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Contract No. DE-AC36-08GO28308

Overgeneration from Solar 

Energy in California: A Field 

Guide to the Duck Chart

Paul Denholm, Matthew O’Connell, 

Gregory Brinkman, and Jennie Jorgenson 

National Renewable Energy Laboratory

Technical Report

NREL/TP-6A20-65023

November 2015
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Overgeneration from Solar 

Energy in California: A Field 

Guide to the Duck Chart

Paul Denholm, Matthew O’Connell, 

Gregory Brinkman, and Jennie Jorgenson 

National Renewable Energy Laboratory

Prepared under Task No. TM13.5020

Technical Report

NREL/TP-6A20-65023

November 2015
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Acknowledgements

This work was funded by the Solar Energy Technologies Office and the Office of Strategic 

Programs of the U.S. Department of Energy’s (DOE) Office of Energy Efficiency and 

Renewable Energy (EERE) under contract number DE-AC36-08GO28308.

Thanks to Jimmy Nelson, UCS, Elaine Hart, E3, and Jim Lazar, RAPP for original figures. For 

their reviews and comments, the authors would like to thank Mark Ahlstrom, Aaron Bloom, 

Jaquelin Cochran, Udi Helman, Ookie Ma, Robert Margolis, Mike Meshek, Mackay Miller, 

Dave Mooney, Jimmy Nelson and Charlie Smith.

Abstract

In 2013, the California Independent System Operator published the “duck chart,” which shows a 

significant drop in mid-day net load on a spring day as solar photovoltaics (PV) are added to the 

system. The chart raises concerns that the conventional power system will be unable to 

accommodate the ramp rate and range needed to fully utilize solar energy, particularly on days

characterized by the duck shape. This could result in “overgeneration” and curtailed renewable 

energy, increasing its costs and reducing its environmental benefits. This paper explores the duck 

chart in detail, examining how much PV might need to be curtailed if additional grid flexibility 

measures are not taken, and how curtailment rates can be decreased by changing grid operational 

practices. It finds that under business-as-usual types of assumptions and corresponding levels of 

grid flexibility in California, solar penetrations as low as 20% of annual energy could lead to 

marginal curtailment rates that exceed 30%. However, by allowing (or requiring) distributed PV 

and storage (including new installations that are part of the California storage mandate) to 

provide grid services, system flexibility could be greatly enhanced. Doing so could significantly 

reduce curtailment and allow much greater penetration of variable generation resources in 

achieving a 50% renewable portfolio standard. Overall, the work described in this paper points to 

the need to fully integrate distributed resources into grid system planning and operations to allow 

maximum use of the solar resource. 
iv

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Table of Contents

Abstract....................................................................................................................................................... iii

1 Introduction........................................................................................................................................... 1

2 Background: Why Ducks Lead to Overgeneration ........................................................................... 3

3 Previous Analysis of the California Duck and Estimates of Overgeneration ................................ 5

4 Study Methods and Data.................................................................................................................... 11

5 Results: Base “Most Conservative” Case ....................................................................................... 13

6 Enabling Greater Solar Penetration: Flatten or Fatten the Duck?................................................. 24

7 The Impact of Improved System Flexibility ..................................................................................... 27

8 Additional Opportunities to Fatten and Flatten the Duck .............................................................. 35

9 Conclusions ........................................................................................................................................ 36

References................................................................................................................................................. 37
v

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List of Figures

Figure 1. The CAISO duck chart................................................................................................................ 3

Figure 2. “Proto-duck” chart of California net load with increased penetration of PV ....................... 6

Figure 3. Example of an analysis of the impact of concentrating solar power (CSP) on the duck 

chart shape............................................................................................................................................ 7

Figure 4. Example of the impact of changing system flexibility on demand shape and curtailment 

from an analysis by the Union of Concerned Scientists.................................................................. 8

Figure 5. Example of an analysis of the impact of high VG on net load shape and 

resulting overgeneration ..................................................................................................................... 9

Figure 6. Example of an analysis of how the duck curve shape can be modified to 

minimize overgeneration ................................................................................................................... 10

Figure 7. Load, solar, and wind profiles for California on March 29 in a scenario with 11% annual 

wind and 11% annual solar assuming no curtailment.................................................................... 14

Figure 8. Modeled net load in California on March 29 in a scenario with 11% annual wind and 11% 

annual solar in a system with a 60% instantaneous penetration constraint ............................... 15

Figure 9. Used and curtailed VG in California on March 29 in a scenario with 11% annual wind and 

11% annual solar ................................................................................................................................ 16

Figure 10. Instantaneous penetration of VG on March 29 with and without curtailment in a scenario 

with 11% annual wind and 11% annual solar .................................................................................. 17

Figure 11. Increase in VG use resulting from schedulable pumped storage in a scenario with 11% 

annual wind and 11% annual solar................................................................................................... 18

Figure 12. Load, solar, and wind profiles for California on July 27 in a scenario with 11% annual 

wind and 11% annual solar................................................................................................................ 18

Figure 13. Instantaneous penetration of VG in California on July 27 in a scenario with 11% annual 

wind and 11% annual solar................................................................................................................ 19

Figure 14. Load in California and VG Profiles on March 29 in a scenario with 11% and 15% annual 

solar assuming no curtailment ......................................................................................................... 20

Figure 15. Net load on March 29 in a scenario with 11% and 15% annual solar considering 

operational constraints...................................................................................................................... 21

Figure 16. Usable and curtailed VG on March 29 in a scenario with 11% and 15% potential 

annual solar......................................................................................................................................... 21

Figure 17. Marginal and average curtailment due to overgeneration under increasing penetration 

of PV in California with a 60% instantaneous penetration limit .................................................... 22

Figure 18. Marginal and average PV LCOE (based on SunShot goals) due to overgeneration under 

increasing penetration of PV in California with a 60% instantaneous penetration limit............. 23

Figure 19. Accommodation of increased penetration of PV by reducing system minimum 

generation requirements and fattening the duck............................................................................ 25

Figure 20. Accommodation of increased penetration of PV by flattening the duck (increasing midday demand)........................................................................................................................................ 26

Figure 21. Impact of flattening the duck on March 29 with 1,285 MW of added storage in a scenario 

with 15% annual solar........................................................................................................................ 28

Figure 22. Net load on March 29 in a scenario with 15% annual solar increasing the maximum 

penetration of VG to 60% to 80% ...................................................................................................... 30

Figure 23. VG curtailment on March 29 in a scenario with 15% annual solar increasing the 

maximum penetration of VG to 60% to 80% and removing the local generation requirement .. 31

Figure 24. Marginal and average annual curtailment due to overgeneration under increasing 

penetration of PV in California after adding mandated storage, removing local generation 

constraint, and increasing maximum instantaneous VG penetration to 80%.............................. 32

Figure 25. Impact of additional demand response on system net load on March 29 and July 27 ... 33

Figure 26. Marginal and average curtailment due to overgeneration under increasing penetration 

of PV in California after additional demand response and increasing maximum penetration 

to 90% .................................................................................................................................................. 34
1

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1 Introduction

In 2013, the California Independent System Operator (CAISO) published a chart showing the 

potential for “overgeneration” occurring at increased penetration of solar photovoltaics (PV).

1

The “duck chart”2 shows the potential for PV to provide more energy than can be used by the 

system, especially considering the host of technical and institutional constraints on power system 

operation.

