Energy Literacy 3: Energy, Power, Carbon.  The basic concepts of energy literacy.


Saul Griffith is an inventor and entrepreneur. He did his PhD at MIT in programmable matter, exploring the relationship between bits and atoms, or information and materials. Since leaving MIT, he has co-founded a number of technology companies including,,,, and

How do we measure energy and power?

If you would like to quantitatively understand the relationship between your lifestyle, global energy use, and climate change, you need to establish the language with which you can translate between these things. There are many different ways we use energy, many different ways we produce energy, and many different consequences environmentally. Power and energy are being measured around us all of the time. You get your electricity bill in kilowatt hours (kWh), your gas bill in Therms or British Thermal Units (BTUs), your car's performance is measured in horsepower, and your lightbulbs are rated in watts. To compare these things you need a common set of units, and we've already encountered 4 different units (kWh, BTU, Hp, W), and two different concepts - energy and power -- and we've only just started.

The first problem with comparing these things is that some of them (BTUs and kWh) are measures of energy consumed, and some of them (horsepower and watts) are measures of power. To add to this confusion, some of them are measures of primary energy (barrels of oil equivalent, or metric tons of coal), some are measures of net electrical power at your outlet (W), some are measures of thermal energy or heat, and some are measures of net mechanical power (Hp at the wheels of your car). To wade your way through all of this, you need an intuition for the difference between energy, and power. Energy can actually be an abstract concept, while people often have a more intuitive understanding of power-- "my car has 200 horsepower!˝

Energy is required to do work. Work is the exertion of a force over some distance. You perform work on an apple when you lift it from the ground to a table. It takes roughly 1 joule of energy to lift an apple from the ground to the table.  It takes 1 watt (1 joule / second) to lift that apple from the ground to the table in one second.  Energy is the measure of how much work can be done, whether it be moving apples, heating your house, or driving your car. You transform energy from one form to another when you do work.  For example, you convert the chemical energy contained within gasoline to mechanical energy of rotating the crankshaft when it is burnt in an internal combustion engine. The energy that doesn't make it to the crankshaft is converted to heat. That's why your engine gets hot.  

Power is the rate at which you consume energy or do work. Lifting the apple onto the table quickly requires more power than doing it slowly, but the same amount of work is performed.  A more powerful car engine can accelerate you to 65 mph faster than an engine with less power, but they both get you to 65mph.

If I were powering the laptop I was writing this on by lifting apples from the floor to the table, I'd have to be lifting a crate of 40 apples every second to do so. That's quite a lot of work. Energy is a quantity, whereas power is a rate.  

Quantitative comparison of aspects of your life (or 7 billion peoples' collective lives) could be made in terms of energy or power (or even carbon). If you use energy, you are bound to ask questions about the time period: is it the amount of energy in a month? Or over a lifetime? It was those questions that convinced me to start thinking in terms of power rather than energy. The rate at which your lifestyle uses energy is a convenient measure that gives you a single number to think about your energy use, power consumption, and ultimately environmental impact. 

But having decided to talk about power, we still needed to decide upon the right units to talk in.  Should it be kilowatt hours per day? Horsepower? BTUs per month? Watts? Kilowatt hours per day measure the use of electricity well. Horsepower measures the use of mechanical power well. BTUs per month describe the use of heat well. Watts, however, are universal, and are in fact the scientific standard as defined by the Système Internationale, so we decided to use them to measure our lives. Even though I'm talking in Watts, you'll still need to think occasionally about energy, especially in the embodied energy of objects. It isn't easy, but it is necessary. At least we are down to only two units, and they are fundamental: Watts (Power - rate), and Joules (Energy - quantity). 

Trying to understand the global energy system requires understanding power use on many different scales. Billions of people each use thousands of watts of power, and the way they use that power and get that power varies enormously. It's very difficult to have an intuition or understanding of all these different units and numbers. We all have a rough understanding of the amount of power in a light bulb. We have a sense of the power of an automobile. We speak of powerful winds. Many people have stood at the side of Hoover Dam or Niagara Falls and have been awed by the raw power in front of them. 

