Need advise

kenr047
kenr047 Registered Users Posts: 3
in Grid Tie and Grid Interactive Systems #1
I would like to install a 4k solar system with grid interactive inverter. Facts on inverters are not clear, I will use 99% grid tie but would like a battery back up for outages. Which inverter will allow me to operate on grid only with or without battery's? Also I would like to be able to switch off the system and not see any difference in powering the house from the grid. So far I have been looking at Outback and Xantrex, Xantrex looks like the one I need but I'm not clear on how they both operate.
Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Need advise

    Welcome to the forum.

    "Grid Interactive" can mean a lot of things, so let's look at the three basic inverters and how they can interact with the grid.

    1). straight grid-tie: solar panels go directly to the inverter which then backfeeds the household/utility with whatever power it can muster from the panels. No back-up ability; grid goes down, you have no power.

    2). off-grid used for back-up: utility and/or panels keep batteries charged. Utility feeds loads when available, inverter feeds loads when the grid goes down. No selling of surplus power to the grid.

    3). hybrid system: panels keep batteries charged, surplus power production is sold to the utility. If the grid goes down inverter supplies back-up power from the batteries.

    All those will utilize grid power for the loads primarily, with the exception that if you have surplus power it will either be sold back to the utility or lost.

    Both the Outback Radian or Xantrex XW systems are of the hybrid variety. They can supply power from the batteries when the grid goes down, or sell surplus panel production when it is up.

    I also have to mention that if your goal is back-up power for occasional, short outages you are better off buying a generator. Solar power systems, especially battery-based, are expensive. As back-up power it is usually not a good choice unless the outages are frequent, long-lasting, or involve critical loads that must be maintained.
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  • kenr047
    kenr047 Registered Users Posts: 3
    #3
    Re: Need advise

    So far the information I see is that the outback inverter won't run w/o batteries, the Xantrex will. This has to be a system that anyone can operate, like my wife if I was not home. My ultimate goal is to sell power and be a little more self reliant. The battery backup makes sense to add at the beginning.
    Looks like Outback runs thru the battery's all the time. If I lost one battery out of 4 (48v) then my transferred loads would all go down. Xantrex looks like it will run on its own w/o the battery's. Is there a better inverter that I haven't researched? I to new at this to risk tons of money and find out that I missed something.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #4
    Re: Need advise

    The hybrid inverters need to have batteries all the time. They just don't need so many of them as a straight off-grid system does. Plus, any loads you want to be battery backed, run on the inverter all the time meaning you can't switch off the inverter system and just be on the grid without rewiring these loads. Most setups do have a bypass switch for doing this however. My advice is to do a straight grid-tied system as it is more efficient, way more reliable and more cost effective - and just get a generator for those infrequent outages. Of course if you have lots of money to spend, give me a call.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: Need advise
    kenr047 wrote: »
    I would like to install a 4k solar system with grid interactive inverter. Facts on inverters are not clear, I will use 99% grid tie but would like a battery back up for outages. Which inverter will allow me to operate on grid only with or without battery's? Also I would like to be able to switch off the system and not see any difference in powering the house from the grid. So far I have been looking at Outback and Xantrex, Xantrex looks like the one I need but I'm not clear on how they both operate.
    Thanks

    You cannot run AC loads off grid without batteries no matter what inverter you buy, if that is what you are asking.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Need advise
    kenr047 wrote: »
    So far the information I see is that the outback inverter won't run w/o batteries, the Xantrex will. This has to be a system that anyone can operate, like my wife if I was not home. My ultimate goal is to sell power and be a little more self reliant. The battery backup makes sense to add at the beginning.
    Looks like Outback runs thru the battery's all the time. If I lost one battery out of 4 (48v) then my transferred loads would all go down. Xantrex looks like it will run on its own w/o the battery's. Is there a better inverter that I haven't researched? I to new at this to risk tons of money and find out that I missed something.

    No: the Outback Radian and Xantrex XW inverters (as well as any other hybrid inverter such as the SMA Sunny Island) need batteries all the time or else they do not work.

