How many kilowatts for full charge?

Discussion in 'Prime Plug-in Charging' started by MollyHU, Oct 7, 2017.

  1. MollyHU

    MollyHU New Member

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    We have net metering/solar power at home. I am looking to purchase a Pruis Prime (because, alas, I cannot afford the Tesla) but really need to know what owners are finding it takes to do a full charge to see if the purchase makes sense for me. Most of my driving is around town < 25 miles/trip so I could potentially be using primarily Electric. I want to see if it would mean needing to get another solar panel up on the roof. I asked at the Toyota dealership and they were completely clueless and could not even understand my question.
     
  2. ChapmanF

    ChapmanF Senior Member

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    I'd like to think they didn't understand it because of the units you used: if you phrase a question "how many kilowatts for full charge", the only answer is something like "who knows? how fast you want to charge it?" I assume the question you wanted to ask was "how many kilowatt-hours for a full charge?" since that's the right unit for an energy question, instead of the power question you asked.

    That's what I'd like to think, but it's also possible they were completely clueless. :)

    -Chap
     
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  3. hill

    hill High Fiber Member

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    Welcome to PC ! !
    IIRC, the Prime's traction pack is ~8.8 kWh's. But it saves at least 2 kWh's when it drops to its bottom 25% of charge, where it enters charge sustain mode. So to cover 6kWh's (per day?) - of car charging - you'd need (for example) just ONE extra panel, producing just over 300 watts for 10 hours a day.
    The problem is - you don't get 10 hours of daylight because the sun's azmyth is seldom directly overhead. Plus, there's typically rainy weather to consider, as well as an overall lower sun azmyth during the entire winter season. Plus there maybe 5% loss conversion from solar DC to AC then back to your car's DC. And, you can't necessarily just add one solar panel because you have to have matching voltages & wattage outputs to properly sync with your existing panels' inverter(s) - presuming your existing solar array doesn't use micro inverters. Lastly, & from experience, good luck trying to find a solar installer that's willing to have their workers go over to an existing PV location to just add on one, or 2 or 3 extra panels. There's just no money in it.
    Now, do you see why your lowly Car Sales Critter couldn't give you a simple answer? They typically don't even know the ins & outs of the very cars they sell - much less understanding PV arrays.

    .
     
    #3 hill, Oct 7, 2017
    Last edited: Oct 7, 2017
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  4. MollyHU

    MollyHU New Member

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    Thank you so much for the reply. So the magic number is approximately 6kWh/ charge. That is what I needed. We are in VT way up north so we are acutely aware of the seasonal changes in the sun. I chuckled to hear you mention rainy weather - picture what happens up here in a good winter storm! We, fortunately, have a very progressive utility company so net metering makes it possible for us valiant souls willing to rake snow off ours roofs to succeed with solar energy. We do have micro inverters and have discussed with our installer from the outset the options of added additional panels - no problem. Ah, the advantages of a little tiny state.
     
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  5. bisco

    bisco cookie crumbler

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    welcome!

    big state with tiny people.:cool:
     
  6. MollyHU

    MollyHU New Member

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    I am appreciating I didn't use the correct term for the energy, kilowatt-hours, but it seems I am the first person who ever asked anything about the energy needed for a charge. They were clueless. I had to tell them about the rebates available from the local utility for purchasing plug in vehicles. It is a great perk for their customers.
     
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  7. bisco

    bisco cookie crumbler

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    yep, dealers = clueless = redundant.:p
     
  8. Prodigyplace

    Prodigyplace Senior Member

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    You sound like Tesla now. :LOL:
     
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  9. bisco

    bisco cookie crumbler

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    i wish.:oops:
     
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  10. RobH

    RobH Senior Member

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    There is a measure called solar insolation that gives the amount of full sun equivalent hours available each day. I didn't see Burlington, but Ithica, NY is rated at 4.57 hours max, 2.29 hour min, 3.79 average full sun equivalent hours per day.

