off grid charging station for Chevy Volt

stefuel Registered Users Posts: 5
I have dabbled a bit in solar hydronic systems but never electric. I have been considering a DIY off grid solar electric system for my house to the point that I even designed and tested (on a small scale) a solar tracking system to get the most out of the system. I kinda shelved the idea when the battery expense was factored in.
Everything that goes on the shelf stays there because ya just never know.
So now I have this smaller scale tracking system and the desire for a Chevy Volt.
In doing some reading I have found that the Volt can go from depleated to full charge with 120 volt connection in about 11 hours and full charge in about 4 hours with a 240 volt supply (20 amps). This car will be for my wife who would only use about 30% of the Volts battery charge daily (at the most) so she would get three days off of a full charge. I would like a system that can support 8 hours of charging. I think I would like it to be able to recharge it's self completely in that three days that it would not be used. I'm looking for panel, battery and inverter/charger suggestions. Thanks,



  • BB.
    BB. Super Moderators, Administrators Posts: 33,474 admin
    Re: off grid charging station for Chevy Volt

    Welcome to the Forum Chip,

    I will make a lot of guesses below--But, roughly, this is how such a system would be designed (feel free to plug in your own numbers).

    Ok, first, figure out how much energy to recharge the Volt's battery bank:
    • 4 hours * 240 VAC * 20 amps * 1/1.25 NEC derating = 15,360 Watt*Hours
    • 11 hours * 120 VAC * 15 amps * 1/1.25 NEC derating = 15,840 Watt*Hours

    Assuming you want to recharge 30% per day (I would suggest using a timer to prevent over discharging your solar power system and taking the battery bank "dead"--which can damage/ruin the solar PV system battery bank).

    For battery bank design, a couple rules of thumb... First 1-3 days of "no sun" autonomy and 50% maximum discharge:
    • 15,840 Watt*Hours * 0.30 Volt charging * 1/0.85 AC inverter eff * 1/48 volt battery bank * 2 days autonomy * 1/0.50 max discharge = 466 AH @ 48 volt battery bank

    Also, need to double check the rate of discharge... Recommend a maximum discharge rate of C/8 hours:
    • 120 VAC * 15 amps * 1/1.25 NEC derating * 1/0.85 inverter eff * 1/48 volt bank * 8 hour discharge rate = 282 AH @ 48 volt minimum battery bank

    So--For a 30% per day @ 120 VAC charge for the volt, you would need a 466 AH @ 48 volt minimum battery bank.

    Next, I need to know how much sun you have--New York vs Las Vegas will affect the size of the solar array. Lets assume you live in a typical North American location with a minimum of 4 hours of "noon time" sun per day for at least 9 months of the year (for winter, I assume you will use more gasoline for the car--or use a genset). The size of array for your load would be:
    • 15,840 WattsHours * 0.30 volt charge * 1/0.85 inverter eff * 1/0.77 panel+solar charger derating * 1/4 hours of sun = 1,815 Watt minimum solar array

    Also, we need to size the minimum array for your battery bank for a range of 5% to 13% rate of charge (another rule of thumb):
    • 466 AH * 59 volts battery charging * 1/0.77 panel+charger derating * 0.05 rate of charge = 1,785 watt minimum array
    • 466 AH * 59 volts battery charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 3,571 watt "nominal" array
    • 466 AH * 59 volts battery charging * 1/0.77 panel+charger derating * 0.13 rate of charge = 4,642 watt maximum "cost effective" array

    So, your array should be around 1,815 to 4,642 watts based on my above guesses.

    In theory, you should be looking at a minimum of 2kW AC inverter... But for a 466 AH @ 48 volt battery bank, the maximum size inverter you could install would be:
    • 466 AH * 48 volt * 1/2.5 maximum surge * 0.50 inverter continuous power derating = 4,474 watt maximum inverter recommended

    If you have a volt, you can use a Kill-a-Watt meter (or similar) to measure it charging from 120 VAC circuit.

