Solar power is awesome.

The problem with solar is that there can be too much of the good stuff when no one needs it and too little or none when power is in high demand.

Net load is the gross load minus the wind and solar resource generation levels. With ever more solar power on the grid, California’s net demand has dropped to near zero around noontime on certain days of the year. Below is a chart produced by the California System Operator (CAISO), which shows the net load on its system for the spring months, March-May, for the years 2015 to 2022.

It is evident that the midday net-load trough created by solar power has become an abyss over the span of only seven years.

CAISO lowest net load day each spring (March-May, 2015-2023), gigawatts 
Source: “As solar capacity grows, duck curves are getting deeper in California,” EIA.

In the past decade, the shape of this load curve has been called the “duck curve.” 

As the net load on the power system fell during times of high solar irradiance, the shape of the load curve began to resemble the outline of a duck in water and lent the name “duck curve.”

For the years 2015-2018, this clever nomer was suggestive.

But as more solar power came online, the midday trough deepened. Now, the shape of the load curve is more suggestive of a swan.

From 2015 to 2018, CAISO’s lowest net load day each spring (March-May, 2015-2023) resembled that of a duck.

Since 2019, CAISO’s lowest net load day each spring (March-May, 2015-2023) has begun to resemble a swan, 
and the name “swan curve” may be more appropriate.

There are two dominant and easy-to-understand implications of this load shape. First, with so much midday solar when loads are generally low, solar energy has little value.

As much as your solar array is virtuous and cranking power, no one needs or wants your solar power at noon.

The result is low economic value for solar energy. California’s new net metering regulation, NEM3, only pays the wholesale value of power. For midday power, this means very little.

Second, as the sun sets in the Pacific and night falls, solar production drops quickly while evening demand kicks in. The result is a rapid rise in net demand.

Both the absolute change in power demand in the evening and its rate have increased. Managing this “ramp” rate is challenging and costly.

It is no surprise that utilities have been ringing the alarm bells and raising their objections to a greater penetration of grid-connected solar energy without energy storage.

The PowerBloc has a large amount of storage and by design never puts solar energy back on the grid. Thus, it does not contribute to the duck curve.