The Power of the Gas Pump, By the Numbers

It doesn’t take much to spark a conversation with an electric car advocate. A casual comment about the drawbacks of an EV—doesn’t it take a long time to recharge?—will quickly spin into passionate arguments about efficiency, kilowatt-hours and states of charge. Electric car detractors are no less vociferous about the superiority of petro-power. Why do folks from both sides of the argument usually fail to win new converts? Because laypeople are clueless about even the most basic technical terms that underpin the discussion. So here comes my attempt to translate EV geek speak into English.

Plug for Tesla Roadster

If you tried to get the same amount of energy from a household outlet as you get from a gasoline pump, it would take about nine days. Of course, it doesn’t take nine days to recharge an EV, because the efficiency of an EV allows the driver to put less energy in the “tank” and still receive an adequate charge. Photo: The electric fuel plug of the Tesla Roadster.

Power to the People

The first important issue to address is the fundamental difference between energy and power. There’s a difference? There is—and if you can’t readily tell the difference between the two, don’t feel bad. Experts, officials and the media frequently confuse these terms. The difference between energy and power is quite simple:

  • Energy is the ability to do work
  • Power is the rate at which work is done

Understanding the difference is very important to the overall understanding of electricity and electrical systems, including electric cars and hybrid vehicles. The amount of energy in your batteries (or in your gas tank, for that matter) indicates the distance you can travel before refueling. That’s known as range, and it’s perhaps the most important issue involved in getting more electric cars on the road. The power rating of your electric motor (or gas engine) tells you how quickly you can turn that energy into useful work, such as vehicle acceleration.

To make this more useful in comparing gas-engine cars and electric vehicles, we need to introduce two more terms:

  • kilowatt (a measure of power, abbreviated kW)
  • kilowatt-hour (a measure of energy, abbreviated kW-hr or kWh)

Let’s begin by flipping on a 100-watt light bulb—better yet, make that 10 of those 100-watt light bulbs. With all 10 bulbs illuminated, you are burning 1 kilowatt of power. Leave those lights on for an hour, and you will have used 1 kilowatt-hour of energy. Get it?

In general terms, that same amount of energy—1 kilowatt-hour—will move an electric car about four miles down the road. The amount of electric fuel that an EV driver can store in batteries depends on a lot of factors: the number and size/shape of batteries, and how willing you are to fully charge and discharge those batteries (thus affecting how long the batteries last). Those are all energy issues. The power issues, on the other hand, are more circumscribed: recharging your batteries from a common household outlet occurs at 1.5 kilowatts. Without getting into complicated math, just know that 1.5 kilowatts is a relatively small power spigot and it’s going to take the driver of an EV a good few of hours, if not seven or eight, to recharge batteries capable of a couple hundred miles of driving. (To be more specific: At 4 miles range per kWh, one can charge from an ordinary wall outlet at a rate of —at most—6 miles of added range per hour of charging, or "6 mph." Then there is the taper-off toward the end of charge, making the last 10-20% of charge even longer).

On the other hand, an internal combustion engine stores its energy in the form of gasoline—and gas packs a 33 kilowatt-hour punch in every gallon. There’s a lot of juice in that juice. Unfortunately, the tank-to-wheels efficiency of the gas engine is five or six times less than that of an electric motor’s battery-to-wheels efficiency. If you consider what it took to extract the petroleum from the well, transport it to a refinery in supertankers and big rigs (both of which are also burning fossil fuels), and then inefficiently burn it in internal combustion engines, then the wastefulness looks even more extreme. (And that’s without calculating the geopolitical and environmental effects of that oil supply chain.)

But for the moment, I’m not concerned about efficiency or sea otters. I need to get to work, my car is running nearly empty, and I need to quickly fill up on those "33 kilowatt-hour" gallons—or my boss will not be happy. Let’s assume that my car’s gasoline tank is 10 gallons. In the five minutes it took to fill up, I would have placed 330 kilowatt-hours of energy in my tank.

If I tried to get the same amount of energy from a household outlet, it would take me about nine days. Of course, it doesn’t take nine days to recharge an EV, because the efficiency of an EV allows the driver to put less energy in the “tank” and still receive an adequate charge. But the comparison shows how gasoline became such a popular fuel over the past century: it allows us to put a lot of energy in our cars very quickly. For the sake of comparing refueling times of gas and electric cars, we need to look at power—again the rate of transferring energy or, in this case, refueling. So, in power terms, I refuel my gas-powered car at 10 gallons per five minutes or 120 gallons per hour. Those 120 gallons—at 33 kilowatt-hours in a gallon—put 3,960 kilowatt-hours in my tank.

If I haven’t lost you entirely, we can at last calculate the power of the gas pump:

  • 3,960 kilowatt-hours per hour, or 3,960 kilowatts

If I did lose you, that’s OK. We have the numbers necessary to compare the power of the gas pump versus the power of the electric cord.

  • The power of the gas pump is 3,960 kilowatts
  • The power of the electric cord is 1.5 kilowatts
  • The gas pump is 2,640 times more powerful than the electric cord

Yes, I’ve disregarded the greater efficiency of the EV versus the gas engine when the fuel is used. But you get the point.

The next equation to consider is cost, but it doesn’t involve another math lesson. The more important equation is not fundamentally mathematical: are we willing forgo the five-minute fillup in exchange for the overnight recharge if it helps us break the 100-year-old dominance of the internal combustion engine, and avert future oil price shocks, oil wars, and global warming? That’s your homework assignment.