    And, if your home is/will be off grid too--Then you should add your other loads and size the system to support those too (I would suggest if you are pretty careful about conservation, that you could look at 3,300 WattHours per day). Note that power use is highly personal--So conservation (picking efficient loads, turning stuff off when needed, using propane/other fuels for "heating", use natural lighting/heating when possible, etc.) is your first step (it is cheaper to conserve power than to generate it).

    As a rough guess (of $10 to $20 per watt of installed power), you are looking at around ~$20,000 to $50,000 (self install vs installer, smaller vs larger system, etc.)...

    Also, I have carried out calculations to many places of accuracy so you can repeat my calculations, I don't get rounding errors. and you can follow the numbers through the multiple equations--In general, if your numbers are within 10% or so of mine--that is close enough for solar work.

    And, solar is a variable resource... If you live near the coast, and/or have a "monsoon" season, dark winter months, etc.--that will all significantly impact seasonal power production. And there is a ~10-20% variation in year over year average power production. You will have to adjust your power usage (or backup genset usage) based on your needs (over discharging an off-grid battery bank can knock years off the life of a battery bank, or even destroy the bank in a few days).

    The above system assumes you will be charging the car at night--if you charge during the day (when the sun is up), it is possible to reduce the size of the battery bank. Also, if you do have grid/utility power available, and the utility supports Grid Tied connected solar systems with a good Net Metering plan, it will be much less expensive to use Grid Tied PV solar vs Off Grid PV solar (off grid power systems cost, very roughly, on the order of $1 to $2+ per kWH vs 1/10th that for utility power).

    Before we go any deeper, questions/corrections/etc.?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: off grid charging station for Chevy Volt

    your idea has a major flaw in it being the sun doesn't give its max power over an 8hr period. it would be closer to half that and that's during the longer warmer months as winter is another problem. if you have enough pv to create the 240vac 40a (9600w) (thanks wayne for pointing out my typo) the volt would want, it is my opinion that this is the better route and this would be better if in a grid tied system so as to alleviate time restrictions on the car's usage. it would be about 30% higher in pv stc ratings on pvs so around 1250w stc in pvs minimally. this is due to the 77% efficiency average most pvs would realize.

    of course this beckons the answers to the questions of if you are grid tied and if you have enough pv on that tracker to cover the power required? this is, of course, not in need of batteries being i am citing the use of a gt inverter, but you weren't specific as to what equipment you have picked up.
  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    Re: off grid charging station for Chevy Volt

    A couple of issues with the offgrid aspects, the included Volt charger is 120V and can be set to either draw 8 amps(13-15hrs) or 12 amps(10-11 hrs). The 240 volt SPX charger draws about 13.5+ amps for a 4 hour charge. The power does ramp up/down smoothly so no starting surge. In either case the chargers expect those kinds of draws to be consistently available. Also keeping the Volt plugged in between charges will help with remote start preconditioning the cabin temps and with the lithium battery thermal management system (TMS). The TMS runs I have seen are about 800-900 watts for a few minutes every few hours in the summer heat to maintain the optimal battery temps. Generally a full charge for 10.4 kWh of car battery takes 13.3 kWh input power.

    There are a couple of guys @ that charge off grid but there whole operation is off grid and one DCFusor has some custom electronics that use the late day excess from his solar system to charge the car. When is battery system has been in float for a period of time he then engages the car chargers to use the solar energy that would normally just fall on the floor. They have large DC lead acid battery banks for buffers and dedicated true sine wave inverters to operate the chargers.

    If you have grid, you are better off to grid tie your panels and just run the watts back into the grid to be used at your convenience. The losses are much less than charging a lead acid battery to buffer your charging of the cars lithium battery. In effect the grid tie is a 100% efficient battery with none of the lead acid battery bank headaches, costs or losses. You get all your watts accounted for. I don't know your location but net metering and TOU plans can actually save you lots of money. Others here are in California and they can back feed in the day time for a high dollar amount and draw back at night for a much cheaper price per kWh. By load shifting to off peak hours they actually get more kWh back than they put in for a given $$$$.

    The light blue line is my Volt chargers (2), you can see a SPX 240 V charge cycle and a couple of TMS runs earlier in the morning from both the Volts. That is for a partial charge for about 17 miles of range we consumed this AM.