    I bring up the SMA because they also make straight grid-tie inverters (Sunny Boy) which can be coupled to the Sunny Island in a very manageable way. You could install a Sunny Boy GT system now and add the Sunny Island later for battery-based back-up.

    But as has been pointed out, no batteries = no back-up power.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #7
    Re: Need advise

    The Outback GVFX3648 is a "grid interactive" inverter that is large enough to support most normal households and would work well with a 4kw array. It's much less expensive than the Outback Radian inverters, Xantrex XW series, or SMA Sunny Island.

    Unlike those however, when used alone - one GVFX will not supply 240 VAC loads without the use of an autotransformer. So - as always - it depends on what loads you are trying to supply/back up.

    It does, of course require batteries!
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  • kenr047
    kenr047 Registered Users Posts: 3
    #8
    Re: Need advise

    I'm not asking the right questions based on the answers i"m getting.
    Will Xantrex still feed the grid (only the grid) with out batteries? Will any hybrid inverter?
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #9
    Re: Need advise
    kenr047 wrote: »
    I'm not asking the right questions based on the answers i"m getting.
    Will Xantrex still feed the grid (only the grid) with out batteries? Will any hybrid inverter?

    No hybrid inverter will work without batteries.
    The only inverter that will work without batteries is a Grid Tied Inverter.

    EDIT: You can connect a Grid Tied inverter to some battery based inverters like the Xantrex XW or SMA sunny Island.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #10
    Re: Need advise

    Xantrex has Grid Tied, Hybrid, and Off Grid inverters... So, it can get a bit confusing. It is the Xantrex XW Hybrid inverter that we are discussing here (and other brands).

    Hybrid inverters are really using the Grid/Utility power as a "dump load". When the batteries are "full", any excess power is sent out to the grid.

    You can setup a GT inverter to "back feed" some TSW type off grid inverters and actually recharge the battery bank (some Off Grid inverters are actually bi-directional regarding AC/DC power... The GT inverter can push current back through an Off Grid inverter and recharge the battery bank. The issue is how to control the charging of the battery bank. Normal Off Grid Inverters (when "back fed" power from a GT Inverter) do not have any way of stopping the recharging of the battery bank--so you either have to setup a dump load and/or controller to turn off the GT inverter when the battery bank is full.

    The Xantrex XW, and (I think) some Magnum inverters can shift the 60 Hz frequency to (for example) 62Hz when the battery is full. That will "knock" the GT inverter off line until the frequency is restored back to 60Hz.

    SMA's Sunny Island series is designed to vary/throttle the GT inverter the farther it gets from 60 Hz (it is actually quite smart and will not mess up your AC powered clocks as it keep track of how far the frequency has drifted).

    There is no "cheap way" of doing this. Basically, you are removing the standard MPPT type battery charger and replacing it with a GT Inverter. There are a few advantages (GT inverters are less expansive and can send high voltage power longer distances than the "standard" MPPT type battery chargers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #11
    Re: Need advise
    BB. wrote: »
    Normal Off Grid Inverters do not have any way of stopping the recharging of the battery bank--so you either have to setup a dump load and/or controller to turn off the GT inverter when the battery bank is full.

    Huh? Perhaps I am interpreting this sentence out of context, but its not correct as written.

    My 'normal' Off Grid Inverter (outback vfx3524) will stop recharging the batteries after bulk, absorb, and float.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #12
    Re: Need advise
    vtmaps wrote: »
    Huh? Perhaps I am interpreting this sentence out of context, but its not correct as written.

    My 'normal' Off Grid Inverter (outback vfx3524) will stop recharging the batteries after bulk, absorb, and float.

    Off grid inverters can't control the charging when they're driven by a grid tied inverter on their AC output. I'll wait for someone more knowledge to explain why... :)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #13
    Re: Need advise

    Vtmaps,

    I have clarified the post to add when "being back fed by a GT Inverter".

