Sunday, 5 June 2011

Why does an electric car generate less carbon dioxide than a petrol powered one?

It is often claimed that an electric vehicle (EV) generates as much carbon dioxide (CO2) & other pollutants as an internal combustion engine (ICE), because coal is burned to generate the electricity.
Is this true?

Short answer: No.

Long answer: 

To understand, it is necessary to take into account all of the CO2 generated, from the time the fuel is mined (or pumped) until "the rubber hits the road".  This is sometimes called a "well to wheels accounting".

Some reasons are listed below:

1. In many jurisdictions around the world, a significant fraction of electricity is generated from non-fossil fuels.

2. Enormous amounts of energy is used in delivering petrol or diesel to the fuel tank of an ICE.  This includes: 

- exploration (finding the petroleum)(edit 17Feb2012);
- drilling (edit 17Feb2012);
- transporting raw petroleum from well head to refinery;
- refining raw petroleum into fuel; &
- transporting fuel from refinery to vehicle.

Some countries have long transportation distances, or have to repeat some of the transportation steps.  This includes those without their own raw petroleum or refining capacity, but also includes some countries which have both.  E.g. USA has its own raw petroleum but imports enormous quantities to feed its even higher demand.

By contrast, coal typically requires little processing before use in a power station.  This is usually limited to crushing to a suitable size & removal of dust.  Transportation is often virtually eliminated, by siting a power station adjacent to a coal mine.

3. Regenerative braking – 10-30%, depending on how & where the EV is driven.  An ICE uses a wasteful solution to slowing a moving vehicle, typical of its era: throw the energy away.  It does this by radiating the kinetic energy stored in the moving vehicle, as heat, through the brakes.  Regenerative braking uses the electric motor in an EV as a generator to convert kinetic energy to electricity & return it to the battery.
4. EVs are substantially "off peak" – coal fired power does not throttle well.  I.e. it has to be run at a substantial fraction of its maximum capacity 24/7/365.  Hence electricity providers' preparedness to sell electricity at a cheaper, "off peak", rate during some hours of the night.  If an EV is charged in the wee small hours (& most are), there is a significant carbon neutral element in that the electricity used would otherwise be converted to heat in the grid.  Note too that this applies, even if the car owner is not able to secure an off peak rate due to the vagaries of how electricity providers market energy.  E.g., off peak pricing often requires that devices be hard wired.
5. EVs are often more aerodynamically efficient than their petrol powered alternatives.
6. EVs are typically fitted with low rolling resistance tires.

7. EVs are an enabling technology for renewable energy.  One of the difficulties with renewable energy is that supply is often intermittent: solar only when the Sun shines, wind only when the wind blows.  Development is in progress for a "smart grid", which can take power from the batteries of EVs when renewable sources are unavailable.  It has been suggested that if 20% of a city's vehicle fleet is electric, the storage in the batteries could be utilized with a "smart grid" to enable 100% renewable electricity.  More detail at:

8. EVs use zero fuel when stopped at lights or in traffic.  An ICE normally "idles" when stopped in traffic.

9. Cold start – starting from cold, an ICE produces disproportionate pollution until its catalytic converter reaches operating temperature.  Petrol powered vehicles run "rich" for some time after a cold start.

10. An EV will be associated with less CO2 & other pollutants the further it is driven.


Renewable sources of electricity are being installed round the world.  The small amount of pollutants which can be attributed to an EV charged from such a grid will reduce over time, as more renewable generation capacity is installed.

11. An ICE on the other hand, has a larger CO2 footprint the further it is driven.

How does this happen?  Most obviously, as an ICE wears, its efficiency falls, fuel consumption increases & so its carbon footprint becomes bigger.

More importantly, over time, the oil industry has utilised sources of petroleum which require more & more energy to produce.  Most obvious is the move to offshore drilling.  Another is use of Canadian oil sands (aka tar sands) or USA oil shale as a source of fuel.  To quantify:

- "In the 1930s, one barrel of oil invested in oil exploration, drilling & pumping produced about 100 barrels. … By the 1970s, one barrel invested only produced about 25 barrels." – Jay Hanson [1]

- One barrel of oil invested in production of, "oil and natural gas now is running around 8 - 11 depending on locale." – Bruce Pile [2]

- Production of gasoline from tar sand, "generates two to four times the amount of greenhouse gasses per barrel of final product as the 'production' of conventional oil." – WikiPedia [3]

- "Gasoline made from the tar sands would give a Toyota Prius the carbon footprint of a General Motors Hummer." – Al Gore [4]

12. Question: How can an EV completely circumvent fossil fuel use?
Answer: Buy "green energy" – if an EV owner signs up for green electricity in many jurisdictions, the electricity provider is committing to installing sufficient renewable capacity to provide for the EV owner's use including, of course, the EV. 

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