Representing Energy

Now we're getting somewhere. We are beginning to develop a little bit of intuition around the idea of "amounts of energy" as represented by KWH. That's actually a pretty big deal. The concepts are a little abstract and certainly not trivial.

So now let's set up a table so that we can convert between various energy measurement schemes. It turns out that we can find conversion factors between these measurement schemes. They're published on the web. At the bottom of this post, I'll list some references that you can chase down if you want, but let's cut to the chase.

• If we heat our home with propane, we're billed for gallons of propane.
• If we heat our home with natural gas, we're (generally) billed for therms.
• If we heat our home with wood, we buy cords of wood.

Just like we multiply how long something is when it's measured in feet by 12 to get it's length expressed in inches, or we multiply pounds by 16 to represent the same amount of weight in ounces, we can look up multiplication factors for the energy contained in all these sources (and more).

Then, for example, we can convert the number of gallons of propane we used last January into an equivalent amount of energy expressed in our preferred units, kilowatt-hours (KWH).

Here's our table of conversion factors:

Converting to KWH

Fuel Type	Unit Type	Conversion Factor
Natural Gas	Therm	29.3
Natural Gas	Cubic Feet	0.3
Propane	gallon	26.5
Heating Oil	gallon	40.6
Firewood	cord	5900

So if we use 10 "therms" of natural gas, we multiply that by 29.3 and we see that we used an amount of energy equivalent to 293 KWH!

A couple of details:

1. Sometimes utility companies tell you the amount of natural gas that you used in terms of "cubic feet" instead of therms. That's okay. You just multiply the number of cubic feet by 0.3 to get KWH.
2. There are lots of different types of wood used for heating. I found a source that listed over 50 different types. There was a big range of values. I just averaged and rounded off to get 5900 KWH per cord. That's probably not all that accurate. If you know what kind of wood you use, you can see the references below and calculate a more accurate number.

In case you'd like to dig into this some more here are some references:

1. Wood Note that the numbers are given in million of BTU's per cord
2. Propane
3. Heating Oil
4. Natural Gas

Measuring Energy, Part II

Okay, in the previous post we explained why it's not easy to talk about how much energy we're using. Now let's settle on one way to represent amounts of energy.

Let's use "kilowatt hours". We'll abbreviate this as KWH. We see this unit of measurement on our electric bill, so it's at least slightly familiar.

So what is a kilowatt hour? That's the amount of energy used if you run a 1000 watt microwave oven for an hour. That's equivalent to the energy used if you burn a 100 watt light incandescent light bulb for 10 hours OR if you burn a 60 what bulb for 16 and 2/3 hours (16 hours and 40 minutes).

We intuitively grasp that there is a "flow" of electricity from the outlets in our home whenever we use an appliance or other piece of electrical equipment. The number of watts an appliance uses is a measure of how big a flow is needed to run that appliance. In fact, we actually express that flow in terms of a "current" of electricity. The number of watts that an appliance uses is calculated by multiplying the electrical current flowing through the appliance whenever it's switched on by the voltage supplied by the electrical company (voltage, continuing with the flow analogy, is roughly like the pressure of the flow). There is a simple mathematical equation that represents this relationship:

P = I x V

Here:

"P" means "Power"

"I" means "Current"

"V" means "Voltage"

So, Power equals the Current times the Voltage.

Now we can keep decomposing these relationships. We can represent Current ("I") in terms of electrical charge and some other things. We can take these relationships all they way down to some very fundamental definitions of things in the physical world, but let's just keep it simple.

We'll represent the energy that we use or save in terms of the units kilowatt-hours or KWH.

Next, we'll see how to represent all of our energy calculations in terms of KWH.

Energy

Energy is the capacity to do work. That work can involve moving things or animals or people around in a car, boat, train, or a plane. The same term, "work", could also refer to heating or cooling a building, lifting people in an elevator, powering a computer, and so on. Basically all of our activities, including life itself, require that capacity to do work -- energy.

Some things require a lot of work to get done. Imagine a large, drafty, uninsulated building on a cold night in northern Montana. Imagine heating that building to a temperature at which most people would feel comfortable sitting still, wearing sandals, shorts, and a t-shirt . That would take a lot of work, a lot of energy. Conversely, some actions require very little energy: a baby is strong enough to bat a beach ball and make it move.

These examples are simple and easy to visualize. Obviously, many situations can be much more complex. How do we deal with that complexity? Well, one way to manage complexity is to avoid qualitative statements such "a lot of work" or "very little energy" and instead measure the amount of energy consumed by a particular activity.

So instead of saying, "It takes a lot of energy to heat our house," we ask, "how much energy did we use to heat our house last winter?" and we try to measure that energy. Then we can compare things more quantitatively. We can determine, for example, how much energy was required to heat a particular house to a particular temperature during the waking hours between November 1st of 2008 and February 28th of 2009. Then we can do the same measurements for a different house and compare the measurements to see which house needed more energy.