TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Help me understand a SolarCity proposal w/r/t Model S charging

Discussion in 'Model S: Battery & Charging' started by HankLloydRight, Apr 23, 2014.

  1. HankLloydRight

    HankLloydRight Fluxing

    Joined:
    Jan 18, 2014
    Messages:
    5,785
    Location:
    Connecticut
    #1 HankLloydRight, Apr 23, 2014
    Last edited: May 8, 2014


    NOTE: New update to my question in Post
    #25 below


    I got a quote today from SolarCity to install solar panels on my house. Sounds like a pretty good deal. Here are some of the specifics:

    Estimated System Size DC 4.17 kW
    System Size AC 3.64 kW
    Estimated Annual Production 4,340 kWh

    I have a P85+. My house currently has 100 amp service from the street, and I use approx 500kwh per month @ $0.16/KwH.

    So if I have the above system installed, and I do not have a HPWC installed but rather a NEMA 14-50 cable to charge my car, how many amps (or MPH) can I expect to get to charge the MS during daylight hours?

    I don't drive many miles per day, let's say rated 40 miles per day (at most on average), so I'm sure whatever it is, I'll have enough power to recharge for the day's usage, but I'm more curious if I return home with a fairly discharged battery, what would my optimal daytime charging rate be? Would I get the full 40 amps from a NEMA 14-50 from the solar array? Or would I be tapping the local utility at that point? I'm just not sure how to calculate the output of the solar array with respect to MS charging.

    thanks.
     
  2. swaltner

    swaltner Member

    Joined:
    Oct 13, 2012
    Messages:
    497
    Location:
    Kansas, USA
    40 Amps @ 240 Volts = 9,600 Watts = 9.6 kW

    So, if you're charging your Model S in the middle of the day, you're still pulling power off the grid. Assuming you are setup with Net Metering, where you're paying your electric company for the net usage of electricity that month, in reality, it doesn't matter. If your panel has a 100 Amp breaker, you may have difficulty having enough capacity to get a 50 amp outlet (required for charging at 40 amps continuous). It all depends upon how many other loads you already have in the house.
     
  3. HankLloydRight

    HankLloydRight Fluxing

    Joined:
    Jan 18, 2014
    Messages:
    5,785
    Location:
    Connecticut
    Thanks, that's puts it in perspective. Perhaps I'll do a NEMA 14-30 socket instead, providing 24 amps or 5.8kw, still gives me 17 mph charging, which for normal usage, is more than enough per daytime charge, and if need be, I can get a near-full charge in 12 hours and a full range charge in 16 hours (from empty).

    For some reason, I thought that the solar array would be additive to the existing 100 amp service, but I guess it doesn't work that way. :(
     
  4. dsm363

    dsm363 Roadster + Sig Model S

    Joined:
    May 17, 2009
    Messages:
    18,235
    Location:
    Las Vegas, NV
    I don't totally understand how all of this works so maybe someone who does can speak to this but maybe installing a 14-50 would still work. That way when you need faster charging you still have it.

    Are they not upgrading the service to your house to 200A or something more? Could charging during the day when AC and other things are on lead to issues?
     
  5. HankLloydRight

    HankLloydRight Fluxing

    Joined:
    Jan 18, 2014
    Messages:
    5,785
    Location:
    Connecticut
    I have not yet seen any work orders or anything that would indicate they need to upgrade to 200 amp service. My (small) house has a mini-split A/C system (I don't know how many amps this draws), an electric stove/oven (which I *NEVER* use), and a few computers and TV stuff and lights. That's it. So I think that trying to add a 50 amp service is stretching it a bit if for some reason, I would need to charge the MS at the same time I'm running the A/C at full blast AND cooking a meal and maybe doing some laundry (once I get my washing machine fixed, that is). That would definitely be more than a 100 amp draw, so I doubt that taking what I have now, and ADDING a 50-amp service would meet the local code. I could probably get away with a 30 amp circuit though, without needing to go through the expense of upgrading the house to a 200 amp circuit. But I'll have to ask the experts about this when the time comes.
     
  6. zwede

    zwede 2013 P85+

    Joined:
    Jan 17, 2014
    Messages:
    501
    Location:
    Plano, TX
    Code doesn't have anything to say about your main breaker in that way. It's up to you to think about what all stuff you turn on at the same time. Just schedule the car to start charging in the middle of the night and pull 40A, no problem.
     
