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

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

  1. Salamander_King

    Salamander_King Senior Member

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    Hypothetically, if I have access to a readily available public charger at a convenient place, I may go for it even at a higher cost than a gas station, but only if I can fully charge the battery in less than 5 min. If it is more convenient, then the higher cost is tolerable. We pay a higher price just for convenience for many other things. But the reality is that they are not available around where I live and the time it takes to charge is way too long. Other than at home or at work, I hardly ever spend 2.5 hours my car parked. Maybe if I am traveling and staying at a hotel where a public charger is available at parking, but that situation has not happened to me in the past two years.
     
    #41 Salamander_King, Jul 12, 2019
    Last edited: Jul 12, 2019
  2. t_newt

    t_newt Member

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    Well I'm just south of San Fran, and I have Time Of Use (TOU) rates, and pay as little as 16 cents a kilowatt hour in the off-hours in summer. That's equivalent to about $1.70 a gallon for gas for a car that gets 50 mpg. Since gas in this area costs over $3.60 a gallon (occasionally topping $4/gallon), that's quite a big saving over running the Prime on gasoline.

    Chargepoint doesn't determine the rate for its chargers--it is the owner of the Chargepoint station. Most are too expensive for a slow charging Prime. But my work has Chargepoint stations and they charge very little. There are a couple of free ones I can go to--one near a park where I can take long walks or sit and have lunch while charging, and another near movie theaters and a gym I frequent. Both are pretty well hidden so there's usually one open, though I've seen more and more Teslas show up, so I think the days of finding open free charging stations may soon be over.
     
  3. Salamander_King

    Salamander_King Senior Member

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    I think you miscalculated something or your assumption is different from mine. How did you get $1.70 a gallon of gas equivalent from the $0.16/kWh electricity?

    For charging PRIME to full, 6.1-6.6 kWh electricity is needed, depending on using L2 or L1, that would be $0.976-$1.056 at $0.16/kWh. If you assume EPA EV range of 25 miles and gas mileage is 50mpg, $0.976-$1.056 of full charged battery electricity used in EV mode is equivalent to half gallons of gas for the same distance to drive in HV mode. That would be equivalent to 2x($0.97-$1.056) of gas or $1.94/gal to $2.113/gal of gas. Yes, EV is still much cheaper than your current $3.60/gal gas for sure, but in a different region, you can find place gas is cheaper.

    I pay $0.195/kWh for electricity and our gas has been below $2.50/gal for a while, this makes HV cheaper than EV. I still plug-in, but to justify my deed, I am wishing the gas price go up above $3 like in your area.;)
     
  4. t_newt

    t_newt Member

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    My calculations:
    For my Prius Prime, it is about 6kWh for 28 miles (about what I get--yes it is a bit better than the EPA range). (6kWh/28 miles)*50= 10.7kWh for 50 miles.

    $0.16/kWh*10.7kWh= $1.71 for 50 miles.

    It is even cheaper if I charge at work ($0.12/kWh) or, of course, at one of the free chargers.
     
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  5. Salamander_King

    Salamander_King Senior Member

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    OK, I buy that. But to be fair, if you are using non-EPA EV range, then you should use your own MPG as well. I bet you are getting better than 50mpg. Mine during summer gets 65mpg easily on the gas-only operation calculated. MPG from the car tops 70mpg quite often. Of course, during summer I can also get an average ~35 miles EV range.
     
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  6. Red Ruby

    Red Ruby New Member

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    Love these plots - thanks! One question: Driving efficiency in mpg (ice) and driving efficiency on electric (mi / kWh) are both measuring the same thing - driving efficiency. So assuming one drives the same way regardless of power source, one could collapse these plots from a 3d contour plot to a simpler 2d plot, showing the line where the two energy sources are cost equivalent. I'm arguing that mpg efficiency and mi / kWh efficiency are not independent variables and thus this could / should be simplified?

    Working it out based on EPA numbers (we could work it out it for the categories of aggressive / normal / efficient driving):

    Prime = 54 mpg, 133 mpge. EPA says 1 gal of gas = 33.7 kWh. So the nominal EPA number for driving efficiency is 3.947 miles / kWh. Right around the 4 mi / kWh line in your plot.

    This implies that unless the price of gas / gal < 13.681 * the price / kWh, it's cheaper to drive electric.

    If the mpg and mi / kWh numbers changed the same way depending on driving style I think this equation would be the only one. Doesn't change any of your conclusions, and in fact your second post is pretty much this assumption mapped into nation-wide averages. So I guess I'm just suggesting that a plot with the single line might be useful in the first post. :)
     
  7. Matt1984

    Matt1984 New Member

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    These graphs are cool, but...

    Is anyone else in southern California wondering why they stop at 25c per kWh? The cheapest rate for SDG&E is currently 29c! :cry:
     
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