Energy 101

In this post, I briefly introduce some basic concepts important for understanding the amount of energy we use.

For any discussion about energy it is helpful to understand how we measure energy, how much we currently use, and where it comes from.  Let’s start with an easy number important for understanding the amount of power used in the world: 17 TW (terawatts). But to understand this number, it is important to understand the difference between energy and power, and the units used to describe these quantities.

Energy is a scalar quantity that is in simplest terms the ability of a physical system to produce changes on another physical system.
The change (or work) one can do on a system can take a variety of different forms, but we will focus on a couple specific types of energy: electric, which is when electrons flow in a circuit to power a device that will ultimately do work and chemical, which is typically the release of energy by breaking chemical bonds in compounds, for example, combustion of gasoline in a car. The SI unit of energy, Joule (J) is equal to the energy expended in applying a force of one newton (N) through a distance of one meter. Instead of using Joules, we’ll stick the unit typically used in electricity, which is the kilowatt hour (kWh). The nice thing about using kWh is it’s easy to go from energy to power (and vice-versa). Power is a rate of which energy is used or produced. For power, we will use a SI unit, which is the Watt (W), equal to J/s. You see that we take a unit of energy (J) and then divide by a time (s) to get a rate and vice-versa to arrive at energy.

For example, one 40 W lightbulb uses about one kWh per day. Again, watt (W) is a unit of power, it’s the rate that you use energy. To get the total energy (which in our case in kWh) we need to multiply the power by the time we’re interested in. If we’re going to use the energy unit kWh, you can guess the unit of time we should use, the hour (h):

Let’s get a feel for the scales of these numbers, for example, a typical lamp has a power between 40-100 W. A kilowatt (kW) is 1000 W (10³) and is in the range of typical small engine, for example one kilowatt is equal to 1.34 horsepower. A megawatt (MW) is a million watts (10⁶). Now we’re starting to get up there in power, this number is typically reserved for power generation facilities, for example, covering a very large rooftop (like the Adelaide Showground in Wayville, Australia, a solar installation done by First Solar) can get you about 1 MW; having a large solar farm can get you up to 100 MW. A gigawatt (GW) is a billion watts (10⁹). This number is reserved for the biggest of energy producers, for example the Hoover dam has a capacity of 2.1 GW. Terawatt is a trillion watt (10¹²), on scale with the total energy use of the planet, 17 TW.

The world uses energy at about a rate of 17 TW or about 2.5 kW per person, the equivalent of keeping 25 lightbulbs on (assuming they are 100 W bulbs).

Fig 1. Historical world energy production (EIA)

Figure 2. US energy production and consumption (EIA)

Of course, this energy use and production is not evenly distributed around the world, some people use more (developed countries) and some use less. The U.S. rate for energy use is about 3.5 TW, about 20% of the total energy use of the planet. With our population, it works out to be about 11.3 kW per person, or the equivalent of 113 lightbulbs on, with our energy use being over 4 times per person that the world average. Several poor countries round out the bottom of the list, for example, Cambodia only uses around 0.02 kW per person or Chad that uses 0.01 kW per person or the equivalent of ten people sharing one light bulb, while Gibraltar used 69 kW per person! Because of technological improvements, however, the amount of energy used per person has actually been steady in the US during the last couple decades, see Figure 3. A plot of energy use per GDP would show a steady decrease in the last several decades due to energy efficiency (and likely other important factors, like changes in the US economy from industrial to services).

Figure 3. Per-capita US energy use (EIA)

We've explored the scale of energy we use, but where does it come from? The short answer is fossil fuels; oil is the most used energy source in the world, see Figure 4. Nearly tied for second and not far behind are natural gas and coal. Tied for fourth are nuclear and hydro-electric.

Figure 4. World energy production (EIA)

The US has a similar energy production, Figure 5. The US gets slightly more of its energy from nuclear than the world average. The right plot of Figure 5 zooms in on the bottom of the left side plot to show how little energy comes from most of the renewables (solar, wind, biomass, and geothermal).  To be fair,   many of these, especially wind and solar, have grown significantly in the last year and is not reflected in this plot. But with the most recent numbers from EIA (March 2011), wind still only represents around 1.5% of the total energy use, while solar is around 0.15%.

Figure 5. US energy production (EIA)

This is by no means a complete introduction. To learn more, I encourage you to check out the US Energy Information Administration (EIA), where all the data came from in this post. A book written by the British physicist David MacKay also has a nice introduction and is free online.