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Musings of an Energy Nerd

Understanding Energy Units

Don't confuse energy and power — it's important to know the difference between Btu and Btu/h, as well as kW and kWh

Energy units can be confusing. Should the annual electricity production of a power plant be reported in megawatts or megawatt-hours? Hmmm... let me think...

If you’ve ever been confused by the difference between 500 Btu and 500 Btu/h, you probably can use a handy cheat sheet to explain energy units. As a guide through the thorny thickets of energy, power, and the units used to measure them, I’ve assembled some questions and attempted to answer them.

Energy is the amount of heat or work that can be obtained by burning a certain amount of fuel. Energy is measured in a variety of units, including kilowatt-hours (kWh), Btu, and joules. A quantity of energy can also be expressed in terms of barrels of oil, gallons of gasoline, or cords of firewood.

A unit of energy can be bought or sold. For example, electricity is usually sold by the kWh. Your electric bill includes a monthly tally of the number of kWh you used. If you are charged $80 for 800 kWh, then each kWh costs you 10¢.

A Btu (British Thermal Unit) is the amount of heat necessary to raise one pound of water by 1 Fahrenheit degree. A joule is the work done by a force of one newton for a distance of one meter.

Power is different from energy. Power is the rate at which energy is burned or used (or, more precisely, the rate at which energy is converted from one form to another). In other words, power is a measure of how quickly work can be performed; Power = Energy/Time, and Energy = Power × Time. Power is measured in watts, kilowatts, horsepower, Btu/h, tons of cooling, and foot-pounds/minute.

Appliances are usually rated by their power consumption. When we talk about a 100-watt light bulb, we are describing it by its power rating. Furnaces also have power ratings — for example, a furnace can be rated at 40,000 Btu/h. If the power rating is…

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20 Comments

  1. Skip Harris | | #1

    Energy tech....mmmm.....MORE! And found Hall of Shame error...
    Hall of Shame has explanation of HowStuffWorks.com error, but that seems to have an error itself:
    "resulting in a respectable average capacity factor of 43% (2,633 TWh / (675,000 MW * 8760 hrs/yr)".

    According to my math, this works out to units of "per hour" rather than dimensionless...or was this another error they were highlighting and I'm simply confused?

    We seem to often fall into forgivable shortcuts that are technically inaccurate, such as writing kw-hr rather than kw.hr, but I always find the common "measure X will save Y gigawatts by the year Z" hard-to-interpret & really irritating.

  2. Cramer Silkworth | | #2

    Hall of Shame math
    Dustin,

    I think Hall of Shame's math works fine, (TWh/yr)/(MW x hrs/yr) fully cancels (once you adjust the Tera- and Mega- to the same units). Though I get 44.5%, not 43%....but anyway.

    Now if you'll excuse me, I need to go turn on my "8,000 Btu" air conditioner... :)

  3. Alex A | | #3

    KW per Hour
    Martin wrote that "there is no such thing as kilowatts per hour"

    Technically there is such a thing... kilowatts per hour is a unit of change in power over time. So if, for example, your wind farm went from 4 MW to 2 MW over 2 hours, you could say it lost capacity at a rate of 1 MW per hour.

    That's definitely "a thing," It's just not what people usually mean when they mistakenly say "KW per hr."

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Alex A
    Alex,
    You're right -- KW/h is a measure of acceleration, like m/sec² (meters per second squared).

  5. Aj Builder, Upstate NY Zone 6a | | #5

    Unit of electrical charge, amp-hour
    Martin, another useful blog thanks. Small correction needed. Nix Amp-hour from your intro to energy unit terms. It is to do with units of electrical charge. Amp-hours your readers know about are ratings on car batteries and can help determine energy within the battery if one knows the voltage of the battery such as a 1 volt battery or a 12 volt battery.

    For example; a higher voltage battery with the same amp-hour rating holds more energy than a lower voltage battery, so knowing amp-hours is not knowing energy amounts if the voltage is not known.

    Might be best just to strike amp-hour from your second paragragh....

    I hope I am explaining this close to right... been close to wrong too often!

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to AJ
    AJ,
    Thanks for noticing my error! As soon as I read your comment, I realized you were right.

  7. Shane Claflin | | #7

    Voltage
    If voltage is water pressure... is amperage viscosity of fluid or is it the other way around?

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Shane Claflin
    Shane,
    It's just an analogy, so there is no correct or incorrect answer to your question. Amperage is the quality of the electric current that causes the conductor to get hot. I can't think of a direct correlation that works with my plumbing analogy -- but I'm open to suggestions.

  9. Skip Harris | | #9

    to Cramer Silkworth, PE: you changed units...
    You changed units from TWh / (MW *hrs/yr) to TWh/MWh....

    But I was wrong as well: the answer comes out in units of years rather than 1/years (as I said) rather than dimensionless (as they said)...

  10. Skip Harris | | #10

    Martin: amperage analog
    Wouldn't amperage (coulombs per second) be analogous to gallons per minute? Each is flow, is limited by capacity of the conductor, and is reduced by internal friction that converts energy to heat. Further, each increases with pressure/voltage and each gallon/coulomb carries more energy with higher pressure/voltage.

