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.

Measuring Energy

Everything in the physical world is energy. That's what Einstein's famous equation says, and most people with at least a high school education are familiar with the general idea of the equivalence between mass and energy. Yet energy remains a somewhat abstract concept and we often toss the word around in imprecise ways. Why is that?

I think it has to do with ease of measurement. If you ask, "How tall is Cindy?" Most everyone knows how to answer the question.  You have Cindy stand against a wall, make a mark on the wall with a pencil, and then use a tape measure to see, with pretty good precision, how tall Cindy is.

Other things are easy to measure. How much milk do I add to this pancake batter? Well, the recipe says "one cup", so I get out a measuring cup, fill the measuring cup up to the line that says "one cup" and I pour it in the batter.

How much does Joe weigh? Ask Joe to stand on a scale and read the result.

So in each of these cases, there is a simple instrument – a tape measure, a kitchen utensil, a scale – that we know how to use to answer our question.

It's not that way with energy, however. We don't "experience" energy directly the way we do someone's height and weight or a volume of liquid. Recall from our earlier post that energy is the potential to do work. If we direct some energy to something, say we turn on a light, we see the result of that application of energy (the light bulb glows). We don't, however, directly "see" the energy.

Not only that, but we are also faced with several units of measurement for energy, and these units don't have nice neat relationships between one another.  

If we're measuring length in the metric system, we know that 100 centimeters equals one meter. In the "English" system, there are 36 inches in a yard. There are 16 ounces in a pound. We see the subdivisions, the relationships.

For energy, we are faced with scales of measurement that are more abstract. Again, that's because we don't directly "see" energy. In addition, it is unclear how the various scales relate to each other. What are the relationships between, BTU's, Kilowatt-Hours, therms, and so on?  Not only that, but we often apply volumetric units (cubic feet of natural gas, gallons of propane or gasoline) to sources of energy.

So we need to set some ground rules for how we describe how much energy we're talking about. We'll settle on one primary way to represent the amount of energy that we're using (or saving or delivering) and then we'll show a way to convert other units of measurement – such as those we find on our utility bills – into that one way.

Sort of like comparing apples to apples.

Where do we go from here?

Welcome!

In this blog, we'll explore information - quantitative information - about what you can do to become more energy efficient. We will want to be confident that we're making sensible choices. Justified confidence is based on knowledge and understanding and thus involves learning, so we'll begin by learning how to measure and track our energy use.

I hope to make this process fun and interesting.