  7. HankLloydRight

    HankLloydRight Fluxing

    Joined:
    Jan 18, 2014
    Messages:
    5,785
    Location:
    Connecticut
    On most days, my car won't be parked at my house overnight.. it will only be there during the day.
     
  8. tga

    tga Active Member

    Joined:
    Apr 8, 2014
    Messages:
    2,214
    Location:
    New Hampshire
    None the less, you want a 14-50 installed, if only for future flexibility. The cost difference will be minimal, unless you need lots of wire. For short runs, the bulk of the charge is labor.

    If you want to charge at a lower rate, fine, you can dial back the charge rate in the car. But don't force yourself to with a lower current outlet.
     
  9. Eberhard

    Eberhard #421 Model S #S32

    Joined:
    Oct 17, 2010
    Messages:
    1,141
    Location:
    Germany
    You can install a control unit at your meter with read in realtime the total consumtion of your house, including the production from the solar.array. Depending on the currently aviable capacitiy it can tell the Model S via the pilot signal, how much current it can draw. If you want, you can set the controll only to use the power from the solar.

    We called this PV-controlled charging.

    Best

    Eberhard
     
  10. drees

    drees Active Member

    Joined:
    Jun 23, 2009
    Messages:
    1,121
    Location:
    San Diego
    Code definitely does tell you many loads you can run off a 100A (or any other size) main breaker and you determine the minimum main breaker size by running a load calculation on the house. All spelled out in the NEC, though of course, your local jurisdiction may vary in how strictly or not it follows those guidelines.

    SolarCity appears to have sized the PV system as large as possible without having to upgrade the service. You are allowed to backfeed up to 20% if your panel's rating assuming your main breaker matches your panel rating which is usually the case. 20% of 100A = 20A. Like an EV charging circuit the maximum PV load is also derated by 20%, so you can have an inverter with a maximum output of 16A. At 240V that's 3840W. In this case, the inverter is rated at 3640W which would be 15A at 240V.
     
  11. RandyS

    RandyS Fan of Elon

    Joined:
    Jul 8, 2012
    Messages:
    360
    Location:
    San Diego
    #11 RandyS, Apr 24, 2014
    Last edited: Apr 24, 2014
    Your mileage may vary, but here is a sample of the load calcs from my house in San Diego right from the approved permit that show a 125 amp panel and an installation of a Blink 30 amp charging station (listed at 7200 VA in the load calculations). It's not about how much power your individual breakers may consume, but these standard NEC load calc formulas...I'm not sure how the solar system changes these calcs. Anyone else have that information?

    evseloadcalcs.jpg
     
  12. drees

    drees Active Member

    Joined:
    Jun 23, 2009
    Messages:
    1,121
    Location:
    San Diego
    Solar doesn't affect your load calcs since you can't assume it'll be on.
     
  13. tga

    tga Active Member

    Joined:
    Apr 8, 2014
    Messages:
    2,214
    Location:
    New Hampshire
    The "20% of panel rating" rule only applies to a backfed system, correct? I can't remember if it's 20% of the main breaker rating, or 20% of the bus bar rating (eg, I've seen panels where the main is 100A, but the bus bars are rated at 125A).

    With a supply side connection, I think this rule no longer applies (I've seen several PV systems with supply side connections where the system output is > 20% of the main breaker). If the OP is looking for a larger system, this may be an option.
     
  14. zwede

    zwede 2013 P85+

    Joined:
    Jan 17, 2014
    Messages:
    501
    Location:
    Plano, TX
    What I was trying to say (unsuccessfully), was that adding a new outlet without upgrading the main breaker is not against code. I'm aware that a load calc is required to determine the initial service/main breaker. But maybe I'm wrong about that? I haven't studied that part of NEC.
     
  15. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

    Joined:
    Jun 21, 2012
    Messages:
    7,019
    A load calc is typically required any time you want to add an additional branch circuit. Most home services typically have room for this and so most electricians don't worry when you tell them to add a 15A circuit. Adding anything bigger becomes a question.

    The 20% of panel rating rule applies to any situation where you are feeding two sides of a panel with a power source - downstream and upstream. This is because you can end up drawing more current across that panel: main breaker size + backfeed breaker size. If the panel is rated for 200A, and your main breaker is rated at 200A, you may only feed up to 40A through a downstream breaker.

    There is a solution if you need it, you can reduce your main breaker size or your feed breaker size so that you can't exceed more than 120% of the panel's rated capacity through circuit breakers. For example, if your panel is rated at 200A, you can reduce your main breaker size to 150A, and that would permit a backfeed of up to 90A. (120% of 200 = 240A; 150A main + 90A backfeed).

