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!
Units you need to know
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):
barrel: Oil reserves are usually measured in barrels,
which are equivalent to 42 US gallons. Barrels are abbreviated "bbl."
Sometimes barrels are given as barrels of oil equivalent (boe) when a quantity of liquid fuel is biofuel or oil mixed with natural gas liquids or something else. A thousand boes are sometimes abbreviated as kboe; a million barrels
as Mb or Mboe; and a billion barrels (one gigabarrel) as Gb
or Gboe. Similar to the Btu conventions described below, millions of
barrels are often abbreviated MMBbl by energy experts, and MBtu
by non-experts. There are even more variations to describe production rates.
All of the following are used to mean "million barrels per day": mb/d,
MMBbl/d, mbpd, Mbd, mbd, Mboe/d and mboe/d.
There are probably more I can't think of right now. All I can say is: it sucks.
Just try to understand the context of the figure you're reading and do a simple
spreadsheet reality check when in doubt. Years ago, I used to use mbpd to express this measure, but I switched to mb/d for consistency with other units. I wish all other
energy journalists would follow my lead, because of course I'm right.
Btu or BTU: A basic heat unit is the British thermal unit,
variously abbreviated as BTU or Btu. It's the amount of energy
needed to cool or heat one pound of water by one degree Fahrenheit, and is approximately the amount of heat put off by burning a single wooden match. One
thousand Btu are often abbreviated MBtu, following the Roman convention
of using M for one thousand. One million Btu are then stupidly abbreviated as MMBtu
(for "thousand thousand BTU"). Don't ask me why, especially
when so many other energy units use "k" for thousand (following the metric
system convention) and "M" for million. Very large (like, global)
amounts of energy are sometimes stated in quadrillion Btu, or "quads."
calorie: A calorie is roughly the amount of energy
needed to raise the temperature of one gram of water by one degree Celsius.
When you read the labels on a food
product, the "calories" shown are actually kilocalories (kcal),
which are equal to 1,000 calories.
joule: Another basic unit of heat commonly used internationally
is the joule, abbreviated J. It is equal to an electric current of one
ampere through a resistance of one ohm for one second. A kilojoule (kJ)
is 1,000 joules, a megajoule (MJ) is 1,000 kilojoules, a gigajoule (GJ)
is 1,000 megajoules, and a terajoule (TJ) is 1,000 gigajoules. After that,
the units are petajoule (PJ), exajoule (EJ), zettajoule (ZJ),
and yottajoule (YJ) ... and that's a yotta joules! At the global scale, energy is often
stated in exajoules (1018 joules).
kWh: The kilowatt-hour is the most common measure of
electrical energy, and represents 1,000 watt-hours (Wh). A watt-hour is
the amount of energy produced by a one-watt source running for one hour. A megawatt-hour
(MWh) is one million Wh or 1000 kWh, a gigawatt-hour (GWh) is
1,000 MWh, and a terawatt-hour (TWh) is one trillion Wh, or 1,000 GWh.
Mcf: Natural gas production is often measured volumetrically
in cubic feet (cf). Thousand cubic feet (Mcf) is the most common
unit (another case where "M" stands for thousand, not million.)
Larger amounts of gas are million cubic feet (MMcf), billion cubic feet
(bcf or Bcf) and trillion cubic feet (tcf or Tcf).
Internationally, these units are often expressed in terms of cubic meters instead of
therm: Natural gas is often represented on utility bills in therms (th or thm). A therm is 100,000 Btu.
toe and MToe: Often used to measure coal, or
to compare different fuels, these units stand for "tonnes of oil
equivalent" and "million tonnes of oil equivalent,"
respectively. They represent the equivalent heat value of a fuel, in terms
of oil. One standard toe is about 40 MMBtu, but in reality different grades of
crude oil contain different amounts of heat energy. So be careful if you're comparing
(for example) the energy in an MToe of Bakken light, sweet crude to the energy
in an MToe of diluted bitumen from tar sands.
tonne: Coal is often measured in by weight in metric
tonnes, which are equivalent to 2204.62 pounds. (So one metric tonne is about
1.1* tons.) The heat value of coal can vary widely by the type of coal; for
example, 1.5 tonnes of hard coal and 3 tonnes of lignite are each roughly
equivalent to 1 toe of oil.
watt: A watt is a unit of power* equal to one joule
per second. Multiply volts with amps and you get watts. A kilowatt (kW)
is 1,000 watts, a megawatt (MW) is 1,000 kilowatts, a gigawatt (GW) is 1,000 megawatts, and a
terawatt (TW) is 1,000 gigawatts.
Common unit conversions
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:
1 boe = 5.8 MMBtu
1 toe = 41.868 GJ = 39.68 MMBtu
1 mb/d = 2 quad/year
1 calorie = 4.1900 J
1 Btu = 251.9958 calories =
1 kWh = 3.6 x 106 J = 3,412 Btu
1 quad = 1.055 EJ
Converting electricity and heat units
Converting electricity to and from heat units can be a very
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
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
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.
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 vs. energy
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 vs. generation
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
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.
Go forth and multiply
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