Energy alternatives series: What’s a watt?



When people talk about power, they often mean energy. There is a distinction here that might seem pedantic, but it is important, and I want to clarify things a bit.

If you are still on the grid, you get an electric bill from the power company that tells you what you used in the past month. This comes as a number (and a price!) reflected in kilowatt-hours. As we will see, this is a measure of the total electrical energy used by your home in the past month.

If you know that ‘kilo’ means a thousand, then you can tell (from your bill) that you used the equivalent of a thousand Watts for a certain number of hours. Or some thousands of Watts for one hour. This can be a bit tricky to unravel, so let’s start slowly and figure out what all this means.

Suppose you use 742 kiloWatt hours in a month (this is the average in the US. Certainly, if you are reading this, you are probably not average, but let’s roll with it to get some numbers to compare.) What does this mean? What does it imply that the power company did on your behalf?

A confusing aspect of this number is that it is in somewhat weird units, which I tried to point out above. To untangle this we first need to know what a Watt actually is.

A Watt measures an amount of energy used in a unit of time. Energy itself is measured in another unit, the Joule. The exact definition of a Joule will be the subject of a later installment of this series, but suffice it to say it is the common currency used to measure energy.

Suppose you use a blow drier for half an hour. Blow driers usually consume about a kilowatt of power. Now, a Watt is one Joule per second. So the way you would see your blow drier usage reflected in your electric bill would be as ½ kilowatt-hour. And we can figure out how many Joules you used from knowing what a Watt is. We’ll return to that.

Suppose you have a 100 W light bulb in your garage that you leave on all the time (I am not suggesting you do that, just that you could.) Since the power usage is 0.1 kW, it takes a while to use the same number of kW-hrs, 10 times as long, or 5 hours.

So power is a rate of energy use, and this multiplied by the time the power is used tells you how much energy you used. That is why kilowatts x hours, or kilowatt-hours, is used to measure it.

Back to the blow drier. A half hour is 30 minutes x 60 seconds per minute, 1800 seconds. 1000 Watts x 1800 seconds is 1.8 million Watt-seconds. From what we said a Watt is, this is 1.8 million Joules. This sounds a lot bigger, maybe, but it is the same as ½ kilowatt-hour. This is the same number of Joules used in 5 hours by the light bulb in the garage.

So summarizing- a kilowatt-hour is a certain amount of energy. The kilowatt part reflects the rate of usage, and the time reflects how long the kilowatt was used. Once the two are multiplied, you get a reflection of the amount of energy, (which is very useful in itself, but that loses the information about the rate and the time, so it seems a little perverse to me to use this unit. I don\’t get to decide what the world uses, but I can convert them to other units, and you can too).  It took a lot less time for the blow drier to consume the same energy as the light bulb. One kW-hr is 3.6 million Joules. Using a blow drier for half an hour or a 100 W bulb for 5 hours consumes the same energy.

So at 746 kW-hrs, the average home used 746 x 3.6 million or 2.7 billion Joules. (Take a deep breath. The bigger number is just putting the numbers into a standard unit, which will be useful to make comparisons.)

A good reason for doing this conversion is that we can now compare various sources of energy (coal, batteries, wind, gasoline, swamp gas, whatever) by knowing the energy in a given amount. This will be the subject of our next discussion.