    The solar insolation value does not include an adjustment for the panels not being at exactly 90 degrees to the sun. So unless you have the panels mounted on a tracker, the available light is going to be reduced by the mismatch between where the sun is and where the panels are pointed. These days, trackers are more expensive than extra panels, so the practice is to compensate for the poor aim by using more panels.

    Then there's the weather. Overcast doesn't zero out the electricity production, but it certainly takes a bite out of it.

    My WAG is that about 6000/3 or 2000 watts of solar panels to cover a daily 6 KWH usage. Anything less is just wishful thinking.

    Since you use micro inverters, you could just add single panels as necessary. But one 300 watt panel isn't going to make much of a dent. Maybe start with 4, expecting to go to as many as 8. Or you may have to stop when the roof is full...
     
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  11. Old Bear

    Old Bear Senior Member

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    I'm probably going to be plunging in over my head, but the way that I would approach the problem is to figure out the typical daily amount of power in kWh that you're selling into the grid. Then make some reasonable assumptions about the Prime in the absence of real measurements: the Prime battery is rated at about 8 kWh but never becomes fully discharged. So,although I understand that that means the Prime requires 6 kWh to fully recharge, your EVSE charger is not 100% efficient, so let's make a reasonable guess that it takes 8 kWh to fully recharge. If you achieve a nominal 30 miles per charge and use 8 kWh for a full recharge, that works out to 3.75 miles per kWh.

    Now, if your trips from home are such that you always return home to re-charge, then your total energy consumption each month in kWh will be the number of miles you drive divided by 3.75. You can compare that number with the number of kWh you sell into the grid and think of it as power you're selling to your car.

    Life gets a little more complicated if your net metering rate is not symmetrical and you're selling at wholesale and buying at retail. It that case, it will make a difference whether you're charging your car at night using retail electricity from the grid versus charging during the day and foregoing wholesale "revenue" from your local utility.

    It would be nice if you could just figure how many kWh you sell into the grid versus how many kWh your Prime uses, but because of all of the variables of day/night, sunny/cloudy/snowy, etc. it gets complicated.

    And if you want to know how much more pv you need to put on your roof without changing your net metering economics, you can be assured that you will have to increase your solar panels by at least the ratio of your (existing household use of PV + your Prime's charging needs) divided by (your existing household use of PV). That's the minimum which assumes all charging is done while the sun is shining. Or, if your Prime is not charging a that time, the excess solar power is being sold into the grid and will offset any power you use from the grid at other hours -- again, assuming symmetrical pricing.

    Finally, I welcome comments on this analysis because I am far from expert. I've tried to explain my reasoning so that those more knowledgeable than I can critique the analysis. I'd also welcome some real numbers telling measured kWh used by EVSE verses kWh delivered by EVSE and the efficiency of the Prime's LiON battery's charge/discharge cycle. As they say, "Your mileage may vary." ;)
     
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  12. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    I consistently pull 6.1 kwh from the wall for a "full" charge. I haven't looked at the numbers for how much of that goes into the battery. I have a 240 V Chargepoint Home charging station that produces the same reports for the home charging station that one gets at public Chargepoint stations.
     
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  13. MollyHU

    MollyHU New Member

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    So appreciative of all this input. Have been doing much figuring about our energy needs and will probably put up 4 more panels now while these precious solar 30% tax credits are still available. If we can manage to get 2/3 of our heating ( and cooling) needs met by our heat pump and 80% of my driving powered through EV all from our solar panels, I will be a very happy camper.
     
  14. JasonG

    JasonG Member

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    Just curious whether you can switch to a time-of-use metering and charge overnight. You'd need to check with your utility but here in San Francisco I was able to do time-of-use metering and still make enough power to cover my car charging at night (off-peak is 11 p.m. - 7 a.m.).