Wikipedia nicely lays out the power consumption of various activities at different orders of magnitude.

Wikipedia provides examples of the energy required to do different things at different scales.

This Wikipedia page contains an excellent converter between various energy and power units.

Now, everyone else talks about "Carbon Footprint." Carbon dioxide is the problem, isn't it? If so, why am I talking about energy and power, joules and watts,  instead of CO2 and PPM?

The best answer to this is that calculating their "carbon footprint" merely makes people want to reduce their carbon footprint. Yes, the carbon is a problem, but let's imagine that it wasn't (perhaps even wish that it wasn't!). Calculating my lifestyle in 2007 on Wattzon, I needed 18kw of power. If 6.6 billion people used that much energy, the world would use more than 100TW. Global world energy production currently is 15-18TW.  It is extremely unlikely that we are going to be able to make more than 100TW of power, fossil-fuel-based, green, nuclear, or otherwise. On top of reducing carbon footprint, people are going to have to simply use less energy -- hopefully while improving their lives.

As I'll explain later, the production of non-carbon emitting energy, say by using solar panels, requires a very large area of land. By talking about power instead of carbon, we will help you understand the trade-offs of all the various methods of producing humanity's power -- even the renewable energy hopefuls aren't perfect. If there is a not so subtle subtext to my blog posts, it is that the energy challenge is a game of trade-offs and compromises. It's actually a design problem; the analogy I like to use is that we are designing the garden that is earth, and we are choosing where to put the rose beds, the organic veggies, the compost heap, and the irrigation system.  The choices we make in the design will effect the quality of the garden, and its variety.

There's another, less obvious reason why I talk about power instead of carbon. The carbon footprint thing leads to a shell game: "I drive a lot, so I have a large footprint. I buy an electric car so now I've reduced my footprint." Well, maybe ... it depends on where the energy came from and how big your electric car is. If you got the power from a coal power plant and it is an electric SUV, you are still using about the same amount of power and producing about the same amount of CO2. If you drive a 6000lb SUV at 75 mph, you're going to burn a lot of energy. (This is also ignoring the embodied energy required to build your shiny new electric car). The hope is that if you do your accounting in energy and power, then there's a better chance of being grounded in a number that's not process-based and so doesn't tempt you just to switch the process (eg. from gas in your tank to coal at a power station). We'd like to inspire people to solve this problem by making intelligent consumer choices, not trying to buy things to solve the problem that ultimately exacerbate it. The solution is as much about more efficient and lower-energy ways of doing things as it is about making carbon-free power.

For reference, here is a table of the amount of CO2 produced making 1 million joules (1 MJ) from different processes:

Natural Gas - 53 g/MJ
diesel - 69 g/MJ
gasoline (petrol) - 67 g/MJ
coal - 83 g/MJ

These emission figures are taken from DEFRA's Environmental Reporting Guidelines for Company Reporting on Greenhouse Gas Emissions.

This Wikipedia page contains an excellent converter between various energy and power units.

To begin estimating your own power consumption, you can use Wattzon


  1. When I was a kid, the Powerhouse Museum here in Sydney ( had an exercise bike rigged to a generator, and as you pedalled various lights and stuff would come on and show the watts you were putting out.

    Even making 100W was hard work, if everyone had to spend a day generating their own power they’d probably think twice about leaving the lights on when they go out.

  2. Dimensional analysis shows units are wrong in first calculation.

    acceleration due to gravity is in units of metres per (second SQUARED).

    metres/second is velocity.

    1. Those people picking holes in the calculations are making basic dimension goofs. Either they are being úber ironic or they really don’t yet have energy literacy.

