This is an extract from a recent report “A Transformative Summer for U.S. Electricity” by IEEFA. 

Key highlights

• Even with heat waves pummeling California and Texas this summer, renewables are reliably and economically transitioning the grid from fossil fuels. 

• Solar generated 16 million MWh of electricity from June 1 to August 31 in Texas, accounting for 11.6% of ERCOT’s total summer demand—a 40% increase from just a year ago. 

• The transformation in California has been equally pronounced, and again points to the reliability enhancing combination of solar and storage. 

• Four years ago, there was essentially zero battery storage capacity installed across the U.S., but now there are more than 20,000 MW and that figure increases monthly.

The evidence from this summer’s sizzling heat is in, and the verdict is clear: Renewables are reliably and economically transitioning the grid from fossil fuels. Heat waves pummeled California (in July and again in early September) and Texas (in August), but grid operators in both states had little trouble meeting demand, in large part due to the rapid growth of renewable energy generation (particularly utility-scale solar) and the installation of thousands of megawatts (MW) of battery storage capacity. For two of the largest power markets in the country, and for the U.S. as a whole, these are the hallmarks of a transformative shift.

Texas: 

Solar’s performance during the Texas summer was nothing short of spectacular. From June 1 to August 31, solar generated 16 million megawatt-hours (MWh) of electricity, accounting for 11.6% of the Electric Reliability Council of Texas’ (ERCOT) total summer demand. This is a 40% increase from just a year ago, when solar produced 11.4 million MWh during the summer. More telling is the reliability of solar’s output during the critical peak demand time each day. These key hours, especially during heat waves, are when the grid is most tested, power prices can skyrocket, and customers’ concerns about brownouts and blackouts soar. This summer, though, a vast increase in solar generation, and its consistent reliability, helped allay those issues. On average, solar met 20.4% of ERCOT demand during the peak hour during this 92-day period, with only a narrow band up or down in daily output. In August, for example, when the average peak demand was more than 80,500 MW, solar generated an average 20.7% of that demand, ranging from 17.2% to 24.2%. Importantly, on August 20 when ERCOT set its latest all-time record of 85,544 MW, solar sent 17,568 MW onto the grid during the peak, accounting for 20.5% of the needed supply. 

Solar’s summer-long reliability undercuts concerns by fossil fuel advocates that renewables aren’t dependable. The data proves that they are dependable. Here, it is worth noting that while solar was reliably available August 20, several gas- and coal fired generation resources tripped offline just as ERCOT was nearing its peak demand for the day. Between 2 p.m. and 4 pm, the 391 MW San Miguel coal plant near San Antonio dropped offline, and then in the early evening, just as the peak neared, roughly 1,200 MW of gas-fired capacity went out of service. A day earlier, the 785 MW Unit 2 at the J.K. Spruce coal plant, also near San Antonio, had gone offline due to a tube leak, leaving system operators short of almost 2,500 MW of supposedly reliable fossil fuel generation on very short notice. The summer-long growth in solar generation has continued into early fall, with solar setting back-to-back output records on September 7 and September 8, hitting 20,990 MW the first day and jumping to 21,667 MW the second day. As important, solar’s contribution to peak hour demand rose as well, with solar sending a record 19,964 MW to the grid the first day and accounting for more than 31% of peak demand both days.

The growth in solar generation, coupled with the system’s existing wind capacity, is reshaping the ERCOT grid, which supplies 90% of the load in Texas, even as it continues to grow. ERCOT generation in 2024 has increased by almost 11 million MWh through August, a 3.6% rise, but renewables have accounted for all the increase (and a bit more). This has pushed wind and solar’s share of the market up to 34.9% and the carbon-free share including ERCOT’s two nuclear plants to 43.2%. Continued growth in solar capacity is almost certain to push renewables’ share even higher over the next two years. According to ERCOT data, 26,087 MW of solar capacity is currently installed on the system. By June 2025, ERCOT expects an additional 6,380 MW of solar capacity will be on the grid, and by the end of the year, twice that—13,000 MW—is expected to be online.

In addition to solar’s growth, the rapid addition of battery storage capacity within ERCOT has added greatly to system reliability, with this new resource playing an increasingly important role in the evening as solar ramps down while demand is still relatively high. Peak demand during ERCOT’s August 20 record occurred at 6 p.m., but demand was still high at 7 p.m. as solar generation began to fall quickly and essentially hit zero by 8 p.m. During that period, batteries across ERCOT began sending electricity back into the grid, reducing the need for additional gas and/or coal-fired generation. From 7 p.m. to 8:30 p.m., battery power accounted for at least 3% of the system’s power needs, climbing to a peak of 3,930 MW at 7:35 p.m., when it met 4.9% of demand.

This battery output is particularly important because of the timing. It occurred when the system was hitting its net demand peak, which is the term system operators use to discuss the time of highest demand after removing solar and wind from the calculation; net peak demand generally occurs after the sun sets, when solar generation has ended. In the August 20 example, net peak demand topped out at just under 71,000 MW at 7:45 p.m. Without the 3,790 MW of battery power flowing back to the grid at that time, additional thermal generation would have been required. By 8:30 p.m., net demand had fallen below 70,000 MW and gas generation started to decline. Unlike years past, ERCOT powered through the August 20 peak—and the entire summer, in fact— without having to issue any calls for consumer conservation. This stands in sharp contrast to 2023, when the system issued 12 calls for conservation from June through September. Clearly, renewables and storage are having a positive influence.

California

The transformation in California has been equally pronounced, and again points to the reliability enhancing combination of solar and storage. This analysis focuses on the region served by the California Independent System Operator (CAISO), which manages roughly 80% of the state’s daily demand, but the lessons are similar across the state. Demand in CAISO’s territory during an early July heatwave peaked at 43,969 MW at 7 p.m. on July 11. In the past, that could have been a problem, since solar generation is rapidly declining that late in the day. In fact, on that day solar had dropped from 11,040 MW at 6 p.m. to 4,370 MW at 7 p.m., according to data from GridStatus.io. But by that point, battery storage injections had ramped up to 4,210 MW and would remain above 4,000 MW until 9:30 pm, enabling system operators to throttle back on gas generation across the state. Significant new clean energy generating capacity, a rapid increase in battery storage resources and better cooperation across the Western Interconnection all helped CAISO and other state power suppliers meet the demands of the two-week-long July heatwave. The CAISO system performed similarly well in late July, hitting another summer peak of 45,426 MW on July 24. A few weeks later, on September 5, when temperatures climbed into the 100s, an even higher peak of 47,753 MW was set. On both days, solar and battery storage recorded strong performance numbers. The symbiotic relationship between solar and storage is clearly evident drilling down into the daily generation numbers.

Conclusion 

The speed of the ongoing transition is often hard to comprehend; the battery storage graphic below illustrates that perfectly. Four years ago there was essentially zero battery storage capacity installed across the U.S., but now there are more than 20,000 MW and that figure increases monthly: That is impressive growth.

The speed of that growth, and solar’s as well, has changed how grid operators run the power systems across the U.S. But that does not mean those systems are less reliable. Indeed, the experiences from the summer of 2024 demonstrate clearly that renewable energy and storage can reliably run power systems transitioning away from fossil fuels.

Access the complete report here