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To plug-in, or not to plug-in? (emissions-wise)

Feb 2023

Plug-in hybrid electric vehicles (or PHEVs, or simply plug-ins) are a proposition for how a vehicle owner can cut back on CO2 emissions from their driving. But just how good are they at this task? If used correctly, they sure (or probably) can bring savings in terms of money spent for fuel for their owners, but even though this aspect is very important, I view the mission to cut back emissions as the primary angle from which they should be looked at. After all, isn't electrification of road traffic and promotion of electric vehicles (of which PHEVs are a subgroup) mainly aimed at reducing CO2 emissions from road transport, as part of wider climate-related actions?

Out of curiosity and just for the heck of it I set out to make an analysis of sorts to see how they compare, purely in terms of CO2 emissions (skipping the considerations about fuel cost savings altogether).

CO2 emissions from plug-ins come from 3 directions:

  1. from fossil fuel burned in the combustion engine component of the plug-in drive system,
  2. from CO2 footprint of electricity generation (characteristic of the power network into which a plug-in is, well, plugged-in to charge the battery for the electric motor component of its drive system),
  3. the emissions created during the production of the plug-in's traction battery (most often than not a lithium ion battery), amortized during the battery's lifetime (for the purpose of this text, 250,000 km driven by the vehicle).

Let's calculate these components.

Calculations

CO2 emissions are expressed in g CO2 per km driven. Here's how to calculate them:

(1) CO2 emissions from fossil fuel burned:
Average fossil fuel consumption of the vehicle (in l/100 km) * 2300 (gasoline engines produce 2300 g of CO2 per litre of gasoline consumed; for diesel fuel, use 2680 instead of 2300 here) / 100 (to express the value per km instead of per 100 kms)

(2) CO2 emissions from electricity consumption
Average electricity consumption of the vehicle (in kWh/100 km) * CO2 emission from electricity generation, characteristic for the network from which the plug-in gets its glow juice from (in g CO2/kWh of electricity produced) / 100 to express the value per km instead of per 100 kms)
The values for emissions from electricity generation vary a lot, for the purpose of this text the following reference values will be used:

(3) CO2 emissions related to lithium battery
Vehicle's battery size (in kWh) * g CO2 emitted to produce 1 kWh of lithium battery1 / 250.000 (to get a value of g CO2 due to battery production per each km driven by the vehicle during its lifetime, which in this text is, rather arbitrarily, set to 250k km)

To get the CO2 emissions for the vehicle, the 3 components have to be summed!

In order to see how fit are plug-ins for their task, I will compare the performance of a plug-in (emissions-wise, that is) to the relevant values of its sibling that comes equipped with an internal combustion engine (ICE) only.

Data rules

All data about vehicle consumption mentioned below are averages of data from a popular site where users log their real-world consumption data. Requirements: cars year of make 2018 or newer; at least 10,000 km of logged data; ideally the same amount of kms logged for gasoline and for electricity consumption and ideally at least one full year of data. After sorting, the top and bottom 5% of sorted vehicles scrapped to avoid extreme values. For the average consumptions for diesel and gasoline-only cars I took the mean value between the average consumprion of all cars and the consumption of the median car, just in case the average is perhaps distorted due to any extreme value on any of the extremes. I am not a data scientist in any way so this list of requirements might not be ideal/necessary/logical, but it is the one I used.

Carz analysis

Volkswagen passat

A VW passat is very handy for this comparison, as you can get it in diesel, gasoline engine only and plug-in versions and they put a similar number of hp on the road.

Plug-in (VW passat GTE)

Add the 3 together and the plug-in passat comes in at 103 to 149 g CO2/km.

Table 1: CO2 emissions for VW passat GTE plug-in (average consumption: 4.0 l gasoline + 11.5 kWh electricity per 100 km; battery capacity: 13 kWh)

VW Passat GTE vehicle emissions from
emissions from
electricity production		 vehicle motion energy
storage		  
electricity
source		 g CO2/kWh powered by
electricity		 powered by
gasoline		 motion
total		 vehicle
battery		 emissions
total
World 440 50.6 92.0 142.6 6.8 149
France 79 9.1 92.0 101.1 6.8 108
PV@roof 40 4.6 92.0 96.6 6.8 103
(in g CO2 per km driven)

