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Thread: PV Grid Tied Power Outage

  1. #1
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    Default PV Grid Tied Power Outage

    In the next few months I'm going to pull the trigger on a 5.6 kW grid tied, roof mounted PV system. Each panel will have MPPT and will all be tied to a central inverter. At the same time, the company will upgrade my electrical from 100 Amp to 200.

    They tell me when the grid goes down, my solar goes down. Short of not being grid tied, what can I do to keep my system productive during a power outage?

  2. #2
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    Default Re: PV Grid Tied Power Outage

    Quote Originally Posted by Vic99 View Post
    In the next few months I'm going to pull the trigger on a 5.6 kW grid tied, roof mounted PV system. Each panel will have MPPT and will all be tied to a central inverter. At the same time, the company will upgrade my electrical from 100 Amp to 200.

    They tell me when the grid goes down, my solar goes down. Short of not being grid tied, what can I do to keep my system productive during a power outage?
    The bad news is that you cannot have a PV system that powers AC loads which does not incorporate batteries and another type of inverter (either additional or alternative) in your system. The worse news is that doing so can double the cost of the system, or more, depending on which and how many loads you want to supply with power when the grid goes down. A whole house backup is for most folks prohibitively expensive.

    The reasons for this are somewhat involved, but it is not just because it's illegal to interconnect with the grid with a GT inverter that does not shut down when grid power goes away, although it indeed is.

    If you peruse this forum and others that are about solar, you will (not surprisingly) find this question asked and answered many, many times. It is disingenuous on the part of any solar installer who does not make this clear to a prospective customer very early in the sales process.

  3. #3
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    Default Re: PV Grid Tied Power Outage

    What about running off of a gas powered generator when grid goes down? Does this prevent me from running my array off of a generator, or running some of my circuits off of a generator or car adapter?

    I currently have neither, but figured it might make sense to do everything possible at once.

  4. #4
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    Default Re: PV Grid Tied Power Outage

    Quote Originally Posted by Vic99 View Post
    What about running off of a gas powered generator when grid goes down? Does this prevent me from running my array off of a generator, or running some of my circuits off of a generator or car adapter?

    I currently have neither, but figured it might make sense to do everything possible at once.
    The problem with that is if you could get it to work (which isn't assured because the output waveform from many generators is too dirty for a GT inverter to synch to), if the current demand from the loads you are running ever falls below the output of your inverter, then you will backfeed the generator, which has a high probability of doing major damage to it. If you have a generator, set up your transfer switch such that the PV inverter is disconnected (i.e., shut down) from the loads when the generator is running.

    And forget about a tiny battery and inverter just to provide a synch pulse for the GT inverter to read; that will not work, either. In an off grid system (or battery backup when the grid is down), you must power the loads from batteries; the only function of the PV is to recharge the batteries. Furthermore, depending on the magnitude of the lumped load you want to supply, the PV system has to be large enough to replace (during the time when the sun is shining) all the energy you are pulling from the batteries, else you will fall behind and run your batteries dry.

    These scenarios have been discussed at great length and the answer is always the same. To run PV off grid, you must have batteries and a battery inverter and your batteries must be large enough to supply your loads. You can be AC coupled (such as with a Sunny Island/Sunny Boy system), or DC coupled (Xantex or Outback), but you have to pony up; there's no free lunch.

    Too many installers do not make this clear because they are afraid of losing the sale. They depend on the customer to think to ask the question, and many don't until after they have bought a system. It gives the industry a black eye when an irate system owner calls an inverter company's tech support demanding to know why they don't have power during an outage; I was in a SMA workshop yesterday and they told me that it happens all the time.
    Last edited by ggunn; June 29th, 2012 at 7:58 PDT.

  5. #5
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    Default Re: PV Grid Tied Power Outage

    As ggunn says--"Simple" GT Inverter+Solar panels is "off" during a power outage and there is no "cheap" way of getting back online (even with a backup AC generator--at least reliably/safely).

    There are "hybrid" inverters that can do both Grid Tied and Off Grid... This inverters still need a full sized battery bank (think the size/weight of 24-36 [fixed] car batteries for a 6kW inverter/solar array or ~600 AH @ 48 volts). And these batteries typically need to be replaced every 5-10 years.

    The hybrid inverter has an internal AC transfer switch that cuts your "protected" loads (sub panel) from the grid and then supplies power to your loads. And they support the connection to the AC2 input for a backup generator too...

    The hybrid inverters are not that much more expensive, but the batteries and interconnections do.

    If you have reliable grid power--then a backup generator is usually the most "cost effective" solution.

    If you have unreliable power, or power that is down for 2+ weeks at a time, then, possible a true hybrid solar system may make sense for you (or if you are disaster planning).

    -Bill
    Last edited by BB.; June 29th, 2012 at 9:11 PDT.
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  6. #6
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    Default Re: PV Grid Tied Power Outage

    Quote Originally Posted by BB. View Post
    As ggunn says--"Simple" GT Inverter+Solar panels is "off" during a power outage and there is no "cheap" way of getting back online (even with a backup AC generator--at least reliably/safely).

    There are "hybrid" inverters that can do both Grid Tied and Off Grid... This inverters still need a full sized battery bank (think the size/weight of 24-36 [fixed] car batteries for a 6kW inverter/solar array or ~600 AH @ 48 volts). And these batteries typically need to be replaced every 5-10 years.
    I know you know this, but FYI to the OP, you cannot use car batteries. Car batteries are designed to deliver high current for short periods of time, while batteries for off grid or grid backup operation are designed to deliver (relatively) low current for long durations. It calls for a different design and using either type of battery for the other application will destroy it in short order. As you might anticipate, the type of battery needed for off grid/grid backup is more expensive.

  7. #7
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    Default Re: PV Grid Tied Power Outage

    Yea--just trying to give an idea on size...

    If you seen electric forklifts in a warehouse type store--A couple of those batteries to support a ~6kW system.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  8. #8
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    Default Re: PV Grid Tied Power Outage

    These guys are right on, this is why a nice fuel efficient inverter type generator for the occasional outage on you critical loads is a better deal when your grid is relatively reliable.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

  9. #9
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    Default Re: PV Grid Tied Power Outage

    well, there is the high voltage charge controller from Xantrex
    http://www.solar-electric.com/xaxwmp80amp6.html

    That will charge batteries off your HV PV array, and then a 48V battery bank can drive critical loads via a small 48V inverter
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  10. #10
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    Default Re: PV Grid Tied Power Outage

    Quote Originally Posted by mike90045 View Post
    well, there is the high voltage charge controller from Xantrex
    http://www.solar-electric.com/xaxwmp80amp6.html

    That will charge batteries off your HV PV array, and then a 48V battery bank can drive critical loads via a small 48V inverter
    That might work, but he would still need another inverter and enough batteries to power his loads, and a switch to take his PV off his GT inverter and put it onto the charge controller (you cannot connect PV both to a CC and a GT inverter; that would interfere with the MPPT function of both the CC and the GT inverter), and an AC battery charger to keep his batteries topped off. It would also mean he would have to learn a whole lot more about how this stuff works to design that system.

    It would be better, IMO, just to add a Sunny Island and batteries if his GT inverter is SMA. Either way, it wouldn't be cheap.

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