battery backup for off-grid systems

solarixsolarix Solar Expert Posts: 713 ✭✭
I'm looking for advice on how to add battery backup capability to on-grid PV systems.
Say some one already has a nice ongrid residential PV system but wants to be prepared for outages. Do you put on a SMA island inverter as it is too late for the real solution of a hybrid inverter like a Zantrex XW. The rub comes in how to make use of the existing solar array during the outage. It is all string wired for high voltage and I don't know of a good way to use it for charging batteries during the outage. Charge controllers seem to max out at around 150Vdc right? Is there a way to set up an array to feed both an on-grid inverter and an off-grid inverter?
Outages are the one weakness of on-grid systems and it would be great to be able to offer a simple off-grid solution to handle critical needs during outages.
Thanks for all your thoughts!


  • icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
    Re: battery backup for off-grid systems

    Just thought I would mention, unless you have a very unstable grid, a cheap generator will in all likely hood be a much cheaper, and indeed a more reliable solution for the grid going down. Batteries get expensive, and even if they are almost never used, they have a finite life span.

    For example, a Eu1000 honda can be had used for ~$500, and will deliver 800 watts 24/7 for ~ 1.5 gallons of fuel per day. If you start it once a month, and keep a bit of fresh gas around you are set. The same $ 500 in batteries might buy 2.7kwh of power without needing to be recharge some how, and will likely only last 5-8 years in standby service, plus you have to maintain them.

    Just a thought,

  • BB.BB. Super Moderators, Administrators Posts: 30,738 admin
    Re: battery backup for off-grid systems

    The big problem with off-grid charge controllers and grid-tied inverter is the Vmp-array operating voltages...

    Typically, off-grid controllers peak out around 100 Vmp maximum (there is an Apollo controller that can hit possibly 140+ volts safely). Grid tied, is pretty much limited to 200 to 450+ VDC Vmp.

    So--the problem with directly using the same solar array for OG and GT operation is that there are few (if any?) controllers that share the same array configuration.

    Solar Guppy, because he designs controllers--has wired a jumper panel that lets him configure an array into different voltage/current configurations--but that is probably more than a typical end-user would want to do.

    The other possible method is to install an Off-Grid true sine wave inverter or a Hybrid Inverter like the Xantex XW + battery bank.

    It turns out that some true sine wave inverters (a few Outback and Xantex inverters are known to be possible--others may be too--but I have no information on that) can be "back driven" by a GT inverter and the OG inverter will actually share the 120/240 VAC load with the GT inverter--and any "extra power" will charge the battery bank.

    So far, only the SunnyIsland family is really setup to do this. Apparently the Xantrex XW Hybrid inverter just got a firmware upgrade to support "AC Coupling" too (I think that is the "official" name of coupling a GT+OG inverter together).

    Note that the OG inverter needs to be larger than the maximum power of the GT inverter.

    The OG inverter "sets the off grid voltage and frequency" and the GT invert just ties into the OG inverter's output (OG + GT inverters must be behind a transfer switch if AC main power is used too with GT inverter).

    The OG both supplies power when the sun is down, and absorbs power when the sun is up and the AC loads are small.

    Because the typical OG inverter has no way of controlling battery charge levels (remember, you are driving the inverter backwards)--you have to setup a separate charge controller to manage the power (there are several ways to do this).

    Or, if the Xantrex XW firmware works the way we think it might--the XW will monitor the battery state of charge and adjust the line frequency to >61 Hz... This will cause the GT inverter to sense an "AC Line Fault" and shut down for ~5 minutes. The XW will continue to supply 120/240 VAC power and the loads will never see a glitch. Once the XW decides it is OK for more GT power--it will simply drop the frequency back to 60 Hz.

    This is purely a guess on my part--You will need to contact Schneider Electric--they now own this part of the Xantrex Product Line for more information.

    The end result is that you need to add an OG inverter/Hybrid inverter + battery bank (and transfer switch for some configurations). This is not a cheap project and battery maintenance costs are not insignificant (replace batteries ~5-10+ years?).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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