During overgeneration conditions, the supply of power could exceed demand, and without 

intervention, generators and certain motors connected to the grid would increase rotational speed, 

which can cause damage. To avoid this, system operators carefully balance supply with demand, 

increasing and reducing output from the conventional generation fleet.3 The overgeneration risk 

occurs when conventional dispatchable resources cannot be backed down further to 

accommodate the supply of variable generation (VG). Overgeneration has a relatively simple 

technical solution, often referred to as curtailment. Curtailment occurs when a system operator 

decreases the output from a wind or PV plant below what it would normally produce. For wind, 

this is performed by changing the energy captured from the wind (by changing the blade pitch 

angle) (Aho et al. 2012). For solar, generation is curtailed by either reducing output from the 

inverter or disconnecting the plant altogether. This of course requires a plant or system operator 

to have physical control of the generation resource, which is typically available for large 

renewable power plants but uncommon for smaller systems, particularly distributed or rooftop 

systems. While curtailment is technically easy, it has the obvious undesirable trait of reducing 

the economic and environmental benefits of VG. Each unit of VG curtailed represents a unit of 

energy not sold on to the grid and a unit of fossil fuel not avoided. As the amount of curtailment 

increases, the overall benefits of additional solar may drop to the point where additional 

installations are not worth the cost (Cochran et al. 2015).

Neither the potential for overgeneration, nor the resulting curtailment of variable generation 

resources is a new concern (Bird et al. 2014). However, the significant attention paid to the duck 

chart signals an important change in attitude toward integration of variable generation (VG). The 

duck chart represents perhaps the first major acknowledgement by a system operator that solar 

energy is no longer a niche technology (at least in California) and that curtailment will be a 

significant issue in the not-too-distant future. The chart has also raised general awareness of the 

issues associated with renewable curtailment and system flexibility.

The duck chart is largely illustrative in nature, representing only one day of the year, and it does 

not quantify the actual curtailment that may occur at increased penetration of solar energy. Nor 

does the chart reflect the impact of mitigation options.

In this work, we examine how the duck chart shape illustrates potential overgeneration risks in 

California at increased penetration of PV. We first review previous analyses of the impact of PV 

 1 https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf 2 The name is derived from its resemblance to the profile of a duck.

3 Throughout this document, we use the term system operator to refer to the balancing area authorities responsible 

for balancing supply and demand through generator scheduling and dispatch. In California, the largest of these is the 

CAISO, but there are several other system operators, including Los Angeles Department of Water and Power, 

Imperial Irrigation District, Balancing Authority of Northern California, and Turlock Irrigation District.
2

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on load in California and potential PV curtailment at increased penetration. We then use a 

production cost model to quantify the amount of overgeneration that can result from the 

increased PV without measures to increase system flexibility. Finally, we show how the duck 

shape can be accommodated with different measures to increase flexibility—including flexibility 

that is provided by the PV itself under appropriate market rules—and how overgeneration risks 

can be dramatically reduced by introducing multiple flexibility measures.
3

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2 Background: Why Ducks Lead to Overgeneration

The CAISO duck chart itself illustrates the general challenge of accommodating solar energy and 

the potential for overgeneration and solar curtailment. In the chart, each line represents the net 

load, equal to the normal load minus wind and PV generation. The “belly” of the duck represents 

the period of lowest net load, where PV generation is at a maximum. The belly grows as PV 

installations increase between 2012 and 2020. While the amount of PV in 2020 is not shown 

directly, it can be estimated by comparing the 2012 curve to the 2020 curve. In this case, the 

normal load (i.e., no PV and adjustments for load growth) at about 1-2 p.m. on March 31, 2020 

appears to be about 22,000 megawatts (MW), while PV is generating about 10,000 MW, leaving 

about 12,000 MW to be met with other resources. In this case, PV provides perhaps 45% of the 

total demand in this one hour. The duck chart also points to the period of overgeneration risk, 

which could result in curtailed energy.

Figure 1. The CAISO duck chart

Source: CAISO 2013

The CAISO duck chart document does not explicitly quantify the amount of expected

curtailment during this period, but it describes two main causes: 

The first occurs as the ISO [independent system operator] prepares to meet the 

upcoming upward ramps [using conventional generation] that occur in the 

morning and in the late afternoon. The existing fleet includes many long-start 

resources that need time to come on line before they can support upcoming ramps. 
4

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Therefore, they must produce at some minimum power output levels in times 

when this electricity is not needed.4

The second source of overgeneration and curtailment “occurs when output from any nondispatchable/must-take resource further increases supply in times of low electricity need, 

typically in the nighttime hours. Historically, this condition was most likely to occur in the early 

morning hours when low demand combines with electricity and generation brought on line to 

prepare for the morning ramp.” This second challenge includes the need to accommodate output 

of all generation resources such as wind and hydro, and plants that produce heat and electricity. 

Overgeneration can also result from “must-run” plants that are needed for local voltage support 

and reliability issues, and also from a number of institutional constraints, such as long-term 

contracts and self-scheduling from certain power plants (GE Energy 2015; Bouillon 2014).

Combined, these issues create an operational challenge which can be described as the “minimum 

generation” problem which represents the technical and economic limits of thermal and hydro 

power plants to reduce output or turn off, especially during relatively short periods, such as the 

few hours of peak solar output. Other factors can produce curtailed VG, including transmission 

constraints, and at increased penetration of VG, conventional generators that must be online to 

maintain system stability. (This latter issue is discussed in Section 6).

Because of the economic challenges posed by curtailment, it becomes important to examine how 

much curtailment may occur, as well as methods to reduce curtailment. The ability to 

accommodate VG is largely determined by the flexibility of the power grid, and flexibility can be 

changed over time. Examining the relationship between system flexibility and curtailment can 

help determine the potential contribution of solar to meeting the energy requirements of a region 

such as California.

 4 https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf
5

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3 Previous Analysis of the California Duck and 

Estimates of Overgeneration

There are a number of discussions of the California duck chart, and several estimates of 

curtailment that may result in California from increased penetration of PV. Several of these 

discussions are part of larger planning and integration studies that consider broader impacts of 

VG on the system (e.g., system economic and environmental benefits), areas of operational 

challenges (e.g., additional reserve requirements), and integration costs. Integration studies, 

along with general grid planning studies use grid-simulation tools that model the operation of the 

entire generation fleet (Sterling et al. 2013). These have a number of names, including 

“production cost” and “security-constrained unit commitment and economic dispatch” models

(Denholm et al. 2014).

5

We use the term production cost model (PCM) to represent the class of models that simulate the 

chronological operation of the power grid, determining which power plants to commit and 

dispatch during each time interval on the basis of forecasted fuel costs, heat rates and other 

operational characteristics. In each time interval, the model selects the least-cost mix of 

generators needed to meet load while maintaining adequate reserves to meet contingency events 

and other reserve requirements. Such models typically simulate the grid for one year of operation 

in 8,760 one-hour time steps. PCMs calculate the total cost of system operation, including cost of 

fuel and cost of operation and maintenance.6 To model the grid realistically, these tools require 

extensive databases of generator properties, transmission capacity, and system operational 

requirements, such as reserve requirements. In theory, a properly designed and implemented 

PCM simulation should produce results (such as generator dispatch, emissions, and total 

production costs) close to the dispatch resulting from the market operations or dispatch software 

used by independent system operators or balancing areas to actually control the grid. However,

PCMs cannot completely simulate market environments because they typically do not capture 

self-scheduling, bilateral contracts, scarcity pricing, bidding strategies, and other factors that can 

alter system dispatch from the “least-cost” dispatch produced by a model.

