Overgeneration from Solar Energy in California - A Field Guide to the Duck Chart: NREL (National Renewable Energy Laboratory)

Research output: NRELPresentation


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 percent of annual energy could lead to marginal curtailment rates that exceed 30 percent. 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 percent 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.
Original languageAmerican English
Number of pages38
StatePublished - 2015

NREL Publication Number

  • NREL/PR-6A20-65453


  • California
  • curtail
  • curtailment
  • duck chart
  • flexibility
  • flexible
  • grid
  • penetration
  • photovoltaic
  • PV
  • renewable portfolio standard
  • RPS
  • solar


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