The Eclipse Was ERCOT's Easiest Solar Test. The Hard Version Comes Every Evening

On April 8, the moon passed across the sun along a path from Texas to Maine, and for a few minutes the United States ran a controlled experiment on its solar fleet. The Energy Information Administration had estimated beforehand that the eclipse would fully block sunlight to 6.5 gigawatts of utility-scale solar in the path of totality and partly block it to 84.8 GW across a much wider area, close to the time of peak solar output.

In the event, the Electric Reliability Council of Texas lost about 8.9 GW of solar capacity. Gas plants replaced roughly 80% of the lost output, and the lights stayed on. The defining feature of an eclipse is that it is perfectly predictable. Grid operators knew the date, the minute and the expected depth of the solar loss in each location. ERCOT five-minute data show solar generation falling from 12:20 p.m. central time, with the eclipse ending in the state at 3:07 p.m.

What made it manageable was not only the size of the gas fleet but the fact that it had been told exactly when to be ready. The EIA's analysis of ERCOT's changing supply shows how quickly this evening ramp is growing. Gas plants in Texas already do what they did on April 8, but daily. There are three differences between the eclipse and the nightly ramp that matter for planning. ERCOT has added battery storage rapidly, and its role in the evening is growing.

The April 8 data show gas did most of the work this time. The eclipse confirmed that ERCOT's thermal fleet can absorb a fast, deep, known loss of solar. The resources that matter most for that risk are those available in the evening hours on short notice: flexible gas plants, batteries with enough duration to cover the ramp, demand response, and imports, of which ERCOT has very few because of its limited interconnection with neighboring grids.