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How much solar to charge

Discussion in 'Model S' started by nursebee, Jun 30, 2015.

  1. nursebee

    nursebee Member

    May 21, 2013
    I need a 8kW system for my house, how much for charging a car over normal usage?
  2. MichFin

    MichFin Member

    May 8, 2015
    Detroit, MI
    8kw system should generate you about 40 KWH per day depending on where you live.

    You should need 1 KWH for 3 miles (depends on how you drive). So if you drive 30 miles a day you should use about 10KWH per day.

    But should not think that way if you're on the grid. Because you probably can get a separate meter for your EV and get off peak rates (about 1/2 the price). At those prices your solar is not cost effective. I have solar power that just about covers my home use and 2 EV's that I pay about $30/month each to drive with my reduced rate.
  3. MorrisonHiker

    MorrisonHiker S 90D 2018.26

    Mar 8, 2015
    One other consideration is that you may not be allowed to add as much solar capacity as you would like, even if you know your electrical usage is going to increase greatly.

    Currently, I live alone and have replaced all bulbs with LED bulbs and done everything possible to reduce my electric bill. Currently, my electric bill is very low, about $30 a month. There will be two additional residents moving into the house in the next few months. We plan on setting up a home business which will use a lot of electricity and will also be adding one to three electric cars in the next 5 years. This will easily cause my electrical usage to be three to four times my current usage. Unfortunately, in order to tie into the grid with XCEL's "Solar Rewards" program, I would only be allowed to add capacity equal to 120% of my current electrical usage. There is no allowance for future planned electrical usage (other than the 20% above current). While I would like to be proactive and install sufficient solar capacity to cover my future needs, I am not allowed to on a grid-tied system bigger than what I currently use. I believe I could install a non grid-tied system but any extra production would go to waste without a battery system. I don't want to have to go through the process and hassle of adding more panels every year.

    One option is to wait a few years and add solar after my average usage has greatly increased. Then I would be able to install many times as many panels as I am allowed now. Another option might be to be very wasteful for the next year, replacing all LED bulbs with halogens, etc. and leaving them on 24/7. This would cause my average usage to skyrocket and after a year, I would be able to install a sufficient number of panels. Of course this goes against the whole idea of being efficient and 'green' so I would never actually do that. A third option is the use of community solar. There may be community solar gardens in your area which would allow you to buy a share of a solar farm. Based upon how many panels you purchase, you would receive a percentage of the production from the solar farm, offsetting your electric bill. The Clean Energy Collective has set up many such community projects across the country. They seem to offer almost all of the benefits of solar installed on your home and almost none of the disadvantages. While they wouldn't help power your house during an outage, a battery backup such as the Powerwall could be used. Currently, the CEC solar farm in my county is sold out so I've put my name on the waiting list. Once panels become available or a new solar garden is built, I'll be able to purchase panels and instantly offset my electrical usage. As my usage increases, I'll be able to add more panels by just giving them my credit card number. No hassle about having an install done at the house every year.
  4. scottm

    scottm Active Member

    Jun 13, 2014
    #4 scottm, Jun 30, 2015
    Last edited: Jun 30, 2015
    this thread sh/could probably be moved to battery and charging

    Do you actually plan on being home during the day when charging the car? And only charge on sunny days?

    If so, going from solar -> 240VAC inverter -> car mobile or HPWC, the amount to generate depends on you having one or two on-board chargers in the car.
    One charger -> 10kW. So maybe 15kW of solar panels, by the time you add all the inefficiency drops, will yield 10kW at the car.
    Two chargers -> 20kW. Here you'd probably want 30kW of solar panel to get full use out of your dual chargers.

    Anyway, this will be the dominant load for the "whole house" while charging. Add your house needs to this.
    8kW, sounds right.. if you don't have air conditioning.

    So you're looking at probably 28kW minimum of panels if you want to be "off grid" for house plus car.
    But buy a bunch of Tesla powerwall batteries, so you can get through periods of clouds passing over the solar panels.

    Or, backfeed the grid ("trickle charge" the grid all day) and do with a lot less solar panel, and no need to store power locally.
    In that case, probably 15kW is fine over a period of a year to approach net-zero billing. But you can add more panels later if not.

    What I'd like to see from Tesla is powerwall-to-car direct DC charging. Power up the powerwall battery all day, using whatever sources of power you've got excess of, then dump to car DC-to-DC.

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