Energy Efficiency of Electric Cars vs Gas Cars

Q: What is the “fuel” efficiency of a modern electric car?

A: A typical energy usage number for an electric car is 25 kWh per 100 km (40 kWh per 100 miles) traveled.  At today’s electricity and gas costs that works out to the same cost per 100 km as purchasing 2.41 liters of gasoline.  2.41 liter/100km or 117 mpg (for us old folks).

Calculation: O.K. where do the numbers come from, and what are the calculations used?

  • 25 kW per 100 km.  measurements on my own plug-in Prius show a consumption of about 25 kWh per 100 km.  The Nissan EPA label lists the Leaf at  34 kWh per 100 miles, which equates to 21.1 kWh per 100 km (divide by 1.61 to go from 100 miles to 100 km)  This 21.1 value is typical of today’s production EV’s, and is a bit less than my measured Prius value of 25 kWh/100km. So the 25 kWh/100km number is a bit conservative, and it is likely a “city miles” value obtainable by most light-footed EV drivers.
  • $0.1225 per kWh from Saskatoon Light & Power – Reading from my electric bill, I am charged $0.1167 per kWh, and then charged 5% GST = bringing the cost up to $0.1225 per kWh after tax. To confuse this number a bit, you could add 2.5 cents per kWh ($0.1488 including GST) if you are purchasing “green” power from the grid, which is an option through both SaskPower and the City of Saskatoon, or you may value your energy at $0.0961/kWh ($0.1009 with GST), which is the rate the City of Saskatoon will pay for grid-tied solar or wind power.  For the purpose of this posting, $0.1225/kWh is used.
  • $3.06 per 100 km Multiplying the 25 kWh / 100 km by $0.1225/kWh yields the total electrical cost to travel 100 km.
  • $1.269 per liter for gasoline http://www.saskatoongasprices.com lists gas prices for most major providers in Saskatoon at $1.269/liter. Consumers can likely pay up to 10% less than that when considering various discount or membership programs available.
  • 2.41 liters for $3.06 – this is the amount of gasoline which could be purchased for the same amount of money spent on the electrical energy used to drive 100 km.  $3.06 / $1.269 per liter = 2.41 liters.
  • 2.41 liters per 100 km – based on dollar equivalence.  Compared to almost all gas vehicles this is fantastic economy. 5 liter/100km is considered very good for city driving in most gas-only vehicles.
  • 117 mpg – using the website: http://www.calculateme.com/cGasMileage/LitersPer100kmtoMPG-imperial.htm to convert 2.41 liter/100 km into mpg (imperial).

There are a number of contentious issues around EV’s including road taxes, range, charging stations, up-front costs and subsidies, energy production subsidies, environmental aspects, maintenance an other operating costs, etc. but these issues have been ignored for this article, and it tries to provide a comparison of the simple costs of operating a plug-in electric vehicle vs. a gas vehicle in Saskatchewan today. We look forward to any questions or comments, and please let us know if we have made any serious errors or omissions in this post.

8 Responses to Energy Efficiency of Electric Cars vs Gas Cars

Ken says:

How do the emissions from the generation of the electricity compare to the emissions of the gas? How many pounds of carbon per hundred kilometers?

    Brent says:

    Hi Ken, good questions!

    I thought about including that info in the original post, but wasn’t sure what to use for a normal gas car.
    On average, gasoline produces 2.3 kg CO2 per liter (based on 8.92 kg/US gal from the EPA page: http://www.epa.gov/cleanenergy/energy-resources/refs.html)

    So using 5 liter/100km, which is good mileage will produce 11.5 kg/100km
    SaskPower claims 0.62 kg/kWh (from SaskPower email – I’m trying to get a web reference for this value), so at 25 kWh/100km I’d be causing 15.5 kg/100km
    However, I generate 80% of my electricity from solar, and pay extra for the remaining 20% to be “Greenpower” – so in theory that should be CO2 free.

