For a moment last Saturday, CAISO was able to serve nearly 100% of its native load with renewable energy, beating a record set just a month earlier.

The peak occurred at 2:50 p.m. on April 30, when the California grid operator served 99.87% of its momentary demand with renewables, breaking the previous record of 97.6% set on April 3, the ISO confirmed Tuesday after reviewing generation data. (See CAISO Sets 98% Renewables Record.)

In a tweet Tuesday, CAISO called the event “a significant milestone along the path to a carbon-free power grid.”

Preliminary data from the ISO indicated that output from renewables reached 18,629 MW during the peak, nearly matching demand. At the same time, gas-fired plants were generating 2,434 MW, nuclear 2,239 MW and large hydro 590 MW. Exports from the CAISO system hit about 4,400 MW during the interval.

The exports did not match total generation from those resources because a portion of the ISO’s natural gas generation consists of reliability-must-run units operating behind transmission constraints and combined heat and power units that cannot be curtailed.

Spring is typically a period of low demand in California, accompanied by relatively high output from solar, wind and hydro, often leading to energy surpluses.

CAISO’s installed renewable energy mix consists of about 57% solar, 30% wind, and smaller amounts of geothermal energy, small-hydro resources and biofuels. While emissions-free and technically renewable, large hydro resources are not included in the mix.

About 32% of California’s energy mix came from renewable power in 2020, the most recent year for which figures are available, according to the state Energy Commission.