An early attempt to model the increased penetration of PV in California using a production cost 

model is Denholm et al. (2008). This work uses the PROSYM PCM and demonstrates a “protoduck” chart showing a deep drop in mid-day net demand (Figure 2) that is similar to that in the 

CAISO duck chart. The net load during this two-day period is from slightly later in the spring 

(May 6–7), and with the higher solar output, represents the lowest net load of any point during 

the year. Despite the lower net load and higher ramp range compared to the CAISO duck chart, 

this analysis did not demonstrate any significant overgeneration or PV curtailment. This is due to 

a variety of favorable assumptions, including “frictionless” exchange of energy with the 

surrounding regions without restrictions other than the thermal limits of the transmission 

network. At the highest level of penetration, nearly half of the incremental PV generation in 

 5 Various entities, including independent system operators use these models to simulate system operations for 

planning purposes. However these are a number of differences between how these models work and the market 

management software used for actual unit commitment and dispatch. An example is the generation of the 

commitment and dispatch “stack” (or merit order). Production cost models use plant-level estimates of variable costs 

while ISO operations use market bids from individual generators

6 Production cost models only consider the variable costs of operating the system. Fixed costs (capital costs, fixed 

O&M) are not considered.
6

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California is displacing out-of-state generation. In addition, while the model did include standard 

generator parameters (e.g., minimum generation and start-time constraints), it did not consider 

any institutional, contractual, or local reliability constraints that may exist within California.

Figure 2. “Proto-duck” chart of California net load with increased penetration of PV

Source: Denholm et al. 2008

The 2013 duck chart and much of the concern regarding overgeneration originates with a series 

of studies published by the CAISO (CAISO 2010, 2011a, and 2011b, Liu 2014a, 2014b and 

2014c) and by emergence of negative prices in the CAISO market driven in part by growth in 

wind generation (CAISO 2012). The CAISO studies have examined the impacts of an increasing 

renewable portfolio standard (RPS) starting with 20%, increasing to 33%, and then 40%, and 

they have demonstrated increasing levels of overgeneration risk. These studies use the PLEXOS 

production cost model, which is one of several commercially available grid simulation tools. 

CAISO produces and maintains a database for this model as part of the Long-Term Procurement 

Plan (LTPP).

7 This database includes generator-level details of California’s electricity sector as 

well the rest of the Western Interconnection. The CAISO has made its PLEXOS databases 

publicly available, and in addition to California utility studies (Mao and Galjanic 2014), several 

non-utility research groups have used them directly or in modified forms to analyze sensitivities 

to various assumptions. Table 1 summarizes several of the previous analyses using some form of 

the LTPP model.

 7 http://www.cpuc.ca.gov/PUC/energy/procurement/LTPP/ltpp_history.htm

0

10000

20000

30000

40000

50000

60000

Winter Spring

Minimum

Summer

Peak

Season and Hour

Net Load (MW)

Base

(no PV)

2%

4%

6%

8%

10%
7

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Table 1. Previous Studies Using the Long-Term Procurement Plan Database

Study Lead Organization Cite Study Focus

CAISO CAISO 2011a, CAISO 

2011b Liu 2014a,

2014a

Multiple studies of a variety of renewable 

portfolio standards considering production cost, 

fuel use, emissions, system flexibility 

requirements and other factors

Argonne National Laboratory Koritarov et al. 2013 Value of advanced pumped hydro storage

NREL Denholm et al. 2013 Value of concentrating solar power

Lawrence Livermore National 

Laboratory

Edmunds et al. 2014 Value of multiple storage options and demand 

response

DNV KEMA (now DNV-GL) Abrams et al. 2014 Value of multiple storage technologies

NREL Jorgenson et al. 2014 Impact and value of multiple solar technologies

Southern California Edison Mao and Galjanic 2014 Operational flexibility and flexible capacity 

requirements 

Union of Concerned Scientists Nelson 2014, Nelson 

and Wisland 2015

Multiple aspects of VG integration including 

options to minimize overgeneration

Several of the studies listed in Table 1 have identified the impact of various individual 

technologies on the duck chart shape of net load. For example, Jorgenson et al. (2014) examined

the impact of two different solar technologies on imports into CAISO under increased VG 

penetration, as illustrated in Figure 3.

Figure 3. Example of an analysis of the impact of concentrating solar power (CSP) on the duck 

chart shape

Source: Jorgenson et al. 2014

Other groups have used the versions of the LTPP database to examine how increased grid 

flexibility could be used to minimize curtailment and enable higher levels of renewable 

penetration. For example, the Union of Concerned Scientists modified the LTPP database to 

simulate how increased flexibility could substantially reduce overgeneration risk in 40% and 

50% RPS scenarios (Nelson 2014; Nelson and Wisland 2015).
8

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Figure 4 illustrates an example from a 50% RPS scenario, where increasing the amount of “nonfossil” sources of flexibility—including demand response, storage, provision of reserves from 

renewables8

, and exports—reduced curtailments by more than 75% compared to a base 

“inflexible” scenario and by 63% compared to a flexible gas scenario (Nelson and Wisland 

2015). This figure shows an example day where adding flexibility options, including providing 

reserves with non-conventional resources, can reduce the minimum generation needed from 

hydro and gas generation, thereby reducing renewable curtailments.

Figure 4. Example of the impact of changing system flexibility on demand shape and curtailment 

from an analysis by the Union of Concerned Scientists

(modified from Nelson and Wisland 2015)

Similarly, NREL has also examined higher renewable penetration scenarios in California using 

PLEXOS with a Western Interconnection database derived from the Western Electricity 

Coordinating Council (WECC) Transmission Expansion Policy Planning Committee (TEPPC), 

with additional modification based on the LTPP database (Brinkman et al. 2015). The NREL 

study examined cases where California achieves greater than 50% reduction in electric sector 

carbon dioxide emissions by 2030 with a variety of renewable energy technologies and 

flexibility assumptions, such as increased export limits and reduced minimum local generation 

requirements. Total annual curtailment estimates range from 0.2% (with a balanced portfolio in a 

more flexible grid) to almost 10% (with a high-solar portfolio in a less flexible grid).

Other modeling tools have been applied to examine the impact of PV on overgeneration in 

California. A study by Energy & Environmental Economics (E3 2014) using the ProMaxLT

production cost model examined RPS levels higher than the previous CAISO studies. It 

 8 Reserves from renewables, as discussed in later sections, involves using curtailed VG energy to provide upward 

reserve capacity, which is traditionally provided by partially loaded conventional generation.
9

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identified a significant increase in solar curtailment, particularly when mitigation options are not 

deployed. In one 50% RPS scenario evaluated (with PV penetration equal to about 26%), about 

8.9% of available RPS energy is curtailed. The E3 study also observed that at the point that PV 

achieves this high level of penetration, the marginal curtailment (reflecting the curtailment rate 

of the last unit of PV added to the system) is as high as 65%. Figure 5 provides an example from 

the E3 study showing a duck-shaped chart with a significant hump representing overgeneration.

Figure 5. Example of an analysis of the impact of high VG on net load shape and 

resulting overgeneration

Source: E3 2014

This list includes only studies that have used commercial production cost models; however 

several other studies demonstrate the challenges associated with PV overgeneration in California.

These include a study by Mills and Wiser (2012) that examines the overall decrease in value of 

PV as a function of penetration including the impacts of overgeneration, and a follow-on analysis 

that examines the impact of mitigation strategies including energy storage and demand response 

that effectively change the net load shape (Mills and Wiser 2014).

Finally, an extensive discussion of the duck chart shape and mitigation approaches is provided 

by Lazar (2014). While the analysis does not perform detailed operational simulations or 

estimate curtailment, it does provide a conceptual framework for changing the duck chart shape 

and flattening the net load through a total of 10 strategies including multiple types of energy 

storage and load shifting/demand response. An example of the analysis is provided in Figure 6, 

where the original duck shape is “streamlined” with the likely result of decreasing 

overgeneration and increasing the ability to integrate greater amounts of PV. Other discussions 

of mitigation options include Lew et al. (2015).
10

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Figure 6. Example of an analysis of how the duck curve shape can be modified to 

minimize overgeneration

Source: Lazar 2014

It should also be noted that the impact of PV on net load and corresponding overgeneration risk 

have been studied in other parts of the United States, including Texas (Denholm and Margolis 

2007), the entire Western Interconnection (GE 2010; Lew et al. 2013), and the Eastern United 

States (Bloom et al. 2015).
11

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4 Study Methods and Data

The goal of this study is to explore the duck curve in detail and identify the overgeneration and 

curtailment challenges associated specifically with increased deployment of solar energy in the 

California system. The study uses the PLEXOS production cost model9 to simulate grid 

operation with as more PV is added. It examines curtailment and considers how curtailment may 

change with alternative operational practices and technology deployment scenarios. The 

modeling framework and methods in this study are derived from the California 2030 Low 

Carbon Grid Study (Brinkman et al. 2015). The dataset for the analysis is based on a 

combination of the WECC TEPPC 2024 Common Case and the CAISO 2014 LTPP PLEXOS 

dataset. This dataset represents the power system in the entire Western Interconnection, while 

representing the California power system (transmission and generation) in more detail. Hurdle 

rates are included in the model based on the WECC 2024 Common Case to represent friction 

between balancing authorities. The analysis and results in this document represent all of 

California, including CAISO and the municipal utilities in California that are not part of CAISO.