    I realize that everything – including the solar power – does have some CO2 footprint, but I do not know how to calculate the more complex lifetime CO2 number.

    It seems if you are using SaskPower’s electric grid, running an EV may cut your “fuel” cost in 1/2, but may actually increase your emissions by 34.7%. We are very interested in coupling solar electric arrays with EV charging stations as this seems to provide financial AND CO2 reduction benefits.

Michael says:

Hi Brent,
Thank you for going through the calculation. The coal-powered electric car discussion is an interesting one. I’d like to point out some sensitivities of this calculation and how it can easily produce different conclusions:

Let’s compare the Nissan LEAF to the Nissan VERSA (almost identical except for power train).

                              LEAF                     VERSA
EPA Combined Eff.        34 kWh/100 miles	   33 MPG (US)
converted to km          21 kWh/100km              7.13 L/100km
CO2 rate                 0.62 kg/kWh               2.3 kg/L
**CO2**                  13 kg/100km               16.4 kg/100km

So I’m showing that EVs in SK may actually save CO2 emissions, even with our coal power. The main differences from your calculation were that I used the more optimistic energy efficiency of the Nissan LEAF EPA rating, but I was fair in comparing it to the EPA rating of a very similar car, the VERSA, which had a combined efficiency of around 7 L/100km vs the 5 L/100km you used.

In addition, with SaskPower’s carbon capture program, we may see SaskPower’s emissions reduced further (the practicality of CCS is another discussion…), while gasoline will continue to output 2.3 kg/L. (Aside: I found an EPA note saying that it is fair to add an extra 25% to this for extraction and processing, bringing the VERSA up to 20.5 kg/100km. I’m not sure if SaskPower includes extraction for their coal?).

I’m sure another calculation could again show something else. Still, I’d like to add my voice to yours in saying that EVs have *huge* potential for cost savings, and in addition, GHG savings, even in Saskatchewan.

Sparing a slight chance of cellulosic ethanol working out, a trifecta of electricity in transportation, electricity for heating and cooling through ground source heat pumps, and generation of electricity from renewable sources, all done together while employing conservation, enables a real possibility for a sustainable energy system, as I see it.

All the best.

Randy says:

Brent,
I’ve a 2004 Prius and am considering purchasing a plug in package. Any recommendations?

Randy
Kelowna

    Brent Veitch says:

    Hi Randy,

    No, I don’t really have a recomendation on plug-in conversions for the prius. The one I’ve used (Enginer) has been full of bugs failures and problems. It has been very educational to install and use, and a lot of fun to drive, but certainly not quite ready for an “average user”.

    I think the Toyota stock plug-in Prius will be the way to go. Their engineering seems extremely sound. Unfortunately it will likely be 4 years before any of those are available on the used market, and the new price for all (plug-in) vehicles is so expensive.

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  • fast caluculation says:

    So if I do a quick calculation:

    If an electric car is going at 100km/h it consumes only 21kwh.

    So 21kw would be the average motor consumption.

    1hp is 746w so 21Kw is 28.15HP

    I think you’re underestimating.

      Brent says:

      Hi fast caluculation,

      Your math looks right, with a couple of notes:
      1) although the average consumption per 100km traveled is 21kWh, the average speed traveled is not 100km/h Lower speeds require less power (HP) to maintain them.

      Most of the values reported are either measured values or EPA figures. They are likely less than a “typical driver” who will be more heavy-footed than the test drivers, or the “green nuts” trying to squeeze the most km out of every kW.

      How many HP or kW should it take for an aerodynamic auto to sustain 100km/h for 1 hour?

      2) when talking HP – people are often talking about the power available at the crank shaft, or sometimes at the wheels. Whereas when talking kW, we have been talking about the power required from the battery. So the two values are a bit different, and the 21 kW needed to travel 100 km would equate to an even smaller average HP being delivered to the wheels (because of electrical and drive train losses.

      Thanks for your post!

      -Brent

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