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Dustin Harris
    Dustin,
    My house has two types of wiring: 12 VDC and 120 VAC. My motto is, "A watt is a watt is a watt." But it isn't true that "an amp is an amp is an amp."

    10 amps at 120 volts is a lot more power (1,200 watts) than 10 amps at 12 volts (120 watts). So the gallons per minute analogy doesn't hold. As I said in my earlier example, if I want 10 gpm, I don't really care if it comes at high pressure through a thin tube or low pressure through a fat tube. But if I want to make toast with a toaster, 10 amps at 120 volts is going to work a lot better than 10 amps at 12 volts.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Viscosity would be like
    Viscosity would be like resistance

  13. Shane Claflin | | #13

    amperage
    amperage could be turbidity of water :)

  14. Skip Harris | | #14

    Martin, I think we are using different units...
    I think our difference stems from our watts analogy.
    gallons = coulombs & voltage = pressure, thus
    gpm=amps
    gpm X pressure = watts

    I believe you are using gpm while I am using gpm X water pressure.

    Imagine running a power distribution system of pipes and water. The power delivered would be gallons/minute (amps) times pressure (volts). You wouldn't really care (as far as watts goes) how if it is 1 gpm at 100psi or 10 gpm at 10psi....but the plumbing would need to be far larger to deliver the same watts at 10psi than at 100psi, just as you need fatter wire for 1000watts at 12 volts (80amps) than at 120 volts (8amps).

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Dustin Harris
    Dustin,
    Analogies are a kind of poetry, not mathematics, so I don't know if there is a single answer to this analogy disagreement.

    However, I'm still don't think I agree with your suggestion, "Wouldn't amperage be analogous to gallons per minute?" Depending on the voltage, a fixed amperage results in either low power (few watts) or high power (many watts). Watts are what I want when I turn on my toaster.

    When I am filling my bathtub, what I want is gallons per minute. As long as I'm getting so many gallons per minute, I'm happy.

  16. Aj Builder, Upstate NY Zone 6a | | #16

    GPM is analogous to amps but
    GPM is analogous to amps but we are straying off topic just a bit.

    Just thinkin Martin, don't be filling that tub, with amps.... stick to the GPMs...

  17. Pat Murphy | | #17

    Watts
    You note that a person using 12,000 watts, which I must acknowledge that I don't understand since I only consume kWhs, would have to pay $405,000 to get that from PV. I would appreciate a more detailed analysis of that, for example how my 1,000 kwH per year and my 500 gallons of gasoline per are calculated and how they relate to 12,000 watts.

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Pat Murphy
    Pat,
    Q. "You note that a person using 12,000 watts, which I must acknowledge that I don't understand since I only consume kWhs."

    A. To convert watts to kW, you divide by 1,000, so 12,000 watts = 12 kW.

    In 24 hours, an American who uses 12,000 watts (12 kW) is using 288 kWh (12 kW x 24 hours).

    Each month, that person uses about 8,928 kWh (288 kWh x 31 days).

    Each year, that person is using 105,120 kWh (288 kWh x 365 days).

    Q. "For example, how [is] my 1,000 kwH per year and my 500 gallons of gasoline per year calculated and how [do] they relate to 12,000 watts?"

    A. Gasoline has about 120,000 Btu/gallon, so your 500 gallons of gasoline are equivalent to 60,000,000 Btu. Since 3,413 Btu = 1 kWh, your 500 gallons of gasoline are equivalent to 17,580 kWh. If you want to combine your annual electrical consumption (1,000 kWh) and your gasoline consumption (17,580 kWh), you get a total of 18,580 kWh.

    As an American, you are consuming 105,120 kWh/year. If 18,580 kWh represents your automobile budget and your residential electricity budget -- that's less than average -- then the rest (86,540 kWh) is your share of the energy used by the U.S. military, Coca-Cola, Archer Daniels Midland, the offices of the State of California, your local public library, Amtrak, John F. Kennedy airport, WalMart, and so on...

  19. Kohta Ueno | | #19

    MMMMMMMMMBtus
    Martin--nice column as always!

    Try to avoid using the abbreviation MBtu, since this unit has two definitions. One MBtu can mean either 1,000 Btu or 1,000,000 Btu; that’s reason enough to stay away from MBtu.

    This MBtu vs. MMBtu system has long driven me nuts, and I agree we should try to get away from it. Just to keep the readers informed of the back story--they are not using M = mega = 1,000,000 times (which would analogously use k = kilo = 1000 times). Instead, they are using it as Roman numeral M = 1000, as in "this film was produced MCMXXXLIV"). So in this system, MBtu = 1000 Btu. However, if you use the metric prefix kBtu = MBtu = 1000 Btu. Argh!

    To make it worse, the IP system uses the terminology MMBtu, which is 1,000,000 Btu. As in M times M = 1000 x 1000 = 1,000,000. Argh!

    So if you interpret things poorly, an MBtu = kBtu, and an MMBtu = MBtu. Argh!

    Also, in case anyone was interested, John Straube and I wrote a piece on explaining Site and Source Energy (Building Science Digest 151: Understanding Primary/Source and Site Energy.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Argh indeed
    Kohta,
    Thanks for the further details on MBTU and MMBTU.

    As far as I can tell, MBTU stands for "muddled British Thermal Units."

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