The renewable generation is based on profiles developed for the Western Wind and Solar 

Integration Study and refined for Phase 2 of that study (Lew et al. 2012). The analysis performed 

hourly unit commitment and dispatch for 1 full year of simulation; however, sub-hourly 

renewable profiles were used to generate the day-ahead reserve requirements for up regulation 

and upward flexibility reserves.10

We begin by considering a scenario where wind provides about 11% of California’s electricity.

This represents a modest growth; in 2013, California generated 12.7 terawatt-hours (TWh) from 

wind in-state and imported another 12.7 TWh of wind for a total of 25.4 TWh, which provides

about 8.6% of the total demand (296.6 TWh).11 We also assume a total of about 1,900 MW of 

concentrating solar power (CSP), which provides about 1.5% of total demand. Most of this CSP 

capacity does not have thermal storage, so it is considered a variable generation resource for this 

analysis. Other qualifying renewables (geothermal, biomass, and small hydro) provide about 

13.6% of total demand. As a result, our initial (base) scenario represents a renewable potential of 

about 36%, not including large hydro. To this base system, we incrementally add PV to analyze 

the progression of the duck chart shape and the resulting overgeneration, considering various 

changes to grid operation and conditions that can effect the net load shape. Table 2 summarizes 

the scenarios analyzed including renewable potential (before curtailment), and reserve 

requirements.

 9 Plexos V6.4 R01 x64 using the Xpress-MP 26.01.04 solver with a MIP relative gap of 0.5% 10 Following Brinkman et al. (2015), we do not enforce a downward reserve constraint, under the assumption that 

downward reserves can easily be provided by curtailing renewable energy generation during times when downward 

reserves are called. This assumption needs further analysis considering the actual curtailment that would result when 

using renewables for down reserves.

11 http://energyalmanac.ca.gov/electricity/total_system_power.html and 

http://energyalmanac.ca.gov/electricity/electricity_generation.html
12

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Table 2. Summary of PV Penetration Scenarios Evaluated 

Solar PreCurtailment 

Potential 

Scenario (%)

Total Solar (PV + 

CSP) Potential 

(GWh)

Total PreCurtailment RPS 

Potential (%) 

Annual 

Regulation Up 

Requirement 

(GW-hr)

Annual Flexiblity 

Up Requirement 

(GW-hr)

11% 35,331 36.0% 3,499 10,590

15% 46,473 39.6% 3,671 11,089

18% 56,438 42.7% 3,947 11,651

21% 66,155 45.8% 4,282 12,240

24% 77,329 49.4% 4,718 12,947

31% 98,964 56.3% 5,652 14,361

37% 119,682 62.9% 6,607 15,746
13

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

5 Results: Base “Most Conservative” Case

We begin with an exploration of PV curtailment in a case with a set of conservative assumptions 

about power system operation based on a “2015 grid” without enhanced grid flexibility. These 

assumptions include:

• Wind and solar cannot provide upward reserves.

• No net exports of electricity from California are allowed and at least 70% of California 

owned or contracted generation (including Hoover, Palo Verde and certain renewable 

generation) from outside of the state must be imported.12

• Up to about 1.3% of peak demand (as much as about 900 MW during periods of peak 

demand) can be shifted via economic demand response programs.13

• No new storage is installed beyond what is in service in 2015.14

• Twenty-five percent of all generation within certain zones must be met with local thermal 

or hydro generation.

15

• Diablo Canyon remains online as a baseload (non-dispatchable) generator. The plant does 

not contribute to the 25% local generation requirement.16

• Instantaneous penetration of VG (including PV, wind, and CSP without thermal energy 

storage) is limited to 60% of the normal load.

It should be noted that the CAISO does not include the 60% penetration limit in their formulation 

of the LTPP model; this limit is based on concerns stated in the CAISO duck chart fact sheet, 

indicating that at 60% penetration:

the grid may not be able to prevent frequency decline following the loss of a large 

conventional generator or transmission asset. This situation arises because 

renewable generators are not currently required to include automated frequency 

response capability and are operated at full output (they can not increase power). 

Without this automated capability, the system becomes increasingly exposed to 

blackouts when generation or transmission outages occur.17

 12 Following Brinkman et al. (2015) we allow non-imported VG to meet the California renewable requirement 

through the purchases of unbundled renewable energy credits (RECs). In the very high penetration cases described 

in the results, up to about 2% of renewable energy is not directly imported and acquired through RECs. 13 This value is about equal to the existing “price response” demand response available from the three investorowned utilities in CAISO, as reported in the “Demand Response Monthly Reports” at

(www.cpuc.ca.gov/PUC/energy/Demand+Response/Monthly+Reports/2015_DR.htm). 14 The impact of storage mandated by California State Assembly Bill 2514 is discussed in the Section 6. 15 In the database from which this study is derived (the Low Carbon Grid Study from Brinkman et al. 2015), the 

zones that require the 25% local generation limit are SDGE, SCE, PG&E (Valley Zone), and LADWP, which 

account for 77% of all California load. For additional analysis of the impact of the local generation requirement, see 

Nelson (2014) and Brinkman et al. (2015). 

16 This is a conservative assumption based on the fact that nuclear power plants typically do not vary load to provide 

operating reserves. 17 https://www.caiso.com/Documents/FlexibleResourcesHelpRenewables_FastFacts.pdf
14

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

We added this limit to our base case explicitly to examine its impact and the importance of 

changing grid operations to allow greater penetration of VG at any moment in time.

The combination of constraints on system operations can result in significant overgeneration,

particularly in the spring. Figures 7–11 demonstrate the drivers behind overgeneration and PV 

curtailment in greater detail. Figure 7 shows the normal load, wind, and solar (combined PV and 

CSP) profiles in a scenario with the potential to meet 11% of annual demand from wind and 11% 

of the annual demand from solar (9.5% from PV and 1.5% from CSP). This figure is for March 

29, which is two days before the CAISO duck chart but actually the “worst” day in terms of PVdriven overgeneration for the load and PV demand patterns for this particularly meteorological

year across all of California. (Because of the relatively low load, the potential generation from 

VG on this day is about 18% from solar and 16% from wind.) The figure also shows the 

resulting net load that would need to be met by the remaining generation fleet, assuming all solar 

and wind generation could be used. In this example, the new minimum load point (of about 

7,700 MW) is shifted from 4 a.m. to noon.

Figure 7. Load, solar, and wind profiles for California on March 29 in a scenario with 11% annual 

wind and 11% annual solar assuming no curtailment

The net load shown in Figure 7 does not consider the operational constraints that actually occur 

in the dispatch, and these constraints do not allow all renewable energy potentially generated on 

this day to be used.

The remaining figures in this sequence are from the results of the power system simulation. 

Figure 8 shows the net load resulting from the VG that can actually be used in the simulated 

system. In this case, the net load met by conventional generation is not allowed to drop below 

about 12,600 MW. This represents a California system-wide minimum generation constraint,

meaning on-line generators in California—and certain contracted generators outside California—

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load without VG Net Load (Unconstrained)

Total Solar Wind
15

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

cannot reduce output to below this level, considering the individual generator parameters and 

system limitations described at the beginning of this section.

