A1. Nomenclature
Throughout the paper, common units will pop up that are highly confusing to a reader not familiar with the subject, largely since the terms power and energy are used seemingly interchangeably in everyday conversations. Energy refers to a quantity; power is a rate of use of energy. The quantity of water in a lake is analogous to energy, whereas the flow rate of a river is analogous to power.
A kilowatt-hour (kWh) is a unit of energy, while a kilowatt (kW) is a unit of power – the number of kWh indicates how much energy we’ll use overall, whereas the number of kW indicates how quickly we use that energy. Continuing with the water analogy, the lake’s size is analogous to kWh, whereas the river flow rate is analogous to kW. Although the units look quite similar, they represent vastly different concepts. In the content of electricity generation, power plants produce a certain amount of energy throughout the year (kWh), but their max capacity (i.e. the most power they could produce at any point in time; the highest rate at which they can produce energy) is measured in kW. A reliable electricity system has to ensure that both power and energy constraints are satisfied. In other words, we need to ensure that the power grid has enough power capacity (in kW) to meet the maximum amount of power that we use at any one given instant, and we also need to ensure that we can produce as much energy as we’ll use throughout the year.
I’ll occasionally refer to other units of energy and power, due to the different scales at which the electricity sector operates. Some units of energy include kWh, MWh (megawatt-hours), GWh (gigawatt-hours), TWh (terawatt-hours), and Quads. 1,000 kWh is 1 MWh, 1000 MWh is 1 GWh, 1000 GWh is 1 TWh. For the sake of scale, an iPhone 12 battery is about one-hundredth of a kWh [i]. Most electric cars sold today have battery sizes between 40 and 100 kWh. Texas uses 1 TWh of electricity every single day [ii]. Unit of power follow a similar pattern: 1,000 kW is 1 MW (megawatt), 1000 MW is 1 GW (gigawatt), 1000 GW is 1 TW (terawatt). 1 kWh represents an hour’s worth of energy at 1 kW of power. Again, to use the water analogy: if your flow rate is 42 gallons / hour, you’ll have 42 gallons after an hour.
Finally, I’ll use the term relative to refer to percentages, and absolute to refer to counts. Relative peaks, therefore, refer to peaks in the percentage of something (e.g. when natural gas made up the largest share of the electricity mix), and absolute peaks refer to peaks in the amount of something (e.g. when natural gas produced the most electricity it had before).