Is There a Battery First then to Grid System?

I've seen lots of grid tied systems that use a battery bank as a backup to grid outages, but is there a system that will run the house primarily from the battery bank and then go to grid when extra can be produced?

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TC

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Is There a Battery First then to Grid System?

    In a way that is how the hybrid system does work, except that it will not pull from batteries when grid is available. This is sensible because grid power is cheaper than battery power.

    Which leads to the question: why would you want to do this?

    In theory it is possible (whole house on hybrid GTI output; export to grid only, no import from grid) by disconnecting the grid whenever the array is not sufficient to export and carry loads (including battery charging).
  • Trickle ChargeTrickle Charge Solar Expert Posts: 28
    Re: Is There a Battery First then to Grid System?
    In a way that is how the hybrid system does work, except that it will not pull from batteries when grid is available. This is sensible because grid power is cheaper than battery power.

    Which leads to the question: why would you want to do this?

    In theory it is possible (whole house on hybrid GTI output; export to grid only, no import from grid) by disconnecting the grid whenever the array is not sufficient to export and carry loads (including battery charging).

    I was asked, and I hadn't heard of that specific config. I suppose they like the idea of running the house on solar as much as possible and you get the impression that it's always grid power unless there's an outage with battery back up systems. I supplement my solar with a generator for 3-4 months of the year but the other 8 I'm capped by my consumption. He'd rather not run a generator by using grid power but then be 100% solar like me and still sell back to the utility.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Is There a Battery First then to Grid System?
    I was asked, and I hadn't heard of that specific config. I suppose they like the idea of running the house on solar as much as possible and you get the impression that it's always grid power unless there's an outage with battery back up systems. I supplement my solar with a generator for 3-4 months of the year but the other 8 I'm capped by my consumption. He'd rather not run a generator by using grid power but then be 100% solar like me and still sell back to the utility.

    Considering that all the power would have to come from the PV array to begin with ...

    During the short opportunity to harvest the solar you would need to pull the full day's worth of power for the household plus additional power to sell to the grid.

    Since the grid is available it can be used as a type of "battery" in itself; harvest during sunlight hours, all surplus in that time 'banked with' the utility, draw from that bank during no-sun times. This is pretty much standard procedure. The exact details will depend on the utility involved.

    The use of the grid as an opportunity load (which is essentially what is described in the OP) doesn't make much sense, especially not economically.
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Is There a Battery First then to Grid System?

    Makes sense ONLY if your area has lots of outages, where you would want to configure a giant UPS for your house. The hybrid inverters will keep a battery bank fully charged, and if there is surplus power, sell it to the grid. some call it load shave. About the same as conventional Grid tie, but you get to maintain and replace a battery bank.
    Usually a generator is a better bet for infrequent power outages, but if they are weekly.....
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  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Is There a Battery First then to Grid System?
    I've seen lots of grid tied systems that use a battery bank as a backup to grid outages, but is there a system that will run the house primarily from the battery bank and then go to grid when extra can be produced?

    Yes, but the only reason to do this that makes sense (IMO) is on an installation that does NOT allow grid tie. Many better inverters will run in a mode called "LBX" (low battery transfer or something similar) that will allow you to run on battery and not use the grid unless the batteries fall to too low of a level. Then the grid charges the batteries (they never send it back to the grid.) This is much harder on the batteries than standby usage but allows you to use more solar energy than you would otherwise.

    However, you can get a hybrid system that uses batteries that will keep the batteries floating almost all the time, and will transfer any extra energy back to the grid. The Outback GTFX line of inverters will do this, for example. If your utility allows net metering this is, by far, the best way to go.
  • RCinFLARCinFLA Solar Expert Posts: 1,469 ✭✭✭✭
    Re: Is There a Battery First then to Grid System?

    If you have grid, you will always lose doing what you propose. The wear and tear on battery and associated periodic battery replacement cost will more then out weigh any grid usage saving.

    My system is primary used as UPS. It cost me about 0.5 kWH's per day just for float charge (without PV assist) which is minor in comparison to periodic battery replacement cost. In Florida, I average about one grid flicker (<100 millisec) dropout per day, primarily due to thunderstorms hitting grid somewhere. Just enough to reset computers, network connection, and DVR's.

    Even if you only sell when batteries are fully charged at float level you still stress the batteries with ripple current selling to the grid.
  • Trickle ChargeTrickle Charge Solar Expert Posts: 28
    Re: Is There a Battery First then to Grid System?

    Thanks for the helpful input everyone.
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