Solar Panels + Home Battery

In order to best lower your carbon footprint, where should you consider using Solar Panels plus a Home Battery? In one word: California. Currently California is undergoing one of the greatest transformations of a power grid in the world. California has all but eliminated its Coal plants, instead relying on a power mix unlike anywhere else in the country. In 2016 electricity was generated in California by:

  • Nat Gas 49%
  • Nuclear 9%
  • Solar 12.5%
  • Hydroelectric Dams 14.5%
  • Wind 6.5%
  • Geothermal 6%
  • Biomass 2.5%

As we have seen in previous posts, when the percentage of Solar generation heads up toward the 10% range things get really difficult for grid operators. California was smart to get rid of its Coal plants, as Coal power plants are not as flexible as Nat Gas power plants -- recall that a Coal plant can only turn down to a minimum of 40% of its capacity, whereas a Nat Gas can turn down to more like 25%. None the less even the majority Nat Gas powered grid of California is being pushed to its limits. Solar generation "squished" into the midday hours floods the grid, causing the Nat Gas power plants to turn way down, but then as the sun goes away around 6pm suddenly the Nat Gas power plants have to ramp right back up. And when does peak demand occur in the fall / winter / spring? Right around 7-8pm!

Let's look at a graph that illustrates this conundrum (known as "the duck curve"):

In the figure above we have California' grid mix with a fall / winter / spring demand curve. There is 9% Nuclear and 15% "Other" which includes Wind, Geothermal and Biomass. There are 34 green dots in the graph paper squares which represent the 9% Solar generation. (There are 375 squares total under the black demand curve line, and 34/375 = 0.09) As you can see the Nat Gas power plants have to turn down to 30% of their capacity around noon - 1pm, but since their minimum is 25%, they are able to do that without cycling (ie, turning completely off then back on). You can also see that any additional Solar would get the grid operators in real trouble.

To make the wild swing this 9% Solar causes more obvious, the next graph shows the Net Load, which is what a grid operator is concerned with. The Net Load is essentially the normal demand curve with the Solar subtracted out. When you do that you get the curve below (see the thick black line):

OK so here we have it. "The Duck Curve." Honestly I think you've got to have some real imagination but the idea is that the Duck's back is the place where the Nat Gas plants are turning down to their minimum (noon to 1pm), the neck of the Duck is the time when the Nat Gas plants have to suddenly ramp up (around 4pm) and the top of the Duck's head is at peak demand (around 7pm). 

To give you an idea that this is a real actual thing, here is a graph of California Net Loads during the month of March from 2011 - 2016:


Can you see the Duck "in the wild" now? In the figure above, look at 2012 (in blue), then compare that to 2016 (in yellow). See how the "Duck's Back" is getting lower? Already in 2016 the Duck's back is pretty much as low as it can go!

The point is this. If you live in California and you put Solar Panels on your roof without a Home Battery, you are only going to make the Duck's back lower. Grid operators could handle a little more Solar, but in order to do so they would have to "lower the Duck's back" by cycling Nat Gas power plants, which would result in a large portion of the carbon emissions saved by your Solar going to waste (see post "Turning Power Plants Off and On?" for further discussion). But if you look back at the top figure ("CA grid in Winter") you can see that even with cycling Nat Gas plants cannot make much more room for Solar in the grid. Again, let's say you put Solar Panels on your roof -- what will happen at this point? The grid operators don't control the Solar Panels on homeowners' rooftops. But they do control large utility scale Solar plants. Therefore, if many more California homeowners put Solar Panels on their rooftops, grid operators will simply turn down the electricity coming from the utility scale Solar plants (known as curtailment). So, yes, the electricity from a homeowners' rooftop Solar will into the grid, but an equal amount of electricity will be turned down from utility scale Solar plants... meaning that the rooftop Solar is not lowering carbon emissions. The rooftop Solar will go to waste.

Enter the Home Battery. Instead of letting that electricity from your rooftop panels flow into the grid, save it in batteries, and then use it later. When should you use it? Just a few hours later, during the evening peak demand! Looking at any of the figures above, you should clearly see that any less power you use between 4 and 10pm will directly lower the electricity needed from Nat Gas power plants. In other words, using the Home Battery to "move" your Solar power from midday to the evening means that all of your Solar is lowering carbon emissions.

So there you have it Californians. Need more evidence? Consider this: as of 2016 the State of California, in conjunction with its public utilities, started adding its own battery storage to the grid.