18

Figure 8. Modeled net load in California on March 29 in a scenario with 11% annual wind and 11% 

annual solar in a system with a 60% instantaneous penetration constraint

These constraints result in curtailed energy, illustrated in Figure 9, which includes the combined 

VG potential, the amount of VG used by the system to meet load, as well as the curtailed VG.

Curtailment is defined as any VG that cannot be used for any reason. Overall, about 5% of the 

potential wind and solar energy on this day is curtailed. However, during most days, higher midday load does not produce a dramatic duck-curve shape and there little or no curtailment. Over 

the entire year, about 0.2% of VG is curtailed.

 18 This minimum generation value is already below a CAISO-only estimate of the lowest net load point of about 

15,000 MW in the current system (Bouillon 2014). The lower minimum generation point in this analysis results 

from several factors including greater flexibility from customer-owned cogeneration assumed in the LTPP model. 

The LTPP model also does not include fixed-scheduling contractual limitations on plant dispatch. Also, Diablo 

Canyon unit 2 was out for maintenance on this day in the simulation, which removed 1,122 MW of non-dispatchable 

capacity. The net load in the system is less than 15,000 MW during only 12 hours of the year in this simulation.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Net Load (With Curtailment)

Net Load (Unconstrained)
16

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 9. Used and curtailed VG in California on March 29 in a scenario with 11% annual wind and 

11% annual solar

Figure 10 shows how the 60% instantaneous penetration limit results in overgeneration and 

curtailed VG. The bottom curve shows the instantaneous penetration of VG from the model, 

while the top curve shows the theoretical penetration if all VG could be used. In this case, the 

VG potential exceeds the 60% threshold for four hours.

19 In this scenario, the vast majority 

(about 95%) of all curtailment occurs during periods where the potential VG penetration would 

exceed 60%. During a few hours of the year there is curtailment at VG penetration levels 

significantly below 60%, indicating that ramping constraints might force some curtailment. 

However the total amount of curtailment during these periods is very small compared to the 

amount created by the 60% limit. While the average net load ramp rate increases, the existing 

system appears to be sufficiently flexible address these ramp rates. The normal load (without 

additional VG) achieves a maximum hourly upward ramp rate of 6,721 MW/hr on December 

22th at 5 pm. In the 11% annual solar case, only 5 hours of the year demonstrate net load ramp 

rates that exceed this value, with the maximum net load ramp rate of 7,379 MW/hr. The 

maximum upward ramp rate on the duck curve day is 3,142 MW/hr. Analysis in later sections 

evaluates the relationship between a lower penetration limit and possible ramp rate constraints at 

higher PV penetration.

 19 The actual penetration of VG is slightly less than 60% because the constraint does not consider a small amount of 

schedulable load within the model.

0

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30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

VG Potential

VG With Curtailment

Curtailment
17

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 10. Instantaneous penetration of VG on March 29 with and without curtailment in a scenario 

with 11% annual wind and 11% annual solar

Of note in these results is the use of existing pumped storage in the California system, which 

represents a total of 2,518 MW of generation capacity20 including 2,264 MW of schedulable 

pumping load that can be used to increase total demand during periods of high solar output.

Figure 11 shows the simulated storage pumping load that occurs, and how storage results in an 

increase in VG used. As noted previously, because this conservative base case considers grid 

conditions that approximate those of 2015, this simulation does not consider the 1,325 MW of 

additional storage that will be deployed as part of the California storage mandates, which is

evaluated in later sections.

 20 These values are for the four existing California pumped storage plants in TEPPC common case (Castaic, 

Eastwood, Helms, and Lake Hodges). The CAISO LTPP model has a combined capacity of 2,728 MW for these 

four plants. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

% Energy from VG

Hour

VG Potential

VG With

Curtailment
18

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 11. Increase in VG use resulting from schedulable pumped storage in a scenario with 11% 

annual wind and 11% annual solar

On most days of the year, significant additions of PV are possible without causing significant 

curtailment. Figure 12 duplicates Figure 7, but for July 27, the day with the highest demand (note 

the scale change on the y-axis due to the significant increase in demand). On this day, there is no 

VG curtailment, and instantaneous penetration is well below the 60% threshold, as illustrated in 

Figure 13.

Figure 12. Load, solar, and wind profiles for California on July 27 in a scenario with 11% annual 

wind and 11% annual solar

0

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10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

VG With Pumping

VG Without Pumping

Pumping Load (Avoided Curtailment)

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load without VG Net Load Total Solar Wind
19

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 13. Instantaneous penetration of VG in California on July 27 in a scenario with 11% annual 

wind and 11% annual solar

The duck chart figures show the challenge of additional PV penetration without increasing 

system flexibility. Adding PV to help reduce the use of peaking capacity on July 27 also 

produces more energy on March 29. Without flexibility changes that will allow additional units 

to reduce output or be de-committed, only a relatively small amount of additional PV generation 

can be accommodated on March 29 (during the shoulder periods in the morning and evening). 

And as more PV is added, there will be a greater number of days with associated PV curtailment.

Figures 14–16 show the progression of the duck curve and associated overgeneration as 

additional PV is added. Figure 14 shows what the net load would be on March 29 without 

curtailment in both the base case illustrated previously and a case where we add sufficient PV to 

meet 15% of total annual demand (pre curtailment). In this case, the pre-curtailment net load 

drops significantly, to below 5,000 MW.

0%

5%

10%

15%

20%

25%

30%

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

% Energy from VG

Hour
20

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 14. Load in California and VG Profiles on March 29 in a scenario with 11% and 15% annual 

solar assuming no curtailment

While Figure 14 shows the belly of the duck growing as more solar as added, the net load 

changes very little at the higher PV penetration due to the 60% penetration constraint in the base 

case. Figure 15 shows how the belly of the duck curve is prevented from growth due to this 

constraint, and very little additional PV can be used in the simulated system on this day. Figure 

16 shows the hourly curtailment and the used PV in the two cases. At the lower penetration, 

nearly all the PV (95%) is used on this day, but in the case with additional PV, most of this 

additional PV is curtailed. Only a small amount of PV in the morning and late afternoon is 

actually useful, and the total curtailment on this day increases from 5% to about 13%. However,

the marginal curtailment on this day, or curtailment of the additional PV added to the system 

between the two scenarios is about 65%. This illustrates the importance of differentiating the 

total curtailment and incremental, or marginal curtailment of PV. On an annual basis, the total 

curtailment increases from 0.2% to 0.9%, while the marginal curtailment is 5.5%. 

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load without VG

Solar Potential 15%

Net Load (Unconstrained - 15% Solar)

Solar Potential 11%

Net Load (Unconstrained - 11% Solar)
21

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 15. Net load on March 29 in a scenario with 11% and 15% annual solar considering 

operational constraints

Figure 16. Usable and curtailed VG on March 29 in a scenario with 11% and 15% potential 

annual solar

0

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10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load without VG

Net Load with Curtailment (11% Solar)

Net Load with Curtailment (15% Solar)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

VG with Curtailment (15% Solar)

VG with Curtailment (11% Solar)

Curtailment (15% Solar)

Curtailment (11% Solar)
22

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

As even more PV is added to the system, more days acquire the duck shape, and overgeneration 

increases. Figure 17 illustrates the resulting fraction of variable generation curtailment due to 

overgeneration as a function of penetration. The bottom x-axis shows the total penetration of 

solar energy sources (PV plus CSP), while the top x-axis shows the penetration eligible 

renewable resources (solar plus wind, geothermal, biomass, and small hydro). Only PV is added 

and the overall penetration is defined as the annual contribution of renewable energy to the total 

energy demand in California,

21 after removing curtailed energy.