    Since the net metering is based on the rate at the time it is produced, I get "paid" the higher daytime rates when my solar is producing excess energy which pays for my charging at night. Over the course of the year, I expect to come out even. Currently, during my current net metering period I've built up $200 in credits from the summer that I will use during the winter months of lower production!

    Of course, we don't heat much here compared to Vermont (and use natural gas) but you should research your options before investing in additional panels IMHO.

    Update: Found this on Burlington electric web site for time of use metering:

    Customer Charge: $13.68

    Summer On-Peak*
    Initial Block (<= 100 kWh): $0.108068
    Tail Block (> 100 kWh): $0.225386

    Winter On-Peak*
    Initial Block (<= 100 kWh): $0.108068
    Tail Block (> 100 kWh): $0.228194

    Off-Peak*
    Initial Block (<= 100 kWh): $0.108068
    Tail Block (> 100 kWh): $0.108419

    Power Miser Credit $1.37
    Residential EEC $0.00905 per kWh
    City Franchise Fee 3.5%
    (exclusive of Initial Block of power)

    Summer On-Peak: June 1st through September 30th, Mon-Fri, 12:01 P.M. to 6:00 P.M.
    Winter On-Peak: December 1st through March 31st, Mon-Fri, 6:01 A.M. to 10:00 P.M.
    Off-Peak: All non-On-Peak times, plus the months of April, May, October, and November
    *Excludes New Year's, Memorial, Independence, Labor, Thanksgiving and Christmas Days.
     
    #14 JasonG, Oct 19, 2017
    Last edited: Oct 19, 2017
  15. Old Bear

    Old Bear Senior Member

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    Molly, I see you are in Vermont. I just came across a very interesting story published online in the Journal of Light Construction about a program in Vermont to install Tesla "PowerWall" home battery systems. The story, which includes several photographs and diagrams, shows an installation done under a program being run by Vermont's electric utility, Green Mountain Power (GMP).

    The article reports that GMP wants to put 500 of these units in customers' homes as an experiment to help balance the utility's loads. The idea is to smooth out demand from customers who generate solar power during the daylight hours and draw power after dark. GMP currently gets 15% of its power from solar and wind.

    See: http://www.jlconline.com/how-to/electrical/tesla-powerwall-not-just-for-solar_o

    And: GMP Launches New Comprehensive Energy Home Solution from Tesla to Lower Costs for Customers - Green Mountain Power

    So, you could use your solar panels all day to charge up your "PowerWall" so that when you come home at night, you could use the energy stored in the "PowerWall" to charge up your Prime.

    Ah, the wonders of technology!
     
    #15 Old Bear, Oct 22, 2017
    Last edited: Oct 22, 2017
  16. MollyHU

    MollyHU New Member

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  17. MollyHU

    MollyHU New Member

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    We are so lucky to have Green Mountain Power as our utility company -- one of the most progressive in the country. Yes, I am in the queue for an installation of the Tesla power Wall 2 which can store 13.5 Kwh. Not bad. If we ever have a power outage, it allows our solar panels to recharge to the power wall directly even with the grid down. With our net metering, there is just one rate so it doesn't matter if I charge day or night. Since 90% of my driving is under 31.5 miles roundtrip (that is what I am getting before winter sets in), I am running almost entirely on EV. I love that my car is primarily solar powered.
     
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  18. Green1

    Green1 New Member

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    MollyHU,

    I live near Buffalo, NY. We got our Prius Prime Plus in March, 2018. I hooked up a Killawatt meter to my charger. It takes from 6.5 to 7.2 kWhrs of power to get a full charge. This is when the battery is used up.
     
  19. jaqueh

    jaqueh Active Member

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    My Prime is now taking about 5.5kWh for a full charge as read from a public Chargepoint station.
     
  20. hill

    hill High Fiber Member

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    See? That's an advantage of traction battery capacity loss. Each subsequent year - even if your pv solar wasn't enough to cover your charging, eventually it WILL be enough.

    .
     
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