What Is a Watt, Really?

Reddy Kilowatt

Clean Energy. We’re all for it. In fact we’re the Southern Alliance for it. But what is it?

I’m not talking about the “clean” part. That one word deserves its own essay. I’m talking about the “energy” part. We all have some idea about what energy is. We all pay utility bills. What we’re paying our hard-earned dollars for is energy, electrical energy, expressed as “kilowatt-hours.” Let’s take a look at what that is.

In the hodgepodge of units we use here in the U.S., there are two main kinds: English and metric. Although the U.S. has steadfastly refused to convert to the metric system (along with Liberia and Burma) the electricity industry is one place where the metric system has crept in. The kilowatt-hour is a metric unit.

Let’s start, though, with another metric unit. In the metric system, energy is measured in joules.

Energy, or work, is what happens when you move a force a certain distance. The joule is defined as measuring the energy involved in moving a force of one newton (the metric measure of force, kind of like a pound) a distance of one meter. In everyday terms, if you pick an apple up off the floor and lift it onto your kitchen table, you’ve produced about a joule of energy.

The joule was named for the English physicist James Prescott Joule who lived in the 1800s. He’s best known for discovering the relationship between work and heat. Since this unit, the joule, is named for a real person, it is treated a little differently by the metric system. Notice that the unit, the joule, is not capitalized. But, since Joule is a person, the abbreviation is capitalized. So it’s a joule, abbreviated J.

The main point here is that the joule is an amount of energy.

Next let’s talk about a rate of energy use. When you picked up that apple and put it on the table, how long did it take? If it took about one second then you had an energy output of one joule (the amount of energy it took to lift the apple) per second (the amount of time it took). So you were producing energy at a rate of one joule per second.

The metric system defines the watt as a rate of energy production equal to one joule per second. So you were producing energy at the rate of one watt.

James Watt

The watt is named for James Watt. Watt was a Scottish inventor who lived about half a century before Joule. James Watt is famous for inventing the steam engine, although he didn’t really do that. What he did was take the bad design of the steam engine in use in his day and make so many improvements that it became a much more useful engine. So much so that it pretty much kick started the industrial revolution. Again, since Watt is a real person, the unit is the watt, abbreviated W.

Main point: the watt is a rate of energy use.

So you would think we were pretty much done at this point. We’ve got a unit, the joule, for an amount of energy. We’ve got another unit, the watt, for a rate of energy use. What more do we need?

Unfortunately, over the years, the electric industry has decided to take it one more step. Remember that a watt is the same thing as a joule per second. So if we multiply the rate at which we are using energy, in watts, by the amount of time we use it, in seconds, we get the amount of energy used over that time period, in joules. But instead of using joules the electric industry uses the watt-second (sort of, hang on for a bit). In other words, instead of multiplying watts times seconds to get joules, they multiply watts times seconds to get watt-seconds.

A joule measures the same thing a watt-second measures – an amount of energy – but joules don’t show up on your utility bill. Actually, neither does a watt-second. What shows up on your utility bill is not the watt-second, which represents using one watt for one second. What shows up on your utility bill is the unit that represents using 1,000 watts for one hour, the kilowatt-hour. That keeps the numbers manageable.

As an example, let’s consider a 100-watt incandescent light bulb.

By the way, the use of the 100-watt bulb as an example is not long for this world. The 100-watt bulb is going the way of the dodo; it’s rapidly being replaced by compact fluorescents and LEDs. So let’s get as much use out of it as an example as we can, while people still know what one is.

When you turn on a 100-watt bulb it uses electricity at a rate of 100 watts. No surprise there. If you let it burn for one hour, you’ve used an amount of electricity equal to 100 watts times one hour, or 100 watt-hours. If you let it burn for ten hours you’ve used 1,000 watt-hours, or one kilowatt-hour. So every hour, the bulb uses one-tenth of a kilowatt-hour.

Here’s where people often get confused. Remember there’s a difference between an amount of energy, in watt-hours, and a rate of energy use, in watts. If you turn the light out for one hour, you’ve saved 100 watt-hours. If you change the bulb from 100 watts to 60 watts, you’ve reduced the rate electricity is used – or the demand – by 40 watts. You can save a watt-hour. You can’t save a watt. A watt is a rate, so you can reduce demand by a watt, but you can’t save a watt.

How does all this relate to your utility bill? Take a look at the graph:

Demand and Consumption

 

Imagine yourself at home in the middle of the night. You’re snug in your bed; the only things on in the house are the refrigerator and your nightlight. Your electric demand – that is, the rate at which you are using electricity – is low. Then your alarm goes off, you start turning on lights and turn up the thermostat. As the day goes on there’s cooking, a load in the washing machine, and, if it’s summertime, air conditioning. Then things start calming down until you’re ready to climb back into bed.

The solid line in the graph is electric demand, or the rate at which you use electricity, in kW. Your demand (in this example) peaks a little after noon. This can vary depending on season, weather, and whether you’re home during the day. Right now, most utilities don’t charge residential customers based on their peak demand, but that could be coming soon with the advent of smart metering.

The area under the curve shows electric consumption. That’s the amount of electricity you’ve used during the day, in kWh. That’s what shows up on your bill. Your utility adds up your usage, in kWh, and multiplies it by your electrical rate. If you live in the Southeast, you’re probably paying between nine and 12 cents per kilowatt-hour. Then they add on fixed service charges, taxes, and the rest of the fine print. Add all that up and you get your monthly electric bill.

If you want to save money on your electric bill, all you have to do is reduce the amount of electricity you use. Turn out lights, turn down your thermostat, weather strip and insulate, install efficient heating and air conditioning. These will all reduce your use of electricity and reduce your monthly bill.

In the future, you’ll have another way to save on your bill. When utilities begin to move towards charging for demand as well as consumption, you’ll be able to save money by lowering your peak demand. You can do your washing at night instead of during the day, for example. You’ll use the same amount of electricity, in kWh, but you’ll reduce your peak demand, in kW. That’s not an option now, but it will be soon when more utilities install meters that can keep track of demand. And when that day comes, you’ll know what it’s all about.

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1 Comment

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Great explanation – easy to understand and entertaining! And I love both the history lesson and the future lesson. Thanks Jimmy.


Comment by Chris Ann Lunghino on March 21, 2013 6:47 pm


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