  3. As a retired nuclear power generation worker, I found it interesting that nuclear was not included in the comparison of CO2 produced per MJ. Aside from this, this was the best article I have read yet

    1. ex-nuclear plant worker– In the uranium fuel cycle in the US, the main enrichment facility at Paducah OH operates off two 1000 megawatt coal plants. Which isn’t to say in the future they can’t turn to wind energy, or electricity from a nuke. The other problem is that nuclear plants release radioactive isotopes every day they operate. Remember Thomas Mancuso & his studies on how much cancer each US N-plant would cause?

  4. By talking about power instead of carbon, we will help you understand the trade-offs of all the various methods of producing humanity’s power — even the renewable energy hopefuls aren’t perfect.

    This pleases me. Energy discussions have long suffered from a paucity of objective analysis.

  5. The author mentions this, but I think it’s worth highlighting: if you could literally power your laptop by lifting 40 apples every second the height of a standard table, that on the face of it doesn’t sound too bad. I mean, you’d get tired after awhile, but it’s feasible.

    But in reality, the 40 watts fed to that laptop requires a much larger energy expenditure further upstream. Losses in the wires, generators, turbines, etc. all reduce efficiency. So that 40 watt laptop (which is on the low end of the power consumption of computers generally) turns out to be a much larger real energy expenditure. I.e. if you were actually lifting apples to generate the electricity, you might have to lift 150-200 apples every seconds for enough power to come out the other end!

    So unfortunately that means that even a relatively thrifty 40W computer is still using a surprisingly large amount of energy as compared to what a human being needs as its bare necessity energy consumption, and that’s not even counting all the other stuff we spend energy on.

    On the bright side, it means that if someone can figure out even more ways to improve efficiency in the power generation/delivery pipeline (and there are some areas where “they” have done a pretty impressive job already), there’s a lot of extra “free” energy available to us.

    If we can improve things at the consumption end by reducing usage 50% (more efficient devices, better conservation by users, etc.), and we can double efficiency at the production and delivery side, the almost 20TW of power we generate now could well come close to the theoretical 100TW the other suggests might be required. Or rather, we’d only need 50TW, and we’d produce 40TW with no additional power production facilities.

    Sure, we’d still need another 25% increase in production, but that seems a lot more feasible than a 5-fold increase. :)

  6. WOW, I’m so dumb, I didn’t realize that (from the graph) my laptop uses 144 kW of electric power per hour! That is 40 watts per second x 60 = 2,4 kW/min x 60 = 144 kW!!!! Just goes to show what an M.I.T. education will do for you.

    1. @ #6 – Anonymous:

      Yes, regardless of your attempts at being condescending, you *are* dumb, 40W is accurate, your math is wrong.

      The “per second” is intrinsic in the definition of a watt. So 40W “per second” would actually be 40W/second = 40 J/second^2 = not the unit you’re looking for.

      40W X 1 hour = 40Wh = 0.04kWh

      1. I think (s)he knows that, but is trying to address the apple-lifting comparison. The real way to make this point is using a great big battery in comparison (no connection to mains power) in which case you’d need 40 watts * 3600 (seconds in an hour) of energy in the battery to power that laptop for an hour.

        So (s)he’s right, as are you: 40Wh = 144 kW (units are everything!)

        There is of course another problem: Mains power is not usually ‘saved up’ so unused capacity is often lost. Imagine if we had efficient and large batteries at every substation — then intermittent power sources like wind and solar might finally be feasible as major sources.

        1. No, he/she is NOT right, so as you aren’t! :-)

          40 Wh = 144 kWs

          (You and he/she let drop the “s”, which is indispensable!)

    2. That is 40 watts per second [sic] x 60 = 2,4 kW/min x 60 = 144 kW!!!! Just goes to show what an M.I.T. education will do for you.

      I would have thought that they taught what a what was at MIT. If you’re going to be sarcastic, at least don’t pretend to have been to a place that would have been unlikely to have let you graduate.

  7. > we are designing the garden that is earth

    Yes! And when enough people accept this heresy, we advance to the next level, and the world becomes a better place.

