Basic Grid Overview Interlude (Part 2.5)

OK now for a little interlude. My goal on this site is to make everything as easy to understand as possible, so this part may seem kind of obvious to anyone who works in this of industry. For me, however, the information in this interlude made a HUGE impact in how I understood how solar power is integrated into today's grid.

So first let's start with a couple of terms which are easy to confuse with one another. The first is CAPACITY. Capacity is defined as the total amount of power a fleet of generators could provide if they were all running 100%. Obviously this scenario never happens, because it would leave absolutely no margin for error. Nonetheless you will often see the amount a power plant could make listed as its capacity. For reasons you'll learn soon, Nuclear plants are the ones that run closest to capacity (i.e. they are pretty much turned on all the way all the time), whereas Nat Gas "Peaker" plants generally run way less than their capacity because they are only running at selected "peak" times.

On the flip side is the amount of electricity that the power plants actually do produce in reality on a given day, which is termed their electrical GENERATION. So you have capacity on one hand, and generation on the other. Typically the electrical generation system generates somewhere around 70 - 80% of its capacity.  This of course varies day by day.

First let's start with a graph that shows the total amount of CAPACITY:

In this graph I have imagined as if all power plants are running as hard as they can all 24 hours. I have shaded in the total amount of electrical power produced with dots. There are 20 boxes going up (on the y axis) so each box represents 5% in the vertical direction, and there are 24 boxes going across (on the x axis) each representing one hour. Therefore one dot represents 5% of the capacity for 1 hour.

Next let's look at GENERATION:

OK so in this scenario I have represented graphically that the amount of power actually produced (i.e. generated) was only 75% of the capacity. 75% is around average, but remember it can vary day to day. Our generation graph has 15 dots going up and 24 across, so it has 25% less dots than the capacity graph.

Next let's go back to CAPACITY, but this time let's show a graph of only 5% capacity:

Notice that the 5% is spread evenly over the whole 24 hours. Pretty simple - there is just one row of dots going all the way across. We can imagine this as 24 units worth of power.

Now some magic (ok not really) - what happens if you have a power plant that produces 24 units of power but only does so over 18 hours?

As you can see I have drawn some little green arrows showing that I have moved some of the dots inward and stacked them toward the middle of the day. But there are still 24 dots total. Count them. I dare you. Next let's squish that 24 units of power even more - this time down to 8 hours:

There are still 24 dots I swear! Now you can see that we're getting something close to what solar looks like to a grid operator. Look back to when the dots were spread over 24 hours. Now look back to here. Notice that at its peak this plant is providing 25% of the capacity of the system! So through squishing something that was 5% overall can make a big difference for several hours. This will become very key later on. Remember squishing.

The graph above is for CAPACITY.  Remember how GENERATION was 25% less?  Let's see that:

Here we can see that the little pyramid is smaller, but still gets up to around 20%. Pretty substantial considering it was only 5% of the electricity made that day!

I'm going to hop back and forth between capacity and generation for a few graphs and show you some different values. So for now let's up the ante.  Let's go to 13% of CAPACITY:

13% Capacity over 8 hrs.jpg

Why 13%? Because, as we'll see in later posts, this is where the grid really starts to have trouble. As you can see, we're getting up there. That sucker goes up to 45% at it's max! You might just say I'm making all of this up so here is a graph from the National Renewables Laboratory showing exactly this prediction for California (see figure 7:

Next let's look at 13% of GENERATION:

Again you can see not quite as high but that 13% of generation gets up to around 35% around noon.

OK you're probably getting bored of all of my little graphs so I'll just show you two more.  We'll go down to 1%:

1% of capacity gets up to around 10%.  Just one little percent!

And finally... 1% of GENERATION:

This time the 1% makes it up to about 7%. Whew!

Initially when I was learning about these issues it seemed crazy to me that only 5% Solar Power could have such an impact on how the grid operates. However, laying it out on graph paper like this with my daughter's markers made it so much clearer. And in the posts to come, it will make it so much clearer as to why home batteries are so crucial to the grid.