[RE-wrenches] Grid-Connect Inverter with battery, AND auto backoff?

Wed Jun 24 09:55:20 PDT 2009

Troy,

Unless you are going to be your client's power plant manager, he or she is going to have to be responsible for at least system operation which means managing production and consumption - or hire someone to manage the system. PV systems are automatic and virtually maintenance-free to some extent. A well-designed, properly installed batteryless grid-tie residential PV system's operation is transparent to the home occupants and requires very little attention. However, a battery-based system regularly requires operator intervention especially in a region with frequent brown-outs and black-outs. There are several Wrenches who live with battery-based grid-tied PV systems. Do any of you power your whole house through the main service panel (no subpanel)? If yes, what size PV array, inverter, and battery bank? Any battery-based, grid-tied PV systems out there in continuous operation since before June 1998?

Joel Davidson
  ----- Original Message ----- 
  From: R. Walters 
  To: RE-wrenches 
  Sent: Tuesday, June 23, 2009 10:29 PM
  Subject: Re: [RE-wrenches] Grid-Connect Inverter with battery,AND auto backoff?

  We've looked at a variety of approaches over the years. My current thinking, is that you want some non critical loads to shut off during an outage, so the customer knows they are on backup power. With whole house backup, they never know till the batteries are gone too. I've found that with creative picking of circuits, the customer can be signaled that the grid is out, without creating any panic or hardship. Most houses have so many circuits that its just not a big deal to have the washing machine not work for a few hours. 
  I'd back off to a proven design, otherwise your going to spend a lot of time with them making this work. Manually switchable double throw breakers are available and work well, but yes, you do have to exert some slight mental and physical effort to operate them. Automatic anything means the installer is always to blame. Good system design puts some responsibility on the operator as well.

  R. Walters
  Solarray.com
  NABCEP # 04170442 

  On Jun 23, 2009, at 10:30 PM, Troy Harvey wrote:

    Load sheding is not a bad idea if the tech exists. Not particularly complex either with networkable breakers and a smart controller in the inverter. Just software. Cost isn't an issue, they are willing to pay.

    The issue on loads isn't battery cut-off (though that isn't a bad idea), but shedding the less-important loads to make sure the house does exceed the inverter power output. A 6kW inverter subpaneled will only give you about four 15 AMP circuits. However a typical house of this size will have 30 such circuits, yet good chance it won't be drawing much more than 50 AMPs for the whole house at any one time. How do you select what is important? Either the inverter is complicated or the electricians wiring is complicated.

    Troy Harvey
    ---------------------
    Heliocentric
    801-453-9434
    taharvey at heliocentric.org

    On Jun 23, 2009, at 10:01 PM, William Miller wrote:

      Troy:

      Let me guess... They want it right away and cheap, too.  Generally speaking, this is an impractical request.  Electrically operated circuit breakers or 20 A relays and controllers are expensive and complicated.  One thing to learn in this trade is when to try and talk a client out of a bad idea, and when a client has such wacky ideas that it is best to walk away.

      I could, however, suggest two ways to do this:

      1. Use two inverters and set the LBCO for one high.  Connect one to critical loads and one (with the high LBCO) to non-critical loads.  When the batteries start getting low, the non-critical-loads inverter shuts down, leaving the critical-loads inverter running.

      Realize that you now need four load centers:  Grid, generator (you have recommended a generator so they can use their wide screen TV during a wind storm, correct?), non-critical loads and critical loads.  This type of design gets complicated fast.  Will the AHJ be able to track this?  Set a clause that allows you to collect hourly fees when they require three different meetings and three re-writes of the permit application.

      2. Use an Outback with external relays to shut off loads when the battery voltage falls below a certain point.   This is a crude approach, the parameters are not flexible (hard coded delay values) and it requires custom built relay panels, time consuming, expensive and a potential service problem.

      Either system is actuated on battery voltage rather than loads.  Inverters I am familiar with have relays and internal controls that operate based a set-able battery voltages, but I know of none that has a programmable relay to actuate at a certain load level.  In addition, loads change so rapidly that this type of switching would be erratic.  Loading is a component of battery voltage, anyway, so you are including that indirectly.

      Good Luck,

      William Miller

      At 07:41 PM 6/23/2009, you wrote:

        Hi folks,

        I need an inverter/charger/controller solution for a grid connect  
        house that:
        1. grid-connects (net-meters)
        2. Islands off of the battery in outages (whole house UPS)
        3. Feeds into the whole house breaker, so the whole house is backed up
        4. Shuts down less important breakers as needed, if the load for the  
        whole house is over the inverter limit

        Instead of guessing which breakers are important to put on a battery  
        backed up sub-panel, my client would like the whole house backed up.  
        But of course, can't guarantee that the house won't be drawing too  
        much for a 6000 Watt inverter at any given time. So would like to have  
        the system intelligently remove less important breakers until the  
        system is below the inverter operation wattage.

        Anyone know of a inverter system that is smart like that?

        Troy Harvey
        ---------------------
        Heliocentric
        801-453-9434
        taharvey at heliocentric.org
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