How can time series information be used to choose a control group?

This post is by Phil Price, not Andrew.

Before I get to my question, you need some background.

The amount of electricity that is provided by an electric utility at a given time is called the “electric load”, and the time series of electric load is called the “load shape.” Figure 1 (which is labeled Figure 2 and is taken from a report by Scottmadden Management Consultants) shows the load shape for all of California for one March day from each of the past six years (in this case, the day with the lowest peak electric load). Note that the y-axis does not start at zero.

Duck Curve

Figure 1: Electric load (the amount of electricity provided by the electric grid) in the middle of the day has been decreasing year by year in California as alternative energy sources (mostly solar) are added.

In March in California, the peak demand is in the evening, when people are at home with their lights on, watching television and cooking dinner and so on.

An important feature of Figure 1 is that the electric load around midnight (far left and far right of the plot) is rather stable from year to year, and from day to day within a month, but the load in the middle of the day has been decreasing every year. The resulting figure is called the “duck curve”: see the duck’s tail at the left, body in the middle, and head/bill at the right?

The decrease in the middle of the day is due in part to photovoltaic (PV) generation, which has been increasing yearly and is expected to continue to increase in the future: when the sun is out, the PV panels on my house provide most of the electricity my house uses, so the load that has to be met by the utility is lower now than before we got PV.

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