Figure 17. Marginal and average curtailment due to overgeneration under increasing penetration 

of PV in California with a 60% instantaneous penetration limit

The rapid increase in marginal curtailment rates as a function of PV penetration is a significant 

limitation for PV to remain competitive with other sources of low-carbon energy once it 

achieves a certain penetration (in this case perhaps 15%–20% of annual demand). This challenge 

can be observed by examining the impact of curtailment on the levelized cost of energy (LCOE) 

of PV. As curtailment increases, and capacity factors decrease, the LCOE increases. This is 

illustrated in Figure 18, which provides PV LCOE as a function of penetration for the base case 

scenario. In this figure, the PV cost is based on the DOE solar program goal of an LCOE equal to 

six cents per kilowatt-hour. This goal is largely dependent on being able to actually use all the 

energy available from PV and on minimizing curtailment.

 21 Where the total demand is equal to the consumer demand plus storage losses associated with pumped hydro

35% 37% 39% 41% 43% 45% 47%

0%

10%

20%

30%

40%

50%

60%

70%

10% 12% 14% 16% 18% 20% 22% 24%

Annual RPS Penetration 

Annual VG Curtailment

Annual Solar Energy Penetration

Marginal

Curtailment

Total Curtailment
23

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 18. Marginal and average PV LCOE (based on SunShot goals) due to overgeneration under 

increasing penetration of PV in California with a 60% instantaneous penetration limit

Figure 18 shows the importance of examining marginal curtailment rates. While average rates 

can remain relatively low, marginal rates determine the cost and value of adding the next unit of 

solar to the grid. Actual investment decisions may be driven by these marginal values, with 

actual allocation of curtailment driven by a variety of factors, including local grid conditions, the 

underlying contractual agreements with suppliers, production tax credits, and other regulatory 

issues. It should be noted that in Figure 18 all incremental curtailments of non-zero cost 

renewable energy resources (CSP, wind, hydro, and geothermal) were assigned to PV. For 

example, if at the lowest penetration of PV there is no curtailment of wind, and when PV is 

added wind is curtailed, this wind curtailment is actually assigned to PV for accounting purposes.

The very high marginal curtailment rates of PV observed in Figure 17 would likely limit 

contribution from solar without changing system operation to accommodate variable generation 

resources. Examination of the duck curve provides insights into how improved flexibility can 

both accommodate and change the net load shape and increase penetration of solar energy 

resources.

6

7

8

9

10

11

12

13

14

15

10% 12% 14% 16% 18% 20% 22% 24%

PV Cost Due to Curtailment 

(Cents/kWh)

Annual Solar Energy Penetration

Marginal Cost

Average Cost
24

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

6 Enabling Greater Solar Penetration: Flatten or 

Fatten the Duck?

Accommodating greater amounts of PV will likely require multiple approaches to increasing the 

overall flexibility of the power system. Previous work by the CAISO (Bouillon 2014) and other 

groups (listed in Section 3) suggest many individual approaches, but these can be summarized by 

two more general approaches, which we illustrate below as fattening the duck and flattening the 

duck.

Fattening the duck represents all approaches that increase the flexibility of the grid and allow

greater instantaneous penetration of variable generation resources. Typically, this means 

(1) changing operational practices to allow more frequent cycling, unit starts and stops, and

(2) minimizing the amount of thermal units held at part load by improving accuracy of VG 

forecasts and not holding excessive reserves. This also means allowing VG to provide operating 

reserves and other services that stabilize system frequency (Gevorgian et al. 2015). These 

changes can reduce the overall system-wide minimum generation requirement, and they allow 

the natural belly shape of the duck to grow larger and provide a greater fraction of the normal 

load during periods of high solar output.

Figure 19 illustrates the change in minimum generation requirements that would be needed to 

eliminate curtailment on our lowest net load day in the 15% solar penetration scenario. The net 

load in this figure is from the constrained system illustrated in the previous section (Figure 15).

In this case, the system’s minimum generation point of about 12,600 MW results in significant 

curtailment. If the system were able to operate at a lower minimum generation level (about 5,400 

MW), curtailment would be eliminated.
25 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 19. Accommodation of increased penetration of PV by reducing system minimum 

generation requirements and fattening the duck 

Alternatively, flattening the duck acts to shrink the belly shape by shifting supply/demand 

patterns to allow solar energy to meet parts of the load that would not normally be provided in 

the middle of the day. This includes either shifting load via responsive demand or shifting supply 

by the use of energy storage (Lazar 2014). 

Figure 20 illustrates the amount of load shifting that would be required to eliminate curtailment. 

In this example, we keep the 12,600 MW minimum generation level associated with the 60% 

instantaneous penetration limit. We add load (from shiftable demand or storage) with timing and 

amounts that exactly match curtailment of PV. As much as 7,200 MW of additional demand or 

storage charging would be required to eliminate all curtailment in this case. The impact of load 

shifting/storage is shown on both the normal (no VG) load (the top curve) as well as the net load 

with VG. On the normal load, additional demand produces a “hump” on the back of duck. This 

stored energy will be used later (or demand later in the day will be shifted earlier), reducing 

demand in the evening (represented by the flat line where the load has been reduced). The impact 

on the net load is to increase the net demand to the minimum generation level, with the added 

benefit of reducing peak demand in the late evening. 

0

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30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load without VG NetLoad (Actual)

Net Load (Unconstrained) Min Generation-Base Case

Min Gen More Flexible

Decrease system 

minimum 

generation
26 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 20. Accommodation of increased penetration of PV by flattening the duck

(increasing mid-day demand)

Increased penetration of PV can occur by applying either approach individually, but the greatest 

impact will occur when the approaches are applied collectively. 

0

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12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

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Hour

Ideal Shifted Load Load without VG

Net Load (Unconstrained) Net Load (Actual)

Min Generation-Base Case

Added 

load

Reduced 

load

Net shift 

in load
27

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

7 The Impact of Improved System Flexibility

Increasing Instantaneous Penetration and the Impacts of the California Storage Mandate

The base case analyzed in Section 5 does not consider several grid changes that will likely occur 

by 2020 that will help reduce the impact of solar generation on grid operations. Among these 

changes is the deployment of new energy storage. In October 2013, the California Public 

Utilities Commission (CPUC) finalized Decision 13-10-040, which adopts procurement targets 

and requirements for 1,325 MW of “viable and cost-effective” energy storage systems by 2020, 

as directed by the California State Legislature in 2010 (CPUC 2010; CPUC 2013).

This new storage can help accommodate increased use of VG by shifting load and flattening the 

duck. We consider the addition of 1,290 MW of storage, following the modeling assumptions of 

the TEPPC 2024 database.

22 The size and characteristics roughly follow CPUC R.13-12-010 and 

include 550 MW with two-hour discharge duration, 520 MW with four-hour discharge duration,

and 220 MW with six-hour discharge duration. The devices are assumed to have 83% round-trip 

efficiency and are distributed among the three California investor-owned utility zones in 

accordance with the storage mandate. We assume all of the storage added is optimized by the 

system operator to minimize the overall cost of system operation and can provide multiple 

services including provision of reserves.

23 This is a critical assumption, and it would require 

optimization either (1) directly by a system operator in the case of utility-scale storage or (2) 

indirectly through real-time pricing or other mechanisms that would optimize behind-the-meter 

storage. Figure 21 illustrates how this additional storage shifts load to flatten the duck and reduce 

curtailment. This figure shows the normal load from the 15% PV case and load with the 

additional storage. The resulting curtailment is also shown, and is compared to the case without 

the added storage.

