April total eclipse to impact solar generation from Mexico to New England


As solar capacity increases, the grid impact of subsequent major solar events also increases. On April 8, a total eclipse will pass from Mexico, across Texas and up the East Coast, with most of the continental U.S. experiencing a significant drop in solar generation. The eclipse will occur from Noon to the early afternoon, when solar generation is at its highest. It is too early to predict weather conditions for the day, in particular high-resolution cloud modeling, so this analysis is based on clearsky data via the Solcast API.

Areas in the totality, where the moon completely blocks the sun, will see a 100% loss in solar generation for the duration of the totality. However, the overall effects of the eclipse will cost up to 16% of daily total clear sky irradiance in areas most affected.

While it is too early to predict the precise cloud impacts on the day, Grid Operators will already be preparing for the maximum potential impact, a temporary total loss of solar generation and a fast ramp of solar decreasing then increasing. For areas directly in the path of the eclipse, the maximum duration will be over 90 minutes of impacted generation, and a total loss of up to 6 minutes. In every grid analyzed, the rate at which solar generation drops off and then picks back up again, is faster than grids normally see in the morning and evening.

Due to the large proportion of utility scale assets in ERCOT, Texas will be heavily impacted by the effects of the eclipse. Individual assets will lose up to 16% of their daily irradiance, but the wide area covered by ERCOT means that the overall loss to the grid will be up to 11.7% of daily utility scale solar generation. At current capacity, that would be 16.9 GWh, though the rapid increase in capacity in ERCOT, and known projects coming online before April makes it likely this number could be higher. Solcast’s grid aggregation model shows that the ramp will be slightly steeper than normally seen in the morning or evening, peaking at a rate of 250 MW/minute. The fast change in generation is what can cause instability in the grid, so asset managers, energy traders and grid operators will be working to maintain stability whilst making the most of volatile energy prices.

As the eclipse moves up the East Coast, it will impact both NYISO and ISO-NE. These regions have less utility scale solar than Texas, so the impact will mostly be seen in ‘behind-the-meter’ residential rooftop solar generation. For each grid, the impacts are fairly similar. NYISO will lose up to 10.91% of their daily rooftop generation, and up to 3.1 GWh of power. Being further south, and hit by the eclipse slightly earlier explains the difference with ISO-NE. New England will lose up to 9.85% of its daily behind-the-meter generation, though differences in installed capacity make this a higher 3.7 GWh. Notably the ramp rate is much higher than the morning or afternoon ramps, as irradiance will drop from almost the daily maximum to zero in approximately 40 minutes. This will require active management from the grid operators to maintain stability.

CAISO in California will also see impacts from this eclipse, though being so far from the path of totality, the effects will be less than seen in the partial annular eclipse in September 2023. Despite seeing a lower proportional effect from grids in the North-East, only 5.72% of daily generation, increased levels of rooftop solar in California mean that the energy losses will be greater than either NYISO or ISO-NE, up to 4.0 GWh.

While the impact of this eclipse is significant, it is predictable, and grid operators are already preparing and planning for the impacts. Large storm events, snow dump events and large heavy cloud fronts are less spectacular but can have even bigger impacts on whole-day solar generation. These events are also harder to plan for and predict, which makes it more important for asset owners and grid operators to plan and manage the impact of weather on solar generation as solar increases in the generation mix.

Solcast produces these figures by tracking clouds and aerosols at 1-2km resolution globally, using satellite data and proprietary AI/ML algorithms. This data is used to drive irradiance models, enabling Solcast to calculate irradiance at high resolution, with typical bias of less than 2%, and also cloud-tracking forecasts. This information is used by more than 300 companies managing over 150 GW of solar assets throughout the world.

This article was amended to correct the figure in this statement: “the wide area covered by ERCOT means that the overall loss to the grid will be up to 11.7% of daily utility scale solar generation”.

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