Energy is a complex subject, loaded with many specialized terms and a confusing array of units and inconsistent abbreviations. For example, natural gas reserves and production are usually measured in cubic feet, but natural gas is usually traded in terms of British thermal units, and it shows up on your power bill in terms of therms.
As a longtime energy journalist, I don't want to know how much of my life has been wasted puzzling over unit conversions. But it's a topic about which we all need to become more conversant. So, let me save you some trouble with a short primer on fundamental energy units and conversions. It's not a comprehensive guide, but once you get familiar with these terms, hopefully you can read and convert them like a pro. Bookmark and refer back to this primer, and never be confused again!
Some fuels are discussed in volumetric units, such as tons (or the metric tonne) or barrels. Others are referenced in terms of heat (thermal) values. (Heat values can actually get quite complicated with various "low heating values" and "high heating values" for different grades of fuels, but we won't go there in this primer.) Here are the most common units (listed alphabetically):
The number of units is dizzying. Fortunately, there are standard formulas for converting all units to other units. Just plug them into a spreadsheet and do some simple multiplication or division.
Here are some of my favorite rules of thumb. They're easy-to-remember, round numbers, so they're handy for doing some quick math in your head, but they're not suitable for detailed calculations.
Here are some more specific common unit conversions:
Converting electricity to and from heat units can be a very confusing business.
If the electricity is produced from a fossil fuel, then represented in (for example) oil equivalent, the actual amount of fossil fuel consumed to make the electricity would be converted to oil equivalent (a combustion equivalent).
Similarly, if the electricity is not produced from a fossil fuel, then it would be converted to a fossil fuel unit using a combustion equivalence.
But if the source of the electricity is unknown, and you're just converting units, or if you're just looking at a total quantity of fuel consumed (for example, natural gas) without knowing how much of it was used for electricity generation and how much of it was used for something else, then you'd use the heat value equivalent of electricity, not the combustion equivalent.
For example, electricity produced from solar would be converted to Mtoe on the basis of how much oil would have to be burned in a modern thermal power station with 38 percent conversion efficiency to make the equivalent amount of electricity. By that calculation, one Mtoe of oil produces about 4,400 GWh (or 4.4 TWh) of electricity. But using a straight conversion of the heat units, one Mtoe of oil is equivalent to 12 TWh of electricity.
So if you find your calculations seem to be off by around a factor of 3, that might be your problem.
For some quick fuel conversions of different fuels, here's a handy table (courtesy Dr. Tad Patzek). To find the comparative heat value of one fuel to another, look up the first fuel in the left-hand column, and get its Btu value. Then find that value in the top row, and look down the corresponding column to find the Btu equivalence of the other fuel.
There are many unit conversion calculators online, like this one from the U.S. Energy Information Association (EIA). (Don't be ashamed to find yourself in the Energy Kids section of the EIA's site. It's a great place to learn about energy, even for adults and energy journalists!)
A comprehensive online resource is Unit Juggler. Its energy converter should do for most purposes.
The American Gas Association also has a good, simple page on measuring natural gas.
A handy Excel file of conversion factors is included in the next-to-last tab, "Approximate conversion factors," of the Historical data workbook included in the BP Statistical Review of World Energy. That workbook is my go-to source when I need to compare different kinds of energy usage over recent history.
If you don't find what you need at those resources, just Google around a bit.
Power is a rate. Energy is a quantity. Put another way, you have the energy in your legs to walk up three flights of stairs, but not the power to leap to the top of a building.
This distinction can be particularly hard to conceptualize with electricity, since it's invisible. With electricity, it's often helpful to think of water metaphors. Volts are like water pressure. Amps are like the diameter of a hose. Watts are like the rate of the water flow.*
Units like kW are measures of power. Units like kWh are measures of energy (like a cup of water). Power multiplied by hours gives you units like kWh. If you run a 1-kW generator for one hour, it will generate 1 kWh of energy.
Capacity is a measure of how much power a power plant can put out.
Generation is a measure of how much energy a power plant actually produces.
The actual generation of a power plant depends on how often it runs. No power plant runs 100 percent of the time, so its actual generation is always lower than its "nameplate" power rating. To denote the amount of energy a power plant actually generates, compared to the energy it would generate if it ran full-time, we use a capacity factor.
A 1-MW plant with a 50 percent capacity factor would have the same energy output as a 2-MW plant with a 25 percent capacity factor.
For example, to calculate the energy output of a 1.5-MW wind turbine with a 30 percent capacity factor, you'd multiply the turbine's nameplate power rating by its capacity factor and the number of hours in a year:
1.5 x 0.30 x 24 x 365 = 3,942 MWh
Capacity factors tend to be fairly consistent from place to place for conventional generators, but for renewables like wind and solar, they can vary widely from place to place, and from machine to machine.**
There can be big differences between technologies too; for example, solar photovoltaic (PV) and concentrating solar thermal (CSP) systems. Making the question even more complicated, solar plants with storage capability can run when the sun is down, giving them even higher capacity factors.
The National Renewable Energy Laboratory (NREL) Transparent Cost Database has some aggregate data on capacity factors that can be useful as rules of thumb, but if you're doing careful analysis, you have to check the vintage of the data (some of it is quite old). In a sector evolving as quickly as renewables, data even one year old can be out of date.
The EIA recently started publishing monthly and annual data for utility-scale fossil fuel generators and non-fossil fuel generators, but even those data are U.S. averages. Actual capacity factors for specific wind or solar systems can vary widely. And, as I explained last July, the benefits of a specific renewable generator also vary widely from place to place, depending on what other kinds of local power it displaces.
Unfortunately, finding location- and system-specific data can be frustratingly difficult. But it's important if you're doing a detailed calculation. For example, a 2012 Lawrence Berkeley National Laboratory (LBNL) survey of U.S. utility-scale solar plants installed in 2010 found capacity factors ranging from 13.8 percent to 30.2 percent.
Capacity factors are often misinterpreted, and old capacity factor data is often the cause of incorrect conclusions about the economics and potential of renewable power systems.
Once you get the hang of energy units, you'll find that it's pretty straightforward stuff. You won't need any more advanced skills than basic arithmetic. Don't be afraid to pop open a spreadsheet and run some simple calculations, especially when in doubt. But mind your decimal places! Many an energy conversion goes awry at the comma or the "point."
If you have any unit questions this little primer hasn't answered, suggestions for other energy primers you'd like to see, or if (gasp) you found an error in this one, just drop me a line in the comments.
* Corrected from original
** In the original version, a comment about the increasing efficiency of renewable power collectors was inserted here. That was confusing. It has been removed. --Chris Nelder
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This post was originally published on Smartplanet.com