 22 The 40-MW Lake Hodges plant is eligible for the storage mandate, and it existed in the base case, so the 

additional storage is less than the 1,325-MW requirement. 23 This is a deviation from the current assumptions in the LTPP model, which assumes a mix of transmission, 

distribution, and customer sited storage, of which only a fraction can provide reserves (Liu 2014a).
28

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 21. Impact of flattening the duck on March 29 with 1,285 MW of added storage in a scenario 

with 15% annual solar

The amount of avoided curtailment in Figure 21 is relatively modest, but it does not consider a 

potentially even greater benefit of distributed storage by provision of grid flexibility services. By 

providing these services (including grid stability), storage can help reduce the need to run 

partially loaded thermal generation to provide reserves. 

The previous section shows the significant impact of the 60% instantaneous penetration limit, 

which creates the flat belly on the duck curve and results in significant overgeneration. While we 

impose the 60% limit in our base case, the CAISO LTPP model imposes a 25% local generation 

limit, which requires 25% of local load in all hours to be met by conventional generators (which 

we also include in our base case in the previous section). Renewables, demand response, and 

storage are ineligible in the CAISO model to meet this requirement. The motivation for this limit 

is described as:

The constraint is necessary for the balancing authority to comply with the NERC 

control performance standards. A balancing authority must have at least 25% of 

its internal generation on-line with adequate available capacity for dispatch or risk 

non-compliance. Within the CAISO’s footprint, a contingency that results in the 

tripping of Path 26 would separate the north from the south. Without a minimum 

amount of generation in southern California, there is a risk that the CAISO could 

completely lose the load if Path 26 were to open.

24

 24 Liu 2014a 

0

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20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Load Before New Storage Curtailment Before New Storage

Load with New Storage Curtailment with New Storage
29

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

The common theme behind these concerns is the ability of a system operator to maintain system 

stability, including voltage stability and frequency stability.25 While there is little direct 

experience in operating grids in the United States with extremely high levels of instantaneous 

VG penetration,

26 studies suggest a variety of approaches to maintaining system stability under 

increased VG penetration. One example is Phase 3 of the Western Wind and Solar Integration 

Study (WWSIS-3) (Miller et al. 2014), which examined frequency and transient stability at 

instantaneous VG penetrations of up to 53% across the Western Interconnection and 62% in 

California. The study simulated the frequency declines after severe disturbances, and found that 

at the levels of penetration simulated, the system was able to maintain enough primary frequency 

response to avoid under-frequency load shedding (blackouts). The study also concluded that the 

use of active power controls in wind turbines and PV could improve frequency response, which 

could allow greater instantaneous penetration of VG. Currently available wind turbines are now 

being deployed that can provide active power control, including both synthetic inertia and 

primary frequency response. Wind turbines can draw stored energy from the rotor to help arrest a 

frequency decline, or they can be operated at reduced output during periods of high VG 

penetration to provide primary frequency (governor) response. PV can also provide these 

services, although both require curtailment.

Fast-responding energy storage, such as batteries and flywheels, can provide rapid response to 

grid events. The amount of new storage in the California storage mandate significantly exceeds 

the WWSIS-3 estimated frequency response obligation for California, and WWSIS-3 found that 

a relatively small amount of storage (less than that in the California storage mandate) could 

provide significant benefits across the entire Western Interconnection.

As active power controls become more common on renewable generators, and if the system 

operator has greater control over the new storage being installed in California, these resources 

could be employed to replace the services now provided by conventional thermal resources. 

To demonstrate how commercially available grid flexibility options can effectively fatten the 

duck, we consider a case where control of distributed resources allows for increased 

instantaneous penetration of VG. We also allow curtailed wind and solar to provide upward 

regulation, contingency, and flexibility reserves. While this provides a system benefit, we do not 

count curtailment that provides upward reserves as “used” energy. However this has a small 

impact as curtailed VG typically provides less than 4% of the total reserve requirement (During 

hours of large curtailment, there is typically a significant amount of partially loaded hydro or 

thermal plants that can provide upward reserves.)

 25 A summary of stability issues is provided by Kundur et al. (2004). They give the following definitions: “Voltage 

stability refers to the ability of a power system to maintain steady voltages at all buses in the system after being 

subjected to a disturbance from a given initial operating condition” and “Frequency stability refers to the ability of a 

power system to maintain steady frequency following a severe system upset resulting in a significant imbalance 

between generation and load.”

26 In October 2014, the Xcel service territory in Colorado provided 61.1% of demand with wind, which was partially 

enabled by utilizing wind to provide regulating reserves. However, because this system is connected to the larger 

Western Interconnection, it does not provide a realistic example of high-penetration of non-synchronous generation 

across a large balancing area or interconnection.
30

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 22 shows an example of a fatter duck that results from increasing the allowable 

instantaneous penetration from 60% in the base case to 80% (with no local generation 

requirement). The top curve shows the net load in the 15% base solar case (the same curve as 

shown in Figure 15). The bottom curve shows the impact of increasing maximum penetration to 

80%, which substantially reduces curtailment on this day.

Figure 22. Net load on March 29 in a scenario with 15% annual solar increasing the maximum 

penetration of VG to 60% to 80%

The corresponding curtailment curves are provided in Figure 23. In this case, the curtailment of 

PV has been substantially reduced from about 13% on this day to about 7%. 

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

60% Max. Penetration

80% Max. Penetration
31

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 23. VG curtailment on March 29 in a scenario with 15% annual solar increasing the 

maximum penetration of VG to 60% to 80% and removing the local generation requirement

While curtailment has been reduced, it has not been completely eliminated due to other 

constraints on the system. The 10,000 MW net load in Figure 22 is close to the minimum output 

of “must-run” capacity in the system. Overall, the model identifies about 8,000 to 9,000 MW of 

minimum generation from nuclear, geothermal, hydro, biomass, and gas-fired combined heat and 

power (CHP) units.27

Overall, these changes to grid operation reduce curtailment and allow greater penetration of VG.

Figure 24 compares the impact of replacing the original base case (including the 60% 

instantaneous limit and the 25% local generation constraint) with an overall 80% instantaneous 

VG penetration constraint. It is important to emphasize that this assumption requires the system 

operatorsto have greater visibility and control of multiple distributed resources, including both 

PV and storage. These distributed resources will likely be needed to perform many functions 

currently met by conventional generation resources, with appropriate controls and market 

mechanisms put in place to compensate owners for providing these services.

Compared to the base case, the curtailment curves are shifted to the right by about 8 percentage 

points, meaning greater energy penetration from solar can be achieved at the same level of 

curtailment. In this case, a solar penetration of 25% is achieved with a marginal curtailment rate 

of about 20%, with the total RPS level approaching 50%.

 27 As noted previously, this low level is made possible in part by the fact that one unit of Diablo Canyon nuclear 

units was out for maintenance on this simulated day.

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

60% Max. Penetration

80% Max. Penetration
32

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 24. Marginal and average annual curtailment due to overgeneration under increasing 

penetration of PV in California after adding mandated storage, removing local generation 

constraint, and increasing maximum instantaneous VG penetration to 80%

Engaging Further Demand Response 

The solutions in the previous section fatten and flatten the duck by applying commercially 

available control technologies to local generation and storage resource. However, the base cases

assume a very small amount of responsive demand that could allow greater PV penetration. As 

with energy storage, demand response (DR) can both flatten the duck (by shifting load) and 

fatten the duck (by providing grid services that reduce need to operate conventional plants at part 

load). Fattening the duck with DR will require provision of services not typically provided by 

loads. While demand shifting can occur through market-based incentives (e.g., time-varying 

prices), using DR to allow for increased VG penetration will likely require DR to provide grid 

stability services (e.g., primary frequency response). This will require loads to sense system 

frequency and automatically reduce load during low frequency events.28 This incurs both an 

implementation cost and any costs associated with paying customers when load is curtailed.

To consider the possibility of how responsive demand could aid in PV integration, we consider 

two steps similar to the previous case. First, we assume a greater fraction of load (up to about 

11% of instantaneous demand) can be incentivized to shift demand to times of lower energy 

prices (corresponding to low net demand).

