Gas or Electric: Which one is cheaper for the Prius Prime!

Discussion in 'Prime Plug-in Charging' started by noonm, May 2, 2019.

  1. noonm

    noonm Member

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    The eternal plug-in question: Is it cheaper to run your Prius Prime on gas or electric?

    Its a pretty simple calculation, but much harder to determine for everyone's specific situation. To help people figure that out, I've turned to the tried-and-true engineering textbook solution: lots and lots of graphs!

    I've put together a series of graphs to help Primers determine if its cheaper to go gas or electric. Here's an example of how to read the graphs:
    [​IMG]
    • Step 1: On the horizontal axis (x-axis) find the price you pay for electricity
    • Step 2: Draw vertically up to the line that best represents your electric driving efficiency (in mi/kWh). This can be found on the Eco Diary screen (figure out either the worst or your average efficiency)
    • Step 3: Draw horizontally until you hit the vertical axis (y-axis). This represents your eGallon price (i.e. the 'per gallon'-equivalent price of your electricity rate)

    In the above example, the eGallon price of an electric rate of $0.15/kWh for an EV driving efficiency of 4mi/kWh is $2.03/gal. If my local gas prices are below $2.03/gal, then running my Prius on gas will be cheaper. If they are above, then electric/EV will be cheaper.

    One slight complication: These graphs will change based on the fuel economy (mpg) of the gas engine too. So I've created additional graphs for those who drive more or less efficient than the EPA-rating on gas.

    More efficient:
    [​IMG]

    Less efficient:

    [​IMG]

    And for Canadian readers, here is the same graph (but for the Prime's standard fuel economy) in metric units and CAD!
    [​IMG]

    For those who are interested in how I've created these graphs or the calculations themselves, I've attached the excel file I used. The method is based on the DOE's eGallon website (see here: eGallon | Department of Energy)

    tl;dr version: Unless your gas is super cheap and electric rate super expensive, its almost always cheaper to run your Prime in EV mode.
     

    Attached Files:

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  2. noonm

    noonm Member

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    While you can find out your specific situation based on the above graphs (or by using the excel file I created), you might be wondering: Are there any places where I should be running on gas instead of electric?

    It just so happens there are! But some caveats:
    • This is just a snapshot of one moment in time (Feb-April 2019).
    • This is based on the average state-wise prices for electricity and gas. If you pay much more or less than the state-wise average for either, the logic may flip on which is cheaper.
    • This is based on EPA-rated mpg (54mpg) and 4mi/kWh electric efficiency.

    [​IMG]
    The good news is that pretty much all of the U.S. (46 states + DC), its cheaper to run your Prime on electric than it is gas. The exceptions are states with generally high average electricity rates (e.g. $0.20+/kWh for Connecticut, Massachusetts and Rhode Island and $0.30+/kWh for Hawaii).

    If you are charging your Prime at a greater than $0.20/kWh rate, but can get gas for sub $3/gal it may be cheaper to run your Prime on gas or you just need to drive your Prime much more efficiently while in EV mode.

    Getting your EV efficiency up to 4.8mi/kWh will make electric charging cheaper for all states but Hawaii.

    You will need to hit 5.6mi/kWh or higher to make electric charging cheaper for Hawaii.
     
    #2 noonm, May 2, 2019
    Last edited: May 2, 2019
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  3. Salamander_King

    Salamander_King Senior Member

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    Great graphs! Thank you for posting those for us.

    The problem I have is that both mpg for HV and mi/kWh for EV changes day to day. When the rate (both gas and electricity) are near the break-even point, it is very hard to determine which is cheaper. In my case, at current $.20/kWh and $2.75/gal, one day I can get cheaper by EV when I don't use any heat and I get 5 miles/kWh, but next day using heat drops it to 3 miles/kWh and gas get cheaper.

    BTW, 65mpg is realistic with PRIME, but 20 mi/kWh??? That would give you ~120 miles of EV range. Reasonable top EV efficiency is 7mi/kWh, or ~42 miles EV range, IMHO.
     
  4. Tideland Prius

    Tideland Prius Moderator of the North
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    Thanks!

    Is the gas conversion correct? Gas hasn’t been under $1/litre in ages.

    $2/gal is 52.8¢/litre assuming 1:1 conversion.
     
  5. bisco

    bisco cookie crumbler

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    too complicated. i just drive as much electric as possible.
     
  6. jb in NE

    jb in NE Senior Member

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    If the electric cost more than running on gas, would you still do the same?
     
  7. john1701a

    john1701a Prius Guru

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    Some people place a high value on reduced emissions. In the case of my electricity provider, we have the option of choosing wind/solar sourced supply for an added cost.
     