    - - - Updated - - -

    This is correct, it depends on how the system is fed.

    The OP's 100A service is likely enough for a load calc that assumed late-night charging, although it would probably depend upon the inspector's evaluation.

    Here's what my solar output looks like in Southern IL (near St. Louis), on a 9 kW system (30x300W panels, 8 kW inverter), yesterday (a very clear day):

    solar.png

    As you can see, during the peak hour (typically 1-2 pm), it's feeding ~30A to the grid. At different times, the current is different.
     
  16. zwede

    zwede 2013 P85+

    Joined:
    Jan 17, 2014
    Messages:
    501
    Location:
    Plano, TX
    Thanks. I feel a bit more educated now.
     
  17. Cottonwood

    Cottonwood Roadster#433, Model S#S37

    Joined:
    Feb 27, 2009
    Messages:
    5,062
    Location:
    Colorado
    It's 20% of the buss bar rating of the circuit breaker panel that is the limit. If you have a 100 Amp panel, you can feed it with a 100 Amp breaker from the Utility and a 20 Amp breaker from your grid tied solar system. The concern is having an over current situation in the buss bars on a nice sunny day with lots of loads running in your house. If you put the Utility feed at one end of the breaker panel and the solar feed at the other, you never get close to this because the currents never add, but 20% is the limit for the uninformed that but both utility and solar feed breakers at one end.

    You could replace your 100 Amp panel without upgrading your utility service and have room for tons of solar. For example, replace the 100 Amp panel with a 225 Amp panel and keep the utility feed at 100 Amps through a 100 Amp breaker. Going all the way to 225 Amps would give you lots of margin for solar when you do upgrade the utility feed to 200 Amps. 120%*225-200=70; 70 Amps of solar is 16.8 kW AC or about 18.7 kW DC!
     
  18. tliving

    tliving Member

    Joined:
    Mar 8, 2014
    Messages:
    636
    Location:
    New England, USA
    I'm a new model s owner and also looking into solar city. Here's my analysis so far:

    http://teslaliving.wordpress.com/2014/04/19/solar-gains/

    Right now waiting on a second engineering site visit.
     
  19. Eberhard

    Eberhard #421 Model S #S32

    Joined:
    Oct 17, 2010
    Messages:
    1,141
    Location:
    Germany
    I am really shocked, how much you have to pay for a solar array. In Germany, your price is about €1.200,-- per kWp +VAT (which you get back) or your 24kWp solar array would cost in Germny
    €28.000,-- including installation ready to run. At current exchange rate for the Dollar is about $40.000,-- You really get heavily overcharged.

    best Eberhard
     
  20. tga

    tga Active Member

    Joined:
    Apr 8, 2014
    Messages:
    2,214
    Location:
    New Hampshire
    In your blog, you mention $144K for a 24kW DC system. That work out to $6/W installed, which is way more than current "real prices". I've seen systems installed recently for $3.25-$3.50/W, installed price. After the Fed tax rebates, that drops to $2.28-$2.80/W.

    If you bought a 24kW system outright, my rough estimate of the numbers look like:

    Installation fee: 24kW @ 3.50/W$84,000
    MA income tax credit (15% of inst price, capped at $1000)($1,000)
    Federal income tax credit (30% of installed price)($25,200)
    Net price of system$57,800
    Depending on site orientation, shading, roof pitch, etc., that system could produce >26mWh/year, which equates to 26 SREC's (Solar Renewable Energy Credits). I don't know what SREC-2 is going to look like, but SREC-1's have traded around $225-$250 in 2013. So that means a 24kWh system could generate over $6,000 in SREC income each year, for 10 years, which pays off the after tax price of the system.

    There are financing programs structured to use the Fed tax refund and SREC income to pay off the system (Sunpower offers a program through their dealers for systems using Sunpower panels). After 10 years, you own your solar system outright, and pay nothing for electricity. With a SolarCity PPA, you're still paying SolarCity.

    I haven't done an in depth model to compare outright purchase vs a lease or PPA, but my gut feel is you come out much better in MA if you purchase because of the SREC income.

    PM me if you want more details on my system and installation experience. I'm happy to let you come look at it, show you the numbers for my system, and refer you to my installer. I'll even let you have a free charge off my 14-50 (but I need a ride in return).
     

Share This Page