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How Much Can Plug-in Hybrid Vehicles Reduce Greenhouse Gas Emissions?

Time Required Very Long (1+ months)
Prerequisites You will need to track your family car's gas mileage for this project.
Material Availability Readily available
Cost Very Low (under $20)
Safety No issues


Are you concerned about global warming and greenhouse gases? Here's a project that shows how hybrid electric and plug-in hybrid electric vehicles can reduce greenhouse gas emissions. Compare your own car's greenhouse gas emissions to hybrid cars using online data from PG&E and Google.org's RechargeIT project.


The goal of this project is to compare the equivalent carbon dioxide emissions of your family car(s) with that of plug-in hybrid electric vehicles from Google.org's fleet.


Andrew Olson, Ph.D., Science Buddies


This project idea is based on the Google.org RechargeIT website:

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Last edit date: 2013-01-10


One of the major challenges facing the world today is to devise more efficient ways to use our energy resources. One of many reasons for doing so is to reduce the amount of greenhouse gases we produce, since these gases trap energy from sunlight in the atmosphere, and most scientists believe that these gases are warming the planet.

This project is about how much greenhouse gas reduction can be achieved by changing the way we get from place to place. We will focus on the greenhouse gas reduction that can be achieved by switching to plug-in hybrid electric vehicles. You'll use data from the Google.org RechargeIT project (Google.org, 2007a), plus data that you collect from your own family car.

You've probably heard about regular hybrid electric vehicles like the Toyota Prius, the Honda Insight, and others. These cars have both a small gasoline engine and an electric motor to power the car. The electric motor is powered by a large on-board battery. The extra assistance of the electric motor means that the gasoline engine can be much smaller (and thus more efficient) than on a gasoline-only vehicle. You can find more information in the "How Hybrid Cars Work" article in the Bibliography (Layton and Nice, 2007). So what is so special about plug-in hybrid electric vehicles?

"A plug-in hybrid is different from today's hybrid because it has a higher capacity battery and the car can be plugged into a regular 120-volt outlet for charging. Because it is operating primarily on electricity for the first 20-40 miles, the car operates much more efficiently and uses dramatically less gasoline. Manufacturers are now designing plug-in hybrids that may not use any gasoline at all for those first 20-40 miles, and since more than 70% of Americans drive less than 33 miles per day, many will not use any gasoline in their daily commutes -- but they retain the flexibility of using the hybrid gas engine for longer trips. In addition, plug-in hybrids will often recharge at night using excess power from base-load power plants that are already running, so they won't generally add to peak electricity demand." (Google.org, 2007b)

If you're a fast thinker, you're probably wondering, "But doesn't that just shift the greenhouse gas problem from the car to the electric generation plant?" That's an important question to keep in mind. There are many possible factors involved, for example:

  • How much carbon dioxide is produced to make electricity?
  • How efficient is the charging process?
  • How efficiently does the electric motor power the car?
Luckily, you don't need to answer all of those questions in order to find out if plug-in hybrids reduce overall carbon dioxide output. As part of this project, you will learn how to calculate the amount of greenhouse gases emitted by a vehicle, whether powered by gasoline or plug-in electricity. The "Vehicle Calculator" webpage (Google.org, 2007c), explains how this is done. You can go there for more details, but we'll show you the basics here. You'll be able to compare carbon dioxide emissions from gas-powered vehicles, regular hybrid electric vehicles and plug-in hybrid electric vehicles.

Another way that plug-in hybrids may help to reduce carbon dioxide emissions is called "Vehicle-to-Grid" (or V2G) technology. Unlike energy sources like gasoline or heating oil, electricity used in the home is not readily stored. Electric utilities must vary their power production over the course of each day and over the course of the seasons to match the changing demand for electricity. If plug-in hybrids were widely adopted, they could help to stabilize the constantly changing demand for electricity. For one thing, they could be charged primarily at night, when demand for electricity is low. Baseline usage power plants that are already in service could supply the electricity for the nightly charging. Plug-in hybrids could also provide electricity back to the power grid at times of peak demand (like hot summer afternoons). This could mean that power companies could avoid firing up reserve generation capacity (which is often older plants that are less efficient or generate more pollution).

"RechargeIT is also exploring ways to develop and deploy V2G technology. With the help of PG&E, Google will be demonstrating how electricity might be transmitted back and forth between plug-in hybrids and the grid. V2G offers the potential to use plug-in hybrids as a battery storage to make better use of our energy and stabilize the grid. Similar to the plug-in hybrid vehicle demonstration, the objective is to collect real world data to understand the benefits of V2G and enable future adoption." (Google.org, 2007b)

"Greenhouse gases" include carbon dioxide, plus other gases (methane, for example) that can also trap heat in the atmosphere. For comparison purposes, greenhouse gas emissions are calculated in terms of carbon dioxide equivalents (CO2e). Each gallon of gasoline produces 23.6 pounds of CO2e when burned (Google.org, 2007c). To figure out how much CO2e a car emits per mile due to burning gasoline, you divide this number by the car's gas mileage, like so:

Equation 1:
2.63 lb CO2e / gal
73.6 mi / gal
=   0.321 lb CO2e / mi

Electric use is measured in kilowatt-hours. A kilowatt is one thousand watts—the amount of energy it takes to light ten 100-watt light bulbs. A kilowatt-hour is the amount of energy it would take to keep those light bulbs going for an hour. This is also about the amount of energy needed (on average) for a typical U.S. household. The amount of greenhouse gases produced when generating electricity can vary greatly, because there are many different ways to generate electricity (burning coal or natural gas to power steam turbines, using nuclear power to make steam to power turbines, or using wind or water to turn turbines). The "Average State-level Emissions Coefficients" resource (EIA, 2002) lists how many pounds of CO2e are created per kilowatt-hour of electricity on a state, regional, and national level. The amounts vary from state to state, depending on the blend of different methods used for generating electricity in each state. To figure out how much CO2e a car produces as a result of electric charging, you multiply the state-level emissions coefficient by the number of kilowatt-hours/mile for the car, like so:

