar and BMT solar are part of the same team, aren't they?


Posted by Steven Nelson, a resident of Cuesta Park,
on Oct 11, 2023 at 3:17 pm

Steven Nelson is a registered user.

Sherry: it really is "no big deal" because unlike smoke or a state-wide cloud cover Solar Eclipses are ENTIRELY predictable. You can go to the Astronomical Almanac and get the Solar Eclipse calculations for the next few years - OR a canon, Five Millennium (to 3000 BCE) from NASA (2006)!

Saturday's, Good to plan for - Very Interesting calculations, predictions and graphs - but "no big deal".
Flex Alert! Please turndown your air conditioner, during the middle of the Eclipse because we will have less Solar Power.
(Oct is someone's EnergyAwarenessMonth :)

Posted by Mondoman, a resident of Green Acres,
on Oct 13, 2023 at 1:55 pm

Mondoman is a registered user.

Regarding the Electric Home Tour, has anyone been successful in accessing the website with the address list? I signed up, but get a cryptic error message when following the link they emailed me. I also get an unknown error when using the Eventbrite Android app's email/password to try to login, although an emailed direct login link works fine. Eventbrite's IT robustness seems to be lacking.

Posted by Mondoman, a resident of Green Acres,
on Oct 13, 2023 at 2:11 pm

Mondoman is a registered user.

Electric Home Tour address link now works.

Posted by Sherry Listgarten, a Almanac Online blogger,
on Oct 15, 2023 at 10:10 am

Sherry Listgarten is a registered user.

I hope everyone was able to go out and enjoy the strange light of the eclipse yesterday morning! You can see the effect it had on the state's solar power here:

The grid responded by increasing imports (I expect from the Pacific Northwest, but I haven't been able to confirm) and natural gas.

They did deploy some battery, but not as much as I thought (I guess it was too expensive), and I don't see much change in hydro. It does look like they slowed the rate at which solar came back on because the solar-return curve isn't much steeper than the solar-depart curve. But generally it looks like this was well within our capacity to handle.

@Steve, you may be underestimating how good we are at predicting weather (e.g., clouds). We have invested a lot in that. Our day-ahead energy forecasts seem very accurate. (You can see them on the CAISO charts.) I'd also note that the goal of the grid here was no impact on people (e.g., no flex alerts). I think you are right that those could be needed with this big a hit on our power supply, but not at this time of day and year.

Anyway, I hope everyone enjoyed the eclipse!

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