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  8. bisco

    bisco cookie crumbler

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    my electric is 24 cents/kwh. i can find gas as low as 2.60 within 10 miles.

    i purchased my first prius in 2004 for geopolitical/environment reasons and haven't looked back.

    frankly, vehicle energy is a minuscule amount of our budget, so i don't give it a lot of thought
     
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  9. Zythryn

    Zythryn Senior Member

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    Don’t know about Bisco. but I sure would.(y)
     
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  10. Salamander_King

    Salamander_King Senior Member

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    Yap, same here. I pay about $16 more per month for renewable energy generated electricity.
     
  11. noonm

    noonm Member

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    Ah, good point. Here's the graph shifted to show the common gas price range for Canada:
    [​IMG]

    To read this graph in reverse, if gas cost CAD$1.30/L, your electric rate would have be ~CAD$0.25/kWh or greater for it to be cheaper to run your Prime on gas.

    I was going to make a similar to the U.S. map for Canada to show which provinces where it might be cheaper to run on gas, but I couldn't find any that it would apply! Put simply, if you're in Canada it will always be cheaper to run on electric.

    I think pretty much most (if not all) of Prius/Prius Plugin-Prime adopters (and especially those on this forum) probably did so for similar reason. The next comparison I want to do is which to compare which is better for Climate Change, running your Prime on gas or electric. I suspect that for environmental reasons, it would probably be better to run your Prime on gas (or pay for renewable energy sourcing for your electric) in the coal heavy-states of the Midwest. However, this type of comparison is a bit more difficult, so it will take me longer to put together than pricing.
     
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  12. jb in NE

    jb in NE Senior Member

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    They aren't running a separate power line to your house. You are getting the same power as everybody else - it's all off the same grid.
     
  13. Zythryn

    Zythryn Senior Member

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    The Renewable Energy Credits are finite. While you are correct, we all use the same grid infrastructure, the RECs are accounted for.

    UCS has done a number of papers on this. Here is the most recent.
    New Data Show Electric Vehicles Continue to Get Cleaner - Union of Concerned Scientists
     
    #13 Zythryn, May 4, 2019
    Last edited: May 4, 2019
  14. noonm

    noonm Member

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    True, but since your incentivizing more renewables by paying a higher rate, I would consider the electricity you get would be from those sources (i.e. renewable). Its a bit indirect, but it does serve the purpose of getting more renewables on the grid.

    I've seen those two, but most of them are reported as in "X vehicle is better than Y". I already have my Prius Prime and I want to know "which areas should I drive on gas vs electric" which I haven't seen done very well. Especially when my EV mi/kWh and HV mpg can fluctuate based on drive or time of year.
     
    #14 noonm, May 4, 2019
    Last edited: May 4, 2019
  15. jb in NE

    jb in NE Senior Member

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    If renewables cost more than existing baseload production (and they appear to), then why aren't the costs incorporated in the utility rate structure?

    Would you pay 5 cents more per gallon for gas that includes ethanol, since that is also renewable?
     
  16. noonm

    noonm Member

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    Oh, they should and some state grids do that (see the ones in the PNW). And honestly, if the fuel actually reduced environmental impacts, particularly climate change, I would pay more at the pump for it. However, although ethanol is renewable, I wouldn't consider it actually better for the environment in the U.S. (where its sourced from corn).
     
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  17. Zythryn

    Zythryn Senior Member

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    The renewables don't cost more.
    The extra payment is to incentivize building more renewables. In addition, in my area (Minnesota) there is a "fuel charge adjustment" line. This is an adjustment for the renewables, which often bring the net cost of the renewables closer to the same price, sometime even cheaper.

    Iowa is another great example:
    Between Iowa, Minnesota and Wisconsin, Iowa has the most renewables, Minnesota in the middle and Wisconsin the least.
    As for retail power pricing, Iowa's is the lowest, Minnesota in the middle, and Wisconsin the highest.
     
  18. jb in NE

    jb in NE Senior Member

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    If they don't cost more, then why are extra payments required to incentivize them? Wouldn't the utility just install them?
     
  19. Zythryn

    Zythryn Senior Member

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    The smart ones are :D
    The additional incentive helps them get built faster than they would otherwise.
     
  20. noonm

    noonm Member

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    I view it as a bit of a hedge since there is great variability in renewable costs based on location. Here's an EIA overview of the levelized cost of energy (LCOE) for electricity in 2015:
    [​IMG]
    There are much bigger ranges on the renewables (particularly the solar thermal, solar PV and offshore wind) than most conventional technologies. However, there are solar PV installations that are actually cheaper than coal.

    This graph also shows why onshore wind is rapidly expanding for purely economic reasons (e.g. cheaper than coal, comparable to natural gas)!
     
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