Equation 2:
0.1189 kWh / mi × 0.61 lb CO2e / kWh   =   0.073 lb CO2e / mi

You can see more details about comparing CO2e from plug-in hybrids vs. gasoline-powered cars on the "Vehicle Calculator" page (Google.org, 2007c). Do plug-in hybrids actually reduce the amount of greenhouse gases produced per mile driven compared to gasoline-powered cars? You can find out by comparing greenhouse gas output per mile from the RechargeIT vehicles with the output of your own family car. The Experimental Procedure section, below, shows you how.

You might also think about other ideas for reducing greenhouse gas emissions from cars and trucks. The Variations section (below) has some ideas to get you started.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

  • Vehicle gas mileage (miles per gallon, or mpg)
  • Electric power units: kilowatt-hour (kWh)
  • Hybrid electric vehicle
  • Plug-in hybrid electric vehicle
  • Equivalent greenhouse gas emissions CO2e


  • How does the value of CO2e/mile for your car compare to the Google.org plug-in hybrid fleet?
  • How does the value of CO2e/mile for your car compare to the national average?


  • These webpages have information on the Google.org RechargeIT project:
    • Here is the main RechargeIT webpage:
      Google.org, 2007a. "RechargeIT.org: A Google.org Project," Google.org [accessed July 9, 2007] http://www.google.org/recharge/index.html.
    • This webpage has an overview of the RechargeIT project:
      Google.org, 2007b. "What We're Doing," Google.org [accessed July 9, 2007] http://www.google.org/recharge/overview.html.
    • This webpage has a calculator for comparing your family car's CO2 emissions and fuel use to the national average, and to Google.org's hybrid electric and plug-in hybrid electric vehicle fleet averages:
      Google.org, 2007c. "Vehicle Calculator," Google.org [accessed July 10, 2007] http://www.google.org/recharge/dashboard/calculator.
  • This webpage shows how hybrid electric vehicles work:
    Layton, J. and K. Nice, 2007. "How Hybrid Cars Work," HowStuffWorks.com [accessed July 10, 2007] http://auto.howstuffworks.com/hybrid-car.htm.
  • This webpage shows how much CO2 is produced (in lbs/kWh) for generation of electricity on a state-by-state and regional level:
    EIA, 2002. "Average State-level Emissions Coefficients," Energy Information Administration, U.S. Department of Energy [accessed July 10, 2007] http://www.eia.doe.gov/oiaf/1605/e-factor.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • Computer with Internet access
  • Family car for gas mileage measurements
  • Lab notebook

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Experimental Procedure

  1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
  2. Here's how to measure your family car's gas mileage:
    1. For this method you need to start with a full tank of gas. So next time the car gets a fill-up, write down the odometer (mileage) reading (from the car's dashboard).
    2. The next time the car needs gas, fill the tank again. Since the volume of the gas tank remains constant, the amount of fuel added to fill the tank will equal the amount of gas that was burned since the last fueling.
    3. Write down:
      • the amount of gas added (in gallons—read from the gas pump),
      • the odometer reading (in miles—from the car's dashboard).
    4. Now you have all the information you need to calculate the car's gas mileage for the time between the two fuelings. Subtract the previous mileage reading from the current one, then divide by the number of gallons of gas added to the car. The result is the number of miles your car traveled per gallon of fuel.
    5. The example table below (with made-up numbers) shows how you can organize your data and calculate your car's gas mileage.
      Gas to fill tank
      Odometer reading
      Distance driven
      Gas mileage (miles per gallon, or mpg)
      14.28 76497 366 25.6
      10.43 76762 265 25.4
    6. To get the most accurate results, you should track your family car's gas mileage for several weeks (at least 3–5 gas tank fill-ups).
  3. Using the conversion factor of 23.6 pounds of CO2e per gallon of gasoline, calculate the CO2e per mile for your car.
  4. From the RechargeIT website (Google.org, 2007a), you can obtain the average fuel efficiency (in miles per gallon) and electric usage (in kWh/mile) for the plug-in hybrid electric vehicles in Google's test project. Calculate the CO2e per mile for the Google vehicles, as described in the Introduction.
  5. How does the value of CO2e/mile for your car compare to the Google.org plug-in hybrid fleet?
  6. How does the value of CO2e/mile for your car compare to the national average?

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  • Although this may be more easily said than done, another way to reduce greenhouse gas emissions from automobiles is simply to drive less. What is the total mileage that your family car is driven in a year? Is it greater or less than the national average (12,000 miles/year).
  • Work with your friends to collect a larger sample of gas mileage for family cars. Calculate the average gas mileage for the family cars of 20 or more families from your school. How does your group's average gas mileage compare to the national average (19.8 mpg in the U.S. as of 2005). You could also extend the data collection to figure out the average total mileage per year for your group. How does it compare to the national average (about 12,000 miles/year). On the basis of your numbers, how much more (or less) greenhouse gas emissions do your group's cars contribute compared to the national average?
  • Compare how your family car's gas mileage changes under various driving conditions (this project is much easier to do with a car that has gas mileage readout on the dashboard):
    • Highway vs. in-city
    • Highway at 65 mph vs. highway at 55 mph
    Calculate the CO2e for these various driving conditions.

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