How many BTUs are in a kilowatt-hour? How many barrels of oil equivalent (BOE) are in a kiloton of TNT? There are a lot of different units of energy and power. Which one is chosen at a particular time depends on the field and the customs of its experts. It can get a little confusing when comparing numbers from practitioners in different fields.
It can be very eye opening to make the conversions. For example, six sixteen watt CFL bulbs lit up for six hours will use as much energy as released by the detonation of one pound of TNT. My preference is to convert powers to watts and energies to watt-hours.
New unit for power
But there seems to be a new unit of power that I can’t find in any of my physics books. Its called a “home.” Here are some examples of its usage…
“The Tatanka Wind Farm, on the North Dakota-South Dakota border, will power 60,000 homes.”
“Limon I Wind Energy Center in Colorado is capable of generating enough electricity to power approximately 100,000 homes.”
“[E]nough clean electricity to power over 60,000 homes.”
“A 230 MW photovoltaic solar station in the Antelope Valley of California that will supply enough energy for 70,000 homes.”
“The new Copper Mountain 3 solar plant, which will be finished in 2015, will be able to generate enough power to supply around 80,000 homes.”
“Chicken Manure to power 90,000 Homes in the Netherlands!”
Mirrors at Ivanpah
Brightsource’s Ivanpah Solar Electric Generating System in California is a case in point. This is a solar thermal site that uses thousands of mirrors to concentrate sunlight to generate heat to run generators. Smithsonian.com says the “$2.2 billion Ivanpah Solar Electric Generating System—the largest of its type in the world—will power 140,000 California homes.” It looks like they are using a “home” as a unit of power.
What does “will power 140,000 California homes” really mean?
According to the EIA, the average home in California consumes about 7000 kilowatt-hours of electric energy each year (most recent data, 2009). That means 140,000 homes would use 9.8 x 108 kilowatt-hours (9.8 x 105 megawatt-hours) of electric energy per year. I think we’re on the right track here, because the National Renewable Energy Laboratory says Ivanpah will produce 10.8 x 105 megawatt-hours per year.
But this unit of power called a “home” is still a little misleading. Although the average California home consumes about 7000 kilowatt-hours of electric energy per year, energy from other sources is also consumed. The other big source is natural gas, which may be used for space heating, cooking or water heating. If you think this is trivial compared to the amount of electricity used, think again. The EIA document on residential energy consumption in California shows these graphs…
I think it is bad practice to use two mix different units for energy (kilowatt-hours and Btu) as the EIA has done with these graphs. How many people can compare kilowatt-hours and Btu by looking a graphs?
The graph on the top left is where I got the estimate of 7000 kilowatt-hours of electrical energy per year for the average California home. Notice that it is labled “ELECTRICITY ONLY.” The graph on the lower left is for “ALL ENERGY average per household,” and indicates about 62 million Btu per California home per year.
How does 62 million Btu compare to 7000 kilowatt-hours? 62 million Btu translates to 18,170 kilowatt-hours! In other words, 11,170 kilowatt-hours of energy consumed in the average California home comes from sources other than electricity. If you find this hard to believe, look at the number of kilowatt-hours you used on a recent winter electric bill and look at the amount of energy, usually in “therms,” on a recent winter gas bill. Convert the “therms” to kilowatt-hours and you will see what I mean. It takes a lot more energy to heat water and air in your house than it does to light your bulbs or power your TV. So Ivanpah really only provides enough energy to power 54,000 (≈140,000 x (7000/18,170)) California “homes.”
You might think that providing enough energy for 54,000 homes is still pretty impressive and makes a big dent in California’s energy needs. Think again. There are 12.5 million households in California. So it would take about 240 (≈12,500,000/54,000) Ivanpahs to power them all. Ivanpah covers about 16 square kilometers. So it would take about 3600 (= 16 x 240) square kilometers to power all these households.
Building 3600 square kilometers of mirror arrays is a big undertaking, but wouldn’t it be worth it to power the entire state of California? The problem is that it wouldn’t power the entire state of California. Residential power consumption is only about 20% (1/5th) of California’s total energy consumption. Far more energy goes into commercial, industrial and transportation needs.
If we assume vast efficiencies then we might say that it only takes 2.5 times (instead of 5 times) the residential energy consumption to run the entire state of California. With these assumed efficiencies Ivanpah would provide the total (not just residential) energy needs for the occupants of only about 22000 (≈ 54000/2.5) homes. It would take nearly 600 (≈2.5 x 240) Ivanpahs, a whopping 9000 (≈ 3600 x 2.5) square kilometers of mirror arrays, and $1.3 trillion (≈ 2.5 x 240 x $2.2 billion) to provide the average energy needs of the entire state.
Why talk in terms of “homes?”
The use of “home” as a unit of power has a warm and fuzzy feeling to it. I guess good and caring people are concerned about “homes,” while cold and uncaring people talk about “kilowatt-hours.” Using “homes” as a unit of power gives the impression (intentionally?) that all the energy needs of the people living in those homes are met. It is much more impressive to say an energy project will “power 140,000 homes” than to say it will compensate for the total energy needs for the people living in 22,000 homes.
I believe this loose use of the English language and lazy, imprecise use of physical values is used precisely because it yields more impressive numbers.