  8. To clarify, lifting a crate of apples by hand would probably consume much more than 40 Joules of energy.

    To be energy literate, someone should understand energy loss at every step. I
    I will give 3 examples:
    1)Human lifts crate of apples. The energy starts at the sun. About 10% of the available energy goes to plants. Assuming the human is a vegetarian, he should get 10% of the energy that the plant got from the sun. Humans are less than 30% efficient, so the human needs 120J to lift that 4kg crate of apples. Roughly 12kJ of energy from the sun actually, if I got everything right.
    2)Running an electric motor to lift the apple. First, we need a source of energy. To simplify, I’ll assume solar, which should be in the range of 5%-40%(I’ll assume 10%, for simplicity) efficiency depending on the quality of the solar cell. In the US, we have a ridiculously complex electrical system (thank Tesla) that can transport electricity from a power plant to your home at over 90% efficiency. The electric motor should have about 80% efficiency in lifting the crate. The overall efficiency should be .8*.9*.1=7.2% efficiency, so we only need around 560J from the sun to lift the crate.
    3)Powering a laptop. Assume solar power again, and the same rate of power transmission. From what I could find, charging a laptop battery seems to have a 75% efficiency. We don’t really lose anything after that, except the 40J required to keep the battery on for 1 second. So .75*.9*.1=6.75% efficiency. So roughly 600J here.

    On top of that, there is the production cost for our solar panel, or our food, and the shipping/construction of our apple lifting motor(which is surely more than 12kJ).

    In fact, one of the benefits of electric cars is that electric motors are vastly more efficient than internal combustion engines. Internal combustion engines lose over 80%(and another 6% in the transmission/driveline) of their energy used before anything is accomplished. Oil and coal power plants lose less than 70% of the energy, so even with a low-efficiency electric motor and inefficient battery charger, we can easily match the efficiency of an internal combustion engine due to the miracles of AC transmission.
    And of course, down the line we can hopefully use wind/solar instead of coal.

  9. I’m glad HarveyBoing already talked about the efficiency losses and such, but as someone who has done a lot of work in energy engineering, I want to re-emphasize the point: in order to measure the energy footprint that something makes, you have to look at the life cycle.

    A simple example is the energy of moving that crate. Recognize that your muscle fibers utilize the chemical potential in ATP at around 25% efficiency, and that the overall harvesting of ATP from, say, glucose, is 38%. Your body isn’t very efficient; the real energy required to do the whole lifting business were your body a continuously operating machine, with a net 40W change in potential, would be more on the order of consuming 421W worth of chemical potential energy in the form of food. Overall efficiency is ~9.5%

    Similarly, the electricity consumed by the computer incurs losses from the transmission (transmission efficiency is around 90%) and from generation (new coal power plants have an efficiency of ~40%). Overall efficiency is 36%, and you’re still burning coal.

    You should also note that each of the energy uses presented – gravitational potential and electricity – can be more or less useful. Intuitively, this makes sense, since it takes mechanical apparatus to get falling water to, say, power my computer. Engineers sometimes speak of the quality of an energy source as a way to describe how useful it is to us. At the bottom of the spectrum is low grade heat – stuff that’s likely high volume, but is only a few degrees hotter than what we have to exchange with it. Since energy requires a gradient to extract it, we can’t get much out of this stuff, and it’s the most difficult form of useful energy to extract to turn into some useful purpose, but the easiest to produce. In practice, low grade heat sources yield so little that they are almost never tapped. The best quality energy is electricity, which can be turned into work, potential, heat, or what have you easily (i.e. with high efficiency), but is also the hardest to produce.

    I think everyone should learn these things, then look at the numbers we face when we talk about the looming energy crisis. Global warming or not, there’s no doubt we have to face peak oil soon enough.

