California solar energy not in sync with electric consumers’ max demand
California Independent System Operator
hourly electric supply
August 26-31, 2021
Southwest United States with California Independent System Operator area highlighted. U.S. Energy Information Administration (EIA) map.
solar electric energy supply declines when total electric consumers’ demand is greatest
solar & wind energy curtailment required when oversupply exists
The California Independent System Operator (Cal-ISO)¹ manages hourly electric megaWatt-hours generated and interchanges (imports/exports) with neighboring regions.
Hourly electric megaWatt-hour (MWhr) consumption demand (blue) and electric energy generated within the Cal-ISO area vary on regular daily cycle. Generation is plotted (below) in yellow, demand in blue. Interchange hourly MWhrs are negative, indicating imports (green). <Fig. 1>
Fig. 1: Cal-ISO hourly electric overview: August 25-31, 2021. U.S. Energy Information Administration (EIA) chart². - Tap/click to enlarge -
Most Cal-ISO hourly MWHr interchanges with neighbors are imports. <Fig. 2>
Total of hourly interchanges is shown in black. August 31 hourly data for neighbor Balancing Authorities was not available for download from EIA Hourly Grid Monitor on September 1.
Fig. 2: CISO hourly interchange with neighbor Balancing Authorities: August 26-31, 2021. U.S. EIA chart
Solar energy declines during daily peak consumption demand
Peak electric generating time was 4-5 PM each day of the August 25-31 period. Total hourly electric MWhrs generated in the Cal-ISO area is shown in black, peak times are highlighted by vertical red lines. Some gaps appear in the EIA data. <Fig. 3>
Fig. 3: Cal-ISO hourly electric energy supply from all sources: August 25-31, 2021. U.S. EIA chart, red lines added by Linecurrents.live.
Daily solar generating output declined sharply during the daily peak hour. (yellow). California electric utilities are installing grid-scale battery energy systems to store electric energy during low-cost periods for release during high cost/demand periods. California is the leader among U.S. States for installed utility-scale battery energy storage capacity.
Cal-ISO major electric generating sources are:
natural gas
solar
wind
nuclear
hydroelectric
Natural gas combustion supplied nearly half of the electric energy generated in the Cal-ISO area August 31, 2021. <Fig. 4>
Fig. 4: Electric MWhrs generated in CISO area from all sources: August 31, 2021. U.S. EIA chart.
Cal solar/wind electric energy occasional oversupply causes curtailments
When demand for electric energy is low and solar energy generation is high, Cal-ISO, individual generating entities, or electric utilities curtail (shut-off) utility-scale solar systems. <Fig. 5>
Fig. 5: U.S. EIA
EIA Today in Energy - August 24, 2021 describes why California’s curtailments of solar electricity generation continue to increase:
Curtailments of solar-powered electricity generation have increased in the California Independent System Operator (CAISO) region, the part of the electric grid that covers most of the state. In 2020, CAISO curtailed 1.5 million megawatthours of utility-scale solar, or 5% of its utility-scale solar production.
Grid operators curtail electricity production from solar and wind generators when supply exceeds demand. In 2020, solar curtailments accounted for 94% of the total energy curtailed in CAISO. Solar curtailments tend to be greater in the spring months when electricity demand is relatively low (because of moderate temperatures decreasing heating and cooling demand) and solar output is relatively high. In the early afternoon hours of March 2021, CAISO curtailed an average of 15% of its utility-scale solar output.
Renewable electric supply curtailments in Cal-ISO area are due systemwide oversupply, economics, local transmission line congestion, or a combination of these factors:
Oversupply is when all anticipated generation, including renewables, exceeds the real-time demand. The potential for this increases as more renewable energy is added to the grid but demand for electricity does not increase. . . . During oversupply times, wholesale prices can be very low and even go negative in which generators have to pay utilities to take the energy.
- California ISO - What the duck curve tells us about managing a green grid (PDF)
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Congestion occurs when available, least-cost energy cannot be delivered to some loads because transmission facilities do not have sufficient capacity to deliver the energy.
- California ISO - Wind and Solar Curtailment - August 31, 2021 (PDF)
Curtailments totaled 1,225,467 MWhr in Cal-ISO year-to-date 2021, according to the August 31 Cal-ISO report.
More info
California ISO - Managing Oversupply
Appendix
other abbreviations synonymous with Cal-ISO are: CAISO and CISO.
U.S. EIA disclaimer: The information submitted by reporting entities is preliminary data and is made available "as-is" by EIA. Neither EIA nor reporting entities are responsible for reliance on the data for any specific use
Hourly electric generating data source for this report is the U.S. Energy Information Administration (EIA) Hourly Electric Grid Monitor. EIA informational notes explain the role of Balancing Authorities and exclusion of small scale generators from collected data:
The generation data represent generators that are managed by balancing authorities or whose operations are visible to the balancing authorities. Generators on the distribution system—both large-scale resources and small-scale distributed resources, such as rooftop solar photovoltaic (PV) systems—are typically not included.