Don’t know anything about electricity prices? Here’s where to start looking at the graph.
First, look at the ‘Load’ graph. This is basically the amount of energy that is being drawn by all the users of the system at any given time. As you can see, it has a pretty regular daily pattern.
Now look at wind generation. Wind, as you can see, does not have a regular pattern. It just goes up and down, fairly randomly. Wind makes up a big chunk of the energy mix.
So it is no surprise to see that price (SMP, the bottom graph) is dependent on two things: Firstly, it is driven by time of day. You usually see the spikes between 5 and 7pm. Secondly, it is driven by how windy it is. If the wind is low, then the price can go high.
Sometimes you see weird stuff. This seems to be caused by the wind falling, or there being unexpected demand. If this happens, expensive generation which can come on very quickly has to be called in.
I am calling SMP the ‘price’. SMP actually stands for System Marginal Price. There is a pretty technical definition of what this means, but it is just the marginal cost (excluding plant costs, depreciation, etc) of the most expensive electricity being generated at a particular half-hour. You can read more about it in this document on the SEM-O website. (SEM-O is the organisation which administers the Single Electricity Market for the island of Ireland.)
There are separate charges for capacity (just paying a generator to be available if needed), transmission (high-voltage lines), distribution (low-voltage lines) and various other things that make up the eventual price you pay for electricity.