Ask an expert what the difference is between a kilowatt (kW) and a kilowatt-hour (kWh), and he or she will tell you that a kW is a measure of power, whereas a kWh is a measure of energy. Great. However, I know I don’t think in terms of power and energy. Until recently, I didn’t even realize there was a difference between the two. What the expert means is that kW is a rate of consumption, and kWh gives you the amount you consumed.

To make these abstract ideas understandable, I’m going to annoy a lot of experts with the following explanation.

Think of kW as the miles-per-gallon (mpg) rating of a car, and kWh as the actual gallons of fuel consumed. (The analogy is imprecise, but arguably effective.) These definitions may be the opposite of what you would expect. I was initially confused because I thought kWh meant “kilowatts per hour” in the same way that mpg means “miles per gallon.” But this is not the case. Mathematically, mpg means miles divided by gallons. But where gallon in mpg is in the denominator, the hour in kWh is actually in the numerator, multiplied by kW. Therefore, kWh means kilowatt-hours, not kilowatts per hour.

Here is a practical example: If you take a trip and all you know is that your average mpg (kW) is 10, you don’t have any idea how much gas (energy) you used. To know your total gas consumption, you also have to know how far you traveled. If you went 10 miles, you used one gallon (kWh) of gas.

Mpg tells you the rate at which your car consumes gas; kW (or demand) is very similar. It tells you the rate at which you’re consuming electrons (electricity is the flow of electrons). Let’s say you have a 0.05-kW (or 50-watt) lightbulb. This rating is kind of like the mpg rating of the car. Just as you don’t know how much gas you’ve consumed until you know how far you traveled, you also don’t know how many electrons (or how much energy, measured in kWh) you’ve consumed until you know for how long you left the lightbulb on. If you leave the lightbulb on for 10 hours, you used 0.5 kWh (0.05 kW x 10 hours). Let’s recap before proceeding.

One problem with kW and kWh is that the units are almost impossible to visualize. We know what one gallon of gas looks like (at least, those of us who have ever mowed a yard do). But what would a kWh look like? I like to think of a kWh as a certain number of electrons, and kW as the rate at which electrons are passing through the wires.

Suppose you have a bucket filled to the top with electrons (this would be really hard to accomplish, but bear with me), and there’s some finite number of electrons in the bucket, let’s say 1 kWh worth. How long will the electrons in the bucket power your lightbulb, and how long will the electrons in the bucket power your clothes dryer? If your lightbulb’s rate of usage is 0.05 kW (again, a 50-watt bulb), you can leave your lightbulb on for 20 hours before your bucket of electrons is empty (0.05 kW x 20 hours = 1 kWh). But your clothes dryer’s rate of energy use is much higher. Let’s say your dryer’s rate of usage is 2 kW (2,000 watts, which is a bit less than an average dryer, but this makes the math easier). Using your bucket filled with electrons worth 1 kWh, you’d only be able to power your dryer for 30 minutes (1 kWh / 2 kW = 0.5 hours, or 30 minutes).

So why care about both kW and kWh? Why can’t we just ignore kW, focus only on kWh, and stop confusing everyone? Unlike the car example, in which you pay the same amount for a gallon of gas regardless of your car’s mpg rating ($3.50 per gallon whether you own a Hummer or a Prius), the amount of energy you consume (kWh) isn’t the only thing that your utility cares about or charges for. This is because even though in the clothes dryer versus lightbulb example, you’ve used 1 kWh either way, it costs more for the utility to provide you with 1 kWh over 30 minutes than it does to do so over 20 hours. In other words, it costs the utility more money to produce electricity at a faster rate (kW). The higher the rate at which you ask a utility to deliver electrons, the higher your demand and the higher the cost.

Alas, you may ask, how can I remember which is which? How can I keep kW and kWh straight? Unfortunately, I don’t have a good solution for memory problems. Perhaps you should consult your physician.