Energy Consumption: A Data Point

Discussion in 'Prime Fuel Economy & EV Range' started by Old Bear, Sep 4, 2018.

  1. Old Bear

    Old Bear Senior Member

    Joined:
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    Location:
    Boston, Massachusetts USA
    Vehicle:
    2017 Prius Prime
    Model:
    Plug-in Advanced
    For the first time since last Spring, we were able to run only in EV mode for the month of August.

    BearMobile-Stats-Aug-2018.jpg

    That works out to 105 kWh for 501 miles of of driving, or about 0.209 kWh per mile.

    From an environmental (and sheep herding) perspective, we were delighted to end the month ranked 47th among the Prius Prime universe. And, of course, with sheep, clouds, sun, rainbow, pinetrees, oranges and turlips.

    Interestingly, from an economic perspective, regular gas in our neighborhood is selling for $2.80/gal and our electric rate is about 21-cents per kWh.

    That works out to $22.05 for 501 miles of driving. $22.05 would have purchased 7.875 gallons of gas. The would yield an equivalent fuel economy of 63.6 mpg.

    During the couple of long trips which we made in May, June and July, calculated our gas-only mileage as about 56 mpg. So, 501 miles at $2.80 per gallon would have cost $25.06 -- which is not much different than our all-electric cost. On an annual basis, that would work out to a unimpressive fuel cost savings of about $36.

    Fortunately, as I've written elsewhere on PriusChat, our motivation for buying the Prime was the feel and appeal of electric driving, environmental considerations, and creature comfort -- not economics. And, of course, if you live in a part of the continent with lower electric rates, your mileage will indeed vary.
     
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  2. burnout8488

    burnout8488 Member

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    Interesting read. It's unfortunate your electric rate is so high, that really kills it. But good to know you can still enjoy the car for what it is on electricity!

    Does your electric company offer a night-rate option? Might be something to look into if you can alter your charging schedule. Then you should net a nice benefit.
     
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  3. Old Bear

    Old Bear Senior Member

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    Vehicle:
    2017 Prius Prime
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    Plug-in Advanced
    Unfortunately, although our utility offers a Variable Peak Pricing option, I am not aware of anyone using it. The economics are not particularly compelling as explained in this post about Time-of-Use-Rates in one of the Tesla online discussion groups:

    There is no charge to install a meter. The Eversource charge for a normal residential service is $6.43/month. The charge for a residential TOU rate is $9.99/month. That is what I have, a separate electrical service and meter for my car charging, but in my case I had to have it because my parking spot is not near my condo and its electrical supply.

    If you are able to do all charging in off-peak periods, then you can potentially save money compared to normal residential rates. First, you'd need to drive enough and charge enough to offset the cost of the monthly service. Right now the standard residential rate (excluding energy) is $0.10774 per kWh. The monthly TOU fee of $9.99 is equal to the cost of 92.8 kWh at the normal rate. So you need to buy more energy than that just to break even on the monthly fee for a separate service on TOU rates. That is not very much driving.

    Assuming you meet that test, the next question is whether the pattern of use will be more economical on TOUR. In winter, the peak rate is nearly 3 times the off-peak rate, so you need to use at least 3 times as much power in off-peak periods as in peak periods, to break even. In summer, it is worse: the summer peak rate is more than 4 times the off-peak rate, and the peak rate starts earlier in the evening, so you need to keep the peak period usage at less than 1/4 of the off-peak usage, to break even.

    I estimated that I could save an average of $10/month on TOUR. Someone who drives more could save more. For me, the challenge will be to avoid pre-heating or pre-cooling the car in peak periods. It would not take too much of that to eat a big portion of my savings. In that sense, winter is more challenging, because the peak period starts at 8 AM. Since I am retired, I rarely leave the house before 8. So if I need to leave the house at, say, 9 AM, and want to preheat the car on a cold winter day, I will be using energy at peak rates.
    . . .
    The Time-of-Use rate is complex where you pay about 23.1 cents/kWh for 4 months June-Sept, 9am-6pm weekdays and only 4.9 cents overnight, weekends, and 12 holidays. During the other 8 months, October-May you pay about 15.4 cents, 8am-9pm weekday and only 3.2 cents overnight, weekends, and holidays. This compares to my standard rate of only 9.5 cents. The TOU plan also has a $3.50/mo extra account fee. While they give you a very attractive off-peak rate, the on-peak rate is dreadful.