  10. It’s not THAT much energy, compared to human energy requirements. The human using the laptop is continuously emitting 60 to 100 watts of heat energy just sitting there. Our food is generally much more expensive and destructive to produce than electrical power (the laptop doesn’t really care what electricity tastes like).

    The laptop doesn’t register compared to other energy expenditures like street lights no one is around to see, junk mail no one reads, food no one eats, driving 60 miles a day to sit in front of a computer and only interact with your coworkers using email.

  11. @complicity

    You are correct. The units for acceleration due to gravity should be m/s^2. The rest of the calculation is valid however and it is probably a simple typo that the superscript “2” on seconds got left out.

  12. g is not gravity, for god’s sake! Gravity is a phenomenon with a variety of ways of measures. You may measure the gravitational force, the gravitational potential energy, the acceleration produced by gravity or the value of the gravitational field, which is what g is. Its local value is 9.8 m/s^2 but elsewhere it takes on different values (which is another reason why “gravity” is not not not “9.8”). And the field, in turn, is only an acceleration by virtue of the equivalence principle.

    Yes. There’s a physicist in the room. Stop talking nonsense.

  13. Where can you even find a laptop that “typically” draws 40W? My ThinkPad typically draws about 12W, or about 8W if it’s just sitting there, or less than 1W when it is asleep, and only about 25W when it’s running full-tilt on both CPUs with the screen brightness turned all the way up. I’m sure I couldn’t get it to draw 40W if I tried. It would probably overheat.

    1. Where can you even find a laptop that “typically” draws 40W? My ThinkPad typically draws about 12W…and only about 25W when it’s running full-tilt on both CPUs with the screen brightness turned all the way up

      Power consumption for a computer is dependent on the exact make/model, specific configuration, and usage, among other things.

      Your numbers for your Thinkpad sound very optimistic to me, but assuming you’re actually measuring energy consumption accurately, perhaps you’ve got some unusual user scenario, combined with a particularly efficient model. Suffice to say, that doesn’t serve as a counter-example to the author’s own observations, which are entirely believable.

      Using Lenovo’s “Energy Calculator” ( I can see that the most efficient ThinkPad model is the SL400 with the 15″ widescreen display, and that in idle mode, they expect it to consume about 15W of power (146KWH annually if in idle mode 24 hours per day). On the other hand, if you get the high-end W700 with the 17″ display, in active mode it has an energy consumption figure of 175W. And even that relatively efficient SL400 is expected to draw over 90W in active mode.

      Direct measurement trumps specifications, of course. But these are the official specifications advertised by Lenovo as part of their “energy-saving” campaign. It seems likely that they are at least in the ballpark, and certainly Lenovo’s got no incentive to inflate the power consumption numbers.

      And as far as direct measurement goes: I have a MacBook Pro Core 2 Duo 2.33, and it draws between 60-80W depending on what I’m doing. A laptop that uses 40W sounds completely reasonable, and even a bit on the low end in terms of power consumption. I find it very difficult to believe that there’s any dual-core laptop out there that can run the display at full brightness, with the CPU utilization pegged at 100% for each core, and yet only consume 25W of power (I’m not even aware of any dual-core CPU that, on its own, consumes so little power at full speed and utilization). Beyond that statement, I won’t waste time questioning your own observations — that’s for you to do, and only if you care to. But there’s plenty of data, both in product documentation and from my own empirical observations that there are laptop computers that easily consume 40W or even more.

      1. HarveyBoing, your numbers are not consistent with reality. There’s NO WAY a MacBook Pro consumes 60-80W because for one thing its battery would be exhausted in less than an hour, and for another it would overheat, and finally its internal power supply is not even capable of supplying that much power. A MacBook Pro has a 73 watt-hour battery and its battery lasts up to 7 hours, so you do the math.

