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Building Science

An Introduction to the Duck Curve

As solar electricity increases, the daily balance of supply and demand is changing dramatically

Electric power transmission lines carry electricity from power plants to commercial, industrial, and residential points of use. Increasingly, they also carry solar-generated electricity, which is having a big impact on how utilities operate.
Image Credit: Image #1: Chris Hunkeler /

You may think there’s no more boring topic than electric utilities. Power plants. Transmission lines. Engineers with flat top haircuts and pocket protectors full of pens in their white short-sleeved shirts.

Well, let me tell you two words that might help make them more interesting: duck curve. If you haven’t heard this term yet, you’re not alone.

A primer on utility load curves

The graph below shows a typical daily load profile for an electric utility. This one’s for a day in New England in 2010, but the basic pattern is similar to what most electric utility load curves look like.

Demand for electricity is at its lowest during the night. When people wake up and start their day, demand rises (morning ramp). Then it stays up at a higher level through the day but rises to a peak after work ends and everyone returns home.

[Image credit: U.S. Energy Information Administration][/caption]

That’s what a typical day looks like in a lot of places. In addition, there are annual changes and utilities are often classified as either summer peaking or winter peaking. That’s just what you think: which season the utility hits its highest peak load for the year.

Utilities meet that demand in two ways. They have baseload plants and peaking plants. Baseload plants are mostly coal-fired here in the Southeast and mostly hydro in the Northwest. Peaking plants are often fueled by natural gas.

Now, what electric utility companies would like their load curve to look like is shown below.

[Image credit: Energy Vanguard][/caption]

If the demand were perfectly flat, a utility’s job would become trivial. (Of course, there’s all that messy stuff about storms and growth and plant outages, but let’s ignore that here.) Push the button and let…

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  1. Dan Kolbert | | #1

    Not so boring..
    Thanks, AB3 - finally getting a chance to read this. I seem to be one of the few people who found the keynote at last year's NESEA conference, a panel on utilities, fascinating, so I may not be the best judge.

  2. User avater GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Dan Kolbert
    You're welcome, Dan. Great seeing you in Portland last week!

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