    As you can see, the rate structure is particularly difficult if you need to use air-conditioning during the day in the summer.

    Our bill provides separate charges for "generation" and "distribution" and it is possible to choose an alternate generation supplier. The state's Department of Public Utilities provides a very useful list of available suppliers, their rates, and their percentage of renewable energy. Energy Switch Massachusetts | The Massachusetts Department Of Public Utilities' Shopping for Electric Supply website

    In our area, using the distribution utility's default provider, our rate looks like this:

    Electric-Rate.jpg

    There is also a fixed connection charge of about $6.43 per month (as noted in the Tesla post above) regardless of the amount of electricity used.
     
  4. Rix

    Rix New Member

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    Madison, Wi
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    2019 Prius Prime
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    Old Bear,

    I’m interested in the environmental advantages to driving with electric power. For your Prius Prime and using your numbers, and converting kwh and gallons of gasoline to Joules, I get 750 kJ/mi in EV mode, and 2330 kJ/mi in hybrid mode. If I assume that 15% of your electricity is from solar and/or wind and/or nuclear, then 85% x 750 kJ = 637 kJ/mi in EV mode.

    Here’s my figuring:

    Prius Prime in EV mode:

    (105kwh/501 mi) x (3.6MJ/kwh) = 750 kJ/mi

    Prius Prime in hybrid mode:

    (56mi/gallon) = .018 gal/mi

    (.018 gal/mi) x (36kwh/gal) = .648 kwh/mi

    (.648kwh/mi) x (3.6 MJ/kwh) = 2.33 MJ/mi = 2330 kJ/mi
     
  5. Rix

    Rix New Member

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    N/A
    Old Bear,

    I failed to include the loss of energy at the plant, and over transmission lines to the Prius Prime owner’s car. Let me try it again. Let’s use your numbers again.

    Your car used 105kwh/501mi, in EV mode. Your car got 56mpg in hybrid mode.

    The Joule equivalent of one kwh is 3.6MJ.

    The energy content of gasoline is 36 kwh/gallon.

    Let’s say 15% of electricity coming from the power plant is solar and/or wind and/or nuclear. Let’s disregard transmission loss (about 10%) for these, since we don’t know the exact percent of non-CO2 energy anyway.

    Prius Prime in EV mode:

    .85 x 105kwh = 89.25 kwh

    Let’s say there is a 35% efficiency of burning fossil fuels (to turn a turbine) in a power plant, and 90% efficiency over transmission lines. (Let’s say natural gas has about the same addition of greenhouse gases as do coal and oil, since if 2% of natural gas [methane] is leaked in mining, etc, then it’s about the same as coal or oil.)

    .35 x .9 = .315 or 31.5% of the fossil fuel burned gets electricity to the customer. 15% is from a non-CO2 producing source; the rest is lost as heat.

    (.315/89.25 kwh) = (1/x) x = 283.3 kwh [This is the electricity generated at plant from fossil fuels that gets to the customer’s house and recharges his/her car.]

    (283.3 kwh/501 mi) x (3.6 MJ/kwh) = 2.035 MJ/mi or 2035 kJ/mi

    Prius Prime in hybrid mode (hereafter called "Prius"):

    (56mi/gallon) = .018 gal/mi

    (.018 gal/mi) x (36kwh/gal) = .648 kwh/mi

    (.648kwh/mi) x (3.6 MJ/kwh) = 2.33 MJ/mi = 2330 kJ/mi

    Here’s judging the relative environmental merit of driving a Prius Prime over a Prius:

    2035 kJ/mi – 2330 kJ/mi = 295kJ/mi

    295/2330 = 12.6%

    So, the Prius Prime in EV mode is about 12.6% less damaging, in terms of greenhouse gases, than is the Prius.
     
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  6. ctscoob

    ctscoob Junior Member

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    These cool charts you posted don’t seem to be available with the 2020 Prius Prime. My entune has barely anything available for features.
     
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