Anyone thought of adding a Electric vehicle charging station to their existing off grid system?

blacknbartblacknbart Registered Users Posts: 20 ✭✭
Building an Off-grid 8-10kW (9,920 watts currently) system and just wanted to figure out what I might need to add on to fulfill charging an electric car here in the future. I attached my current off-grid solar layout for your review.   I was reading I may need 2500 watts or maybe 10KWH's extra to charge and car a day. I figure maybe adding extra panels, a Charge Control, and extra Batteries to capture and deliver for our daily needs.  Thoughts on the process? Shooting from the hip.


  • mike95490mike95490 Solar Expert Posts: 9,363 ✭✭✭✭✭
    Really pointless to add batteries to charge a car after sun hours.  Add another 5Kw of PV panels & charge controller and use your existing setup, or charge in town.  The cost and wear/tear on a battery bank Pb or Li, is cost prohibitive I believe.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • blacknbartblacknbart Registered Users Posts: 20 ✭✭
    Thanks @mike95490 I am enligtened! Thanks. 

  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin

    What you really need to do is define the power you need to harvest from your solar power system... Then do (at least) a back of the envelope calculation to figure out what it would take to support those needs.

    For example, assume 25% of an 85 kWH Tesla battery bank charged every (sunny) day. Having a battery bank at home certainly allows you to charge at night and drive during the day--As most people probably use their cars. Charging options:

    Lets pick the smallest which is 120 VAC @ 20 Amps--Keep the solar system small... There may be options for charging a Tesla from solar panels directly--But I know nothing about them--And I wanted to support charging overnight.
    • 85,000 WH * 0.25 charge = 21,250 WH per day (1/4 charge)
    • 120 Volt * 20 amps charging current = 2,400 Watt AC charging power
    • Rough location Fresno California (hours of sun per day, facing south)
    Assume Flooded Cell Lead Acid battery bank--Certainly can choose something else (AGM, Li Ion, Tesla power wall, etc.)... But to start the math:
    • 21,250 WH per day * 1/0.85 AC inverter eff * 1/48 volt battery bank * 2 days storage * 1/0.50 max planned discharge = 2,083 AH @ 48 Volt battery bank (nominal off grid starting point)
    To charge such a bank, suggest 5% to 13% rate of charge, with 10%+ for full time off grid (daily) use:
    • 2,083 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 7,980 Watt array minimum
    • 2,083 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 15,961 Watt array minimum
    • 2,083 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 20,749 Watt array "typical" cost effective maximum
    And sizing the array for your location/hours of sun per day...

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 52° angle from vertical:
    (For best year-round performance)


    Pick December as "break even" day (may need genset, or fewer electric miles per day during bad weather:
    • 21,250 WH per day * 1/0.52 off grid system eff * 1/3.72 hours of sun per day = 10,985 Watt array "December break even"
    Somewhere between 10,985 and 15,961 Watt array at a minimum, and upwards of 20,749 Watt array are all "justifiable".

    A 2,083 AH @ 48 VDC battery bank would support upwards of a 10-20 kWatt AC inverter very nicely--Perhaps look at 240 VAC charging (faster charging available).

    A very quick back of the envelope calculation--Lots of details need to be worked out (including daily power needs, battery size/chemistry, etc.)... 

    At least it is a start and framework to start working out your needs & costs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • blacknbartblacknbart Registered Users Posts: 20 ✭✭
    @BB. Thanks for the breakdown. I'll step up my research.  
  • SandroadSandroad Registered Users Posts: 2

    I use an off-grid solar system to charge an Audi Q5 plug-in hybrid. All of my local miles are run as an EV; the gas engine runs only on longer trips. This is the third plug-in hybrid vehicle I’ve used on my system, the previous being a Ford and a Honda. 

    The Audi uses an average of 0.44 KWh per mile in EV mode. Other vehicles, other ambient temperatures, other driving styles, etc. will use more or less than that. I run about 12 miles locally each day, so my daily electrical use is a bit more than 5 KWh. The Audi has a 14.1 KWh battery, so I’m using slightly more than a third of the capacity each day.

    My solar system consists of 3000 kW of panels running power into a Magnum Energy PT-100 controller, then into 8 GC2 AGM 224A batteries, and then into a Magnum Energy MS-PAE 4024 inverter. The EVSE that sends power to the car is set for [email protected] 

    My solar panels have a serious shading issue, since I have chosen to not cut trees for full sun. I also live in an area east of Lake Michigan, which has abundant sun during the spring and summer and much less sun in the fall and winter. As a result, my solar system fully supplies the Audi with power for local travel during the spring and summer and the utility supplies most (but not all) power during the fall and winter. If my panels were in full sun all year, this balance would be very different. 

    I really like having the sun charge the storage batteries during the day so I can charge the car overnight when I’m not running around town. It take approximately 4 hours to charge the Audi with the parameters above. As you can imagine, this stresses the battery bank and I’m now due for new ones after just 3 years of use. This time around, I’m going with LiFePO4 and making some changes in the electronics for better battery management. 

    All in all, I’m a huge advocate of using either on-grid or off-grid solar power for an EV charging station. With just a little practical management of the power, it’s doable and fun.  

  • solar_davesolar_dave Solar Expert Posts: 2,382 ✭✭✭✭
    edited November 2020 #7
    here is a guy that charges his Chevy Volt off grid, he build a "bang-Bang" controller to only use excess capacity to charge his Volt.

    We use our grid-tie system to charge our 2 Volts for the last 9 years. Worked out great for us.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,034 ✭✭✭✭✭
    Too bad about the Volt. :'(    I have friend with one who comes up and gets a free charge here. Pretty simple to use the AUX for this. Offgrid folks have been doing that (pumping water) since Trace put an AUX port on their Sine inverters in the 1980's

    Still running the Leaf's?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

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