    From what I have read/understand, what may happen is a back fed off grid inverter may stop charging if the battery voltage gets so high, the OG Inverter "faults" and shuts down AC inverter operation. Then the GT inverter will shutdown too (no AC power for it to "sync" with).

    Basically, the standard Off Grid inverter has no set point electronics/smarts to detect that a battery bank is fully charged and no ability to go from 60 Hz to 58/62 Hz (which would shut down the GT inverter).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #14
    Re: Need advise
    stephendv wrote: »
    Off grid inverters can't control the charging when they're driven by a grid tied inverter on their AC output. I'll wait for someone more knowledge to explain why... :)

    Because the AC OUT of the Outback is not coupled to the internal charging circuitry; it has a "direct route" (backwards) through the inverter circuitry to the batteries with no regulation. It was not designed to be used this way. Shutting it down via the high Voltage disconnect is not desirable as it turns off the power (blackout). This is not the same as regulating the charge.

    The Xantrex XW and SMA Sunny Island can both detect over-charging and skew their output frequencies to cause the GT inverters to drop out without losing AC to loads. Although some frequency-sensitive devices (like digital clocks) may have a problem.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,622 admin
    #15
    Re: Need advise
    Cariboocoot wrote: »
    The Xantrex XW and SMA Sunny Island can both detect over-charging and skew their output frequencies to cause the GT inverters to drop out without losing AC to loads. Although some frequency-sensitive devices (like digital clocks) may have a problem.

    FYI--I believe Stephen said he was at a SMA class and they were very tricky folks. The Frequency would be slewed both + and - from 60 Hz (say 59 Hz for a minute then 61 Hz for a minute) so the average frequency would still be 60 Hz and clocks/timers would be accurate for the long term average.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #16
    Re: Need advise
    BB. wrote: »
    FYI--I believe Stephen said he was at a SMA class and they were very tricky folks. The Frequency would be slewed both + and - from 60 Hz (say 59 Hz for a minute then 61 Hz for a minute) so the average frequency would still be 60 Hz and clocks/timers would be accurate for the long term average.

    -Bill

    I'm not a fan of AC coupled systems, but it never ceases to amaze me how well SMA has thought their's through. Probably explains the price. :D
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #17
    Re: Need advise
    Cariboocoot wrote: »
    I'm not a fan of AC coupled systems, but it never ceases to amaze me how well SMA has thought their's through. Probably explains the price. :D

    Hehe, very typically German. It was funny because after a few beers one of their tech guys admitted that if he was building his own off-grid system, it would be with a sunny island and outback controller ;)
    Don't know why the sunny island is so expensive in the US, over here, it's about the same price as every other 5kW inverter.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #18
    Re: Need advise

    Not sure you got your question answered.

    A XW6048 can sell to grid via the separate battery charge controller but I don't recommend this approach if your primary everyday purpose is to suppliment / sell to grid with only an occasional power outage backup. The wear and tear on the batteries will run the over time system cost up. You stress the batteries just providing the AC ripple filtering for the DC charge controller. The DC charge controllers are only a bit lower cost then a GTI.

    If your primary purpose is to suppliment grid power then you need a battery-less GTI. You can use the GTI inverter to backfeed a XW6048 to charge the batteries during outage but the battery sizing must be large enough to take the full PV / GTI power output.

    If you plan on 4kW PV/GTI then you will generate about 70 amps of maximum battery charging current for a 48vdc battery system. The XW in back feed (AC coupled) has no regulation for the backfeed power other then switching the GTI inverter on or off by slewing the AC frequency outside the tight GTI frequency operating range. During no grid power situation, when battery needs charging you get whatever the PV produces. At full illumination that would be about 70 amps of battery charge current. Assuming a 20% AH rating maximum safe battery charging rate that would be about a 350-400 AH battery. That is a reasonable size for temporary outage backup but it depends on your desired maximum loading and time length you want battery backup. I would recommend having a generator just in case you need charging and there isn't enough solar.
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