PrintUnit Conversions
Mistakes in unit conversions really can cost an immense amount of money. We are NOT going to turn this course into a worksheet on unit conversions, and we won’t require you to memorize unit conversions, but we will explain some of the key points next—enough to let you keep your hypothetical job with the power company...and maybe a real job someday.
Words such as energy, work, and power are tossed around in casual conversation but have very careful definitions in engineering and science, and for the people who buy and sell energy. You can think of energy as the ability to do something. Wind up an old-style alarm clock, and the spring has stored some mechanical energy, which is available to move the clock hands and make the ticking sound. Water above a dam has gravitational potential energy and can flow down under gravity, driving a generator to make electricity, or floating your boat to the sea. The chemical bonds in the gasoline in your car have chemical energy, and if you make the gas hot enough with a spark in your engine in the presence of oxygen, the bonds will change and make the car go.
When you are “using” the energy, it is doing work. Pushing you across the country, or moving the clock hands, or moving your boat down the river, require overcoming friction and wind resistance and such. So does using a plow to break the soil and turn it over so you can grow food and lots of other things. How fast you use energy, or how fast you do work, is power. You do some amount of work in climbing the stairs to the next floor, but doing it in 10 seconds requires more power for a shorter time than doing it in 10 hours.
In terms of units, and how you’ll answer your boss about the Quebec contract, energy can be measured in calories. A Big Mac has just over 500 calories, so 4 sandwiches provide just over the 2000 calories that a typical person would eat in a day. Most of the world measures energy in joules rather than calories, and those 4 Big Macs are just over 8 million joules, which is the same as 8 megajoules.
If you were on a starvation diet, you might make those 4 Big Macs last a month—a low-power diet! But if you eat 2000 calories per day and “burn” them inside you to make you go—normal power for a person—that is the same as 8 million joules per day, or roughly 100 joules per second, which is called 100 watts. Amazing as it may seem, all your skills and brilliance and good looks and charm use energy at the same rate as one old incandescent light bulb! Your energy use—your personal power—is a bit higher than 100 watts when you’re up and doing things, and lower when you’re sleeping, but averages out to 100 watts. We don’t usually define a “people power” unit, but 750 watts, or 7.5 people, is a usual definition of 1 horsepower.
So, energy can be measured in calories or joules and is the ability to do something, while power in calories per day or joules per second is how rapidly you do it, and a watt is a shorthand way of saying joules per second. But, suppose you have 10 old-style light bulbs turned on all the time, so you’re using 1000 watts or 1 kilowatt. Each hour, the computer at the power company says you have spent another hour using 1 kilowatt, so they add the price of 1 kilowatt-hour of electricity to your bill. At the end of 24 hours, you are billed for 24 kilowatt-hours. Kilowatt-hours, like calories and joules, provide a way to measure energy. The Quebec company uses joules, you use kilowatt hours, and you’ll keep your job because you know (maybe with some help from the internet) that 1 kilowatt-hour is 3.6 million joules, so those Québécois are not going to beat you in a deal!
Note:
In case you feel a sudden urge to actually do calculations with these, you might recall that the calorie you eat is sometimes also written as a capital-C Calorie, and is the energy to raise 1 kilogram of water by 1 degree C, distinguished from a calorie that is written with a lower-case c and is the energy to raise 1 gram of water by 1 degree C. So when we read about food calories, we are really talking about kilocalories. This is another reason why most of the world uses joules.
You should also know that there are many more ways to measure these things, which you do not need to learn now, but which you should know exist. People often use British Thermal Units, or BTU's, for energy, and BTU's per hour for power, but occasionally they get sloppy and say “BTU” when they mean “BTU per hour”. Or, they get lazy and say that one quadrillion BTU's is a “quad” and just quit talking about BTU's. People who sell natural gas have figured out how many BTU's, or joules, or calories, can be obtained by burning a particular amount of typical natural gas, and how much space that gas occupies at standard temperature and pressure, so they may measure energy in cubic feet of gas, or cubic meters of gas, even though they know that this depends on temperature and pressure and the particular gas. Barrels of oil can be used the same way. And, it goes on from here—refrigeration workers in the US talk about power in terms of “tons of cooling” linked to the power needed to freeze a ton of water in a day (one ton of cooling is approximately 3510 watts).
Now, we hope it is obvious that unless you are planning to work in cooling, or you have rather strange friends you would like to impress, it is probably not a good idea to clog your brain with the conversion factor between watts and tons of cooling! But you should know that a few fundamental ideas such as energy and power have been made to look very complicated by having a lot of names and units. And you also should know that many jobs you might be hired for will require you to figure out: 1) how things traditionally have been measured; and 2) how to convert to what other people are doing in their jobs. And if you can’t do that reliably, there is a high chance that you will be fired!