29 Second, we increase the VG instantaneous 

penetration limit to 90%. This assumption reflects the possibility that directly controllable 

responsive demand can provide the system operator with increased flexibility including

frequency stability measures such as primary frequency response. 

 28 An example of an existing program that uses frequency-responsive loads is the ERCOT “Non-Controllable Load 

Resource” that provides Responsive Reserve Service. This program pays loads to reduce output automatically when 

the frequency drops below a certain threshold (ERCOT 2014).

29 This and other changes to system operation will likely require new market mechanisms. Hogan and Paulos (2014) 

discuss several of these.

35% 37% 39% 41% 43% 45% 47% 49% 51% 53% 55%

0%

10%

20%

30%

40%

50%

60%

70%

10% 12% 14% 16% 18% 20% 22% 24% 26% 28% 30% 32%

Annual RPS Penetration 

Annual VG Curtailment

Annual Solar Energy Penetration

Base (Marg.)

Base (Avg.)

80% Limit (Marg.)

80% Limit (Avg.)
33

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 25 illustrates the impact of the added demand response on net load shape in the 24% 

potential solar case. Figure 25a shows the result for the duck curve day (March 29), while Figure 

25b shows the result for the peak load day (July 27). The scales are the same for comparison. 

The impact on March 29 is very small due to the assumptions regarding the amount of shiftable 

demand, which is based largely on heating and cooling demand. The basis for this is discussed in 

Brinkman et al. (2015). On March 29, there is little need for either cooling or heating in the 

middle of the day, meaning there is low potential for demand shifting on this day based on the 

model assumptions. The impact on July 27 is more significant, and while DR adds significant 

economic benefit from load shifting, no curtailment occurs on this day.

(a) March 29 (b) July 27

Figure 25. Impact of additional demand response on system net load on March 29 and July 27

Further analysis is needed to estimate the actual potential for demand shifting and associated 

costs; however, there likely are additional opportunities for shiftable loads. As an example, the 

CAISO LTPP model includes about 1,200 MW of schedulable pumping loads. The pumping 

load profile in the LTPP model is pre-scheduled and generally corresponds to match historical 

(low-VG) demand profiles. The scheduled pumping load is highest during the traditional offpeak period in the early morning, and it drops by about 700 MW in the late morning, exactly 

when the PV output increases and overgeneration may occur. Assuming there is flexibility when 

this pumping load can occur, re-scheduling this load could accommodate some additional PV.

Overall, based on the assumptions made in this simulation, the impact of load shifting and the 

increase in maximum penetration has modest impact on avoided curtailments. The impact of the

added DR case on PV curtailment is illustrated in Figure 26.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

60,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Net Load w/o DR

Net Load w/DR

Load Reduction

Added Load

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

60,000

12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM

Megawatts

Hour

Net Load w/o DR

Net Load w/DR

Load Reduction

Added Load
34

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 26. Marginal and average curtailment due to overgeneration under increasing penetration 

of PV in California after additional demand response and increasing maximum penetration to 90%

The relatively modest reduction in PV curtailment observed moving to the 90% penetration 

limits is due to the constraints on thermal and hydro plant operations. The presence of baseload 

non-carbon resources in the system, including nuclear, geothermal, and hydro, in addition to 

must-run combined heat and power plants limits the maximum penetration of wind and solar to 

well under 90%. During days with very high penetration of PV, nearly all the non-CHP fossilfueled thermal capacity in California is turned off for the 11 hours of solar production. However,

the results in this section imply that deploying new communications and control technologies 

that allow distributed resources to participate in grid functions and could significantly increase 

PV potential. In these examples, total penetration of about 25% solar on an annual basis appears 

possible with about 5% annual curtailment.

35% 37% 39% 41% 43% 45% 47% 49% 51% 53% 55%

0%

10%

20%

30%

40%

50%

60%

70%

10% 12% 14% 16% 18% 20% 22% 24% 26% 28% 30% 32%

Annual RPS Penetration 

Annual VG Curtailment

Annual Solar Energy Penetration

Base (Marg.)

Base (Avg.)

80% Limit (Marg.)

80% Limit (Avg.)

90% Limit (Marg.)

90% Limit (Avg.)
35

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

8 Additional Opportunities to Fatten and Flatten the 

Duck 

In the previous section, we indicate that near-term technology options are capable of helping 

mitigate challenges of the duck chart and increase solar penetration to as high as 25% with 

limited curtailment. Moving beyond this point with exclusively solar resources becomes 

increasingly challenging; however, several additional options can help fatten and flatten the 

duck. While we evaluated demand response in this work, further analysis of load shifting 

potential is needed, as we assume that during the hours of high VG output, less than 1.5% of 

total demand may be shifted over a period of hours or more.

Other options that have been suggested to address overgeneration include regional interchange, 

more flexible generation, and energy storage. While this analysis has significant interaction 

between California and neighboring states, additional interchange, including exports from 

California, could potentially further reduce curtailment (Nelson and Wisland 2015). This may 

require broader implementation of various market mechanisms that allow for exchanging energy 

across regions.

30 A long-term challenge may occur when surrounding states also adopt

increasing amounts of wind and PV, leading to regional surpluses of renewable energy during 

spring afternoons.

Finally, additional storage (beyond existing and mandated storage) could be used to shift load. In 

addition to electricity storage technologies such as batteries or pumped hydro, concentrating 

solar power using thermal energy storage can shift solar generation to periods of low PV output. 

Storage with high capacity value could enable further retirements of the thermal generation fleet 

that could reduce minimum generation constraints (Denholm and Mehos 2011). Finally, while 

this analysis focuses primarily on enabling high solar penetration, it should be noted that a more 

balanced portfolio could more generally reduce the challenges of integrating VG. When PV is at 

25% penetration, additional wind (or non-VG renewables such as geothermal) has significantly 

lower levels of marginal curtailment than PV. This has been noted previously (E3 2014;

Brinkman 2015), and it suggests the need for a more comprehensive analysis of different 

renewable portfolios to achieve the most cost effective mix of generation technologies.

 30 An example is the Western Energy Imbalance Market 

(https://www.caiso.com/informed/Pages/EIMOverview/Default.aspx). 
36

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

9 Conclusions

Accommodating increased levels of PV in California will require understanding and addressing 

changes in net load shape created by large power production over a relatively short period in the 

middle of the day. System planners and operators will need to consider changes to a system 

historically dominated by dispatchable thermal and hydro resources. In the near term, changes 

underway or proposed in California and elsewhere—such as shorter scheduling intervals, 

increased interaction across regions, and the creation of new market incentives for generator 

dispatch—will reduce the minimum generation challenge and enable greater utilization of VG. In 

the longer term, grid operators will need non-traditional resources to supply reserves and grid 

stability services. This shift in operating practices will in turn require system operators to have 

visibility and control of distributed PV, storage, and load, and it will likely require new market 

mechanisms to incentivize these resources to participate in providing grid services. Without 

utilizing PV or other distributed resources to provide grid services— which is technically 

feasible—excessive curtailment of PV could occur at penetrations well below 20% on an annual 

energy basis

Because of the limited coincidence of PV supply with demand, additional mechanisms will be 

needed to maximize load-shifting. Simple historical methods, such as time-of-use pricing with 

fixed price intervals will likely be insufficient to address the variability and uncertainty of the 

solar resource, which changes on a daily and hourly basis.

By using a combination of grid flexibility options, the duck shape of net load can be 

accommodated and shaped to allow annual PV penetrations that exceed 25%, with limited

curtailment, even without considering the impact of large-scale energy storage. Many of the 

needed grid flexibility options are already being deployed in various locations around the United 

States. Additional portfolio analysis can assist in designing a mix of VG resources and associated 

“enabling” technologies that could achieve very high penetration while maintaining grid 

reliability.
37

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

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