VW Passat - gasoline

VW Passat - diesel

So, if you charge a passat GTE PHEV in an average corner of the world (in the sense of carbon intensity of electricity generation) and then you go around driving it rather averagely, your car's CO2 emissions would be very much in line with a diesel passat (149 vs 155 g CO2) and less than 10% below the value for a gasoline powered, ICE-only passat (163 g CO2).
If you would drive the same plug-in across India, South Africa, Indonesia or Poland, where the carbon intensity of electricity generation is above world's average2, then your emissions would come out at at least 170 g CO2/km, so above the average value for any version of the same car that comes equipped solely with an ICE. In such places, at least from the perspective of curbing CO2 emissions, driving a plug-in to achieve that mission brings to mind a line from a David Bowie song: "And I've been putting out fire / With gasoline."
If you happen to live in a part of the world where carbon intensity of electricity generation is low though, like France, Norway, New Zealand, Uruguay2, or you mainly fill your car from a PV installation, then the CO2 emissions savings you get (compared to same car that is powered solely by an ICE) can be around 33%, provided, of course, you regularely fill your car with electricity.


Toyota RAV4

Toyota RAV4 comes as a plug-in, a regular hybrid or a normal ICE (gasoline). I will compare the plug-in with the regular hybrid (as they are the bulk of non-PHEV cars available in the database where I got the data from). In RAV4's case though there is quite a substantial difference in horsepower between the PHEV and the non-PHEV vehicles, with some 30% more hp in the PHEV variant, so this might mess the results a bit perhaps.

Plug-in (Toyota RAV4 Plug-in)

Table 2: CO2 emissions for Toyota RAV4 Plug-in (average consumption: 3.5 l gasoline + 13.0 kWh electricity per 100 km; battery capacity: 18 kWh)

Toyota RAV4 Plug-in vehicle emissions from
emissions from
electricity production		 vehicle motion energy
storage		  
electricity
source		 g CO2/kWh powered by
electricity		 powered by
gasoline		 motion
total		 vehicle
battery		 emissions
total
World 440 57.2 80.5 137.7 9.4 147
France 79 10.3 80.5 90.8 9.4 100
PV@roof 40 5.2 80.5 85.7 9.4 95
(in g CO2 per km driven)

Toyota RAV4 - gasoline (hybrid engine, non-plug-in)

Since the Plug-in version of the RAV4 is a 4-wheel drive car, I also took into account only the AWD versions of the regular hybrid RAV4 to even the field a bit.

In this case again, if you charge your RAV4 Plug-in from a well that has a carbon footprint equal to the world's average, there is no difference between the CO2 emissions of the Plug-in and the regular hybrid car (147 vs 145 g CO2/km; sidenote: the plug-in's consumption does not seem to be penalized by the fact that it has more horsepower). If the Plug-in gets its electricity from a low carbon intensitiy source, then the emission savings are substantial (33%, 100 vs 145 g CO2/km).

Sorta conclusion

For a proper analysis more cars would be needed (maybe will be added at a later date), but it would seem that driving plug-ins somewhere where the carbon intensity of the power network from which the car gets its charge is equal to the world's average electricity generation carbon intensity, then the plug-in offers no savings in terms of CO2 emissions compared to same/similar car that is not a plug-in. You might still save money though, if the electricity is cheap, but this is another topic, not touched here.

So if you care about CO2 emissions and are considering buying a plug-in, it perhaps pays to get to know in advance what is the electricity generation carbon footprint in the network where you will be getting your charge from.




1 To best of knowledge set to 130,000 g CO2 for each kWh of battery capacity (sources: https://www.transportenvironment.org/wp-content/uploads/2021/07/2019_11_Analysis_CO2_footprint_lithium-ion_batteries.pdf , Effects of battery manufacturing on electric vehicle life-cycle greenhouse gas emissions (theicct.org) , both accessed Feb 2023)

2 Source: Carbon intensity of electricity, 2000 to 2021 (ourworldindata.org) (accessed Feb 2023)

3 The average of the two: Carbon intensity of electricity, 2000 to 2021 (ourworldindata.org) and https://app.electricitymaps.com/map (past data for 5 years) (Both accessed Feb 2023)

4 Best shot from sources: Life Cycle Greenhouse Gas Emissions from Solar Photovoltaics (nrel.gov) , Re-assessment of net energy production and greenhouse gas emissions avoidance after 40 years of photovoltaics development (nature.com) (Both accessed Feb 2023)


Ivo Makuc, 2023
byguesswork@gmail.com