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  • Ron Jones

    I hope that you will consider the following question. It is seriously more difficult without the math which your comment page will not permit.

    Considering the law of conservation of mass and energy, how does the hybrid that does not plug in and is recharged by burning fuel, get higher gas mileage than does the same vehicle with the same engine running only on engine power. Notably the Ford Escape Hybrid vs. the Escape with both using the 4-cylinder engine.

    I understand that the electric motors are much more efficient at moving the vehicle than is the engine but, if it is more efficient to have the engine turn a generator that charges the batteries than turn a drive shaft that moves the vehicle, why not run on batteries alone and have the engine set up in your garage at night to charge the batteries.

    With the Ford Escape Hybrid both the motor and the engine are connected to running gear. It appears that Volvo has followed the idea of the engine powering the batteries only and we will have to wait to see how that works out.

    OK now to the hard part it that convoluted question was not confusing enough.

    My Physics friends tell me that the hybrid breaks at least to laws of physics. One is the law on entropy. By that I am referring to Lord Kelvin’s comment that it is impossible to convert heat completely to work. If the power goes directly to push of the vehicle then there is one loss of conversion. If the power goes to charge the battery there is still a loss but there is a second loss when the battery power is used to turn the wheels. In the hybrid either of the losses may be less than the loss from direct drive but the total of both losses must be more than the single loss or we are looking at a prepetual motion problem.
    The second problem is outlined by the First Law of Thermodynamics — really need the math here. So lets try again,
    dU= theataQ – theataW
    or in English any net increase in the internal energy U of a thermodynamic system must be fully accounted for in terms of heat (theataQ) entering the system minus work (theataW) done by the system.

    So heat must go somewhere, if it is not used to create motion then it is wasted somewhere and MPG must suffer. . If electricity were a waste product of the fossil fuel engine and it were being salvaged, then the hybrid would be an obvious fix and would cause the mileage to soar.

    Because hybrids do work and produce better MPG or higher efficiency than do non-hybrids my physics powered friends are running on the wrong fuel.

    I realize that this question is poorly stated and may be somewhat difficult to follow but I believe that it its at the core of the move from fossil fuel to plugging into the grid.

    Thanks for a great page on the hybrids. A thourly enjoyable read.

  • lanzdale

    Time is the hybrid’s secret. It does not violate the law of thermodynamics because it never instaneously saves the energy it creaes. Over a driving period the vehicle must accelerate, maintain it’s motion against wind and friction, and then spend a different kind of energy (friction of the breaks) to stop or slow down. Some of the stopping energy is captured by turning a rod with wires in the presence of a magnet. This requires force which is then saved in batteries. The stored power is used later as an auxilliary to the gasoline motor. But there’s another detail which has to be considered.

    The electric motor on a hybrid is relativly low powered compared to the gasoline engine. On the Honda Insight for example the electric motor generates only about 15 hp equivalent while the gas generates about 75. How could 1/5 the power double the effeciency?

    The answer lies in one of a gasoline engine’s inherent limitations. The physical cylinder/cam shaft can only be one size. While valve sizes and rates of air/fuel can be varied during driving, the engine is cast only one time, in the factory.

    An accelerating engine would like one size cylinder so it can deliver energy very fast, but at crusiing speed a different size is better. The compromise wastes energy at both speeds.

    With a second, electric motor which can instantly provide torque handeling the power phase, the physical cylinder size can be optomized for lower power crusing speed. Effeciency is the secret not batteries.

  • Kieth nissen

    Question: if all passenger cars were converted to plug-in electric and were then driven the same miles as in 2008 what portion of the total electric generating capacity of the USA would the cars require?gimib

  • Koblog

    I realize this was posted in 2006 and it’s now 2010, but I have a question, if anyone’s listening:

    My 2005 Acura TL boasts 228 hp out of its six cylinder engine. It gets about 17 mile per gallon around town and up to 30 on the road.

    In terms of performance, most electric cars (except for the hyper expensive exotic electric cars) seem to be built around high “mileage” vs. rubber burning performance.

    The article says electrics are more efficient and thus need less power in the “tank” to move you along, but many gasoline engined cars could be downsized to four, three or even two-cylinder engines capable of “moving” the vehicle.

    Question: if the gasoline and electric powered cars have equal performance (acceleration, top speed, etc), does one consume less power than the other?

  • burt

    How much weight is required for the wheel motors and hugh battery?
    There goes you milage. Next question, how much is the government going to charge to recycle the battery?

  • CO_Sean

    The key inefficiencies in an internal combustion engine as compared to an electric motor of the same output are in the coasting, idling, and deceleration phases of travel. With an electric motor, even one that is not designed to regeneratively charge the batteries, there is almost no energy loss in all but the acceleration phase of travel, whereas with an internal combustion engine, it is continuously generating power, and exhausting energy supply, regardless of the phase of travel.

    As for battery recycling, technically all toxic materials need to be recycled. Conventional lead/acid batteries, and similarly more advanced material batteries, are 95+% recyclable, meaning the materials are not wasted. Ultimately the batteries have a reclamation value and the market has a continuous demand for that material, so there won’t be a recycling charge, and in fact you’ll get (certainly not much) some money back.

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