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