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.