        All of my observations of the ThinkPad are based on the direct measurements of the hardware as reported by the machine’s BIOS. If you are skeptical that Intel makes a low-power, dual-core CPU I suggest reading the catalog more closely. The Core 2 Duo L7300 in my laptop never consumes more than 17W (this is its “Thermal Design Power” or TDP) and Intel also markets a more advanced dual-core CPU with a TDP of only 10W. And remember, the TDP is only reached when the CPU is working full blast. When it’s just sitting there while the owner is reading BoingBoing, a modern CPU hardly even draws 1W.

      2. @HarveyBoing: I agree with jwb: those numbers seem pretty high. Although I don’t quite believe the Apple specs like jwb does.

        I just checked my MacBook Pro’s battery. It claims to be rated for 60Wh, and I see no reason that they’d low-ball this estimate, so we can treat it as an upper bound.

        My computer’s already a couple years old, so it probably isn’t as efficient as newer ones, but here are my numbers: while working (wi-fi on, screen bright etc) it lasts about 2.5 hours. Idle (on but closed), it lasts about 10 hours.

        So my MacBook Pro must be consuming no more than 60/2.5 = 24 Watts while working, and 60/10 = 6 Watts while idle.

  14. Re: electric cars, the author wrote:

    If you got the power from a coal power plant and it is an electric SUV, you are still using about the same amount of power and producing about the same amount of CO2.

    I’m not sure that’s true. If the vehicles weigh the same and have similar aerodynamicl, then the amount of energy required to move it forward is the same.

    But the amount of energy you need to generate is much lower, because the energy is delivered to the wheels much more efficiently. Coal plants are generally 30-40% efficient at turning fuel into joules. Car engines? About 10%. Even accounting for the losses of transmission and charging the battery, electric vehicles compare very well.

    Also, as your energy grid becomes more carbon efficient, the amount of CO2 your driving generates will go down. The combustion engine will always be powered by 100% fossil fuels. Or 200% fossil fuels, if we start getting significant quantities of oil from tar sands or coal-to-oil processes.

  15. One thing I like about our gas company in BC (Terasen) is that our bill comes with gas usage measured in J. (Actually, it’s usually GJ, but we get bi-monthly (or maybe quarterly?) bills.)

    1. That seems nicer that the random-sounding cubic feet of gas at 60 degrees, or whatever mine comes in.

      That said, it’s almost certainly measured the same way: the measure the cubic feet of gas that you use, then they assume the gas is all the same and that they know exactly how many joules are in a cubic foot, and then multiply.

  16. Fine, for everyone talking in watts/time, how many total watts of battery are consumed by a single hour of usage? It’s not 40W!

    1. Fine, for everyone talking in watts/time, how many total watts of battery are consumed by a single hour of usage? It’s not 40W!

      Again, read above for the definition of watts. Watts are a unit of power, not of energy, so you can’t say that a battery has “used up” x watts over an hour, or that it “contains” x watts. Rather, it can “contain” x WattHOURS (or Wattminutes or Wattseconds or wattever).

      But in answer to the question you may have been asking, a regular 1.25V battery is probably rated for 2000mAh, so it could provide 1.25V and 2 amps for a total of 1 hour before running out (it reality the battery probably couldn’t provide that much power, so it would provide less power for more time, but the calculation would end up the same).

      1.25V x 2 amps = 2.5 Watts of power for 1 hour.

      So the battery would contain 2.5 Wh

      (note the important “h” at the end of that unit!)

  17. Energy conversion is the most important subject to the future of the human race. Thermodynamics should be taught in middle school. Thank you for this article.

    P.S. Some uhh pro electric car people here seem to be seriously undercutting the efficiency ICdrivetrains to make their case. By weary of their figures. It is not clear that coal/ch4 powered electric cars are better than petrol engines.

    P.S. One other poster mentioned the 2nd law. Yes. there is lots of crappy energy (low grade heat). We must also recognize that across all industries, the most common application is to heat or cool things just a little bit. Using waste heat would triple the energy out of all heat engines on earth………..

    P.S. 3. Electricity is not cheap enough to use to make low grade heat. To advocate such a position is engineering heresy.

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