Measuring AC energy flow direction

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  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction
    vtmaps wrote: »
    I have no desire to derail this valuable and interesting discussion of analogies, but looking back at the OP:

    Have we come up with a practical solution to the OP's problem that he is being charged for selling to the grid? --vtMaps

    Lateral thinking strikes again! :blush:

    One: if it is possible, turn off "sell" on the inverter (hybrid type). This is the best solution.
    Two: if it is a straight-forward GT inverter suggestions include TED (post #9) and possibly hard to find directional Watt meter (post #11). Neither of which solves the big problem of stopping power being fed back to the grid and getting charged for it. But if you know where the energy is coming from and going to you at least have a better idea of what is going on.

    The rest of the thread is, sorry to say, just a discussion of current flow.
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    Lateral thinking strikes again! :blush:

    One: if it is possible, turn off "sell" on the inverter (hybrid type). This is the best solution.
    Two: if it is a straight-forward GT inverter suggestions include TED (post #9) and possibly hard to find directional Watt meter (post #11). Neither of which solves the big problem of stopping power being fed back to the grid and getting charged for it. But if you know where the energy is coming from and going to you at least have a better idea of what is going on.

    The rest of the thread is, sorry to say, just a discussion of current flow.

    Why are you sorry to say it? Threads drift toward areas of interest to the participants, like any other discussion.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction
    ggunn wrote: »
    Why are you sorry to say it? Threads drift toward areas of interest to the participants, like any other discussion.

    But too often away from answering the question, as vtMaps pointed out.
    One of the tasks of moderator is to try to keep things focused.
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    But too often away from answering the question, as vtMaps pointed out.
    One of the tasks of moderator is to try to keep things focused.
    That's got to be a fine line. You don't want to be so unfocused that soon we are all talking about what tricks our cats can do, but conversely not so focused that discussion of interesting and somewhat relevant points is stifled.
  • BB.BB. Posts: 27,785Super Moderators admin
    Re: Measuring AC energy flow direction

    Short of designing/buying a GT system that supports Net Zero power flow--Negotiating with the utility is probably a place to start.

    See if you can get some sort of net metering (possibly, at least sell excess power at wholesale costs--two unidirectional meters) and offer to support positive newspaper articles/involving a local engineering school, etc...

    But, I am sure, that in most private/public bureaucracies, nobody ever got ahead by doing something "different".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazzalazza Posts: 330Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction

    HI Thanks to all. I appreciate all inputs.

    The origin of the problem, yes, is to avoid losing the excess energy and stop feeding into the grid. Hence we are looking at designing a cheap, easy to apply gadget that will switch on/off loads as and when it recognises an excess of supply from the renewables system i.e. when there is excess power escaping into the grid.

    But to understand how such a gadget could work, obviously we have to know how to measure the DIRECTION of energy flow. And the discussion has begun to give me a clearer picture.

    As far as I can tell so far, the gadget needs to recognise the phases of both V and I waveforms... and perform a calculation to work out whether Power/Energy is being sucked into the building or injected out into the grid. I imagine the equipment needs to be very accurate... :roll:
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction

    So basically you need to sense when current starts to flow to the grid and use that as a trigger to either turn on loads or turn off GT.

    For some reason I think "simple" probably won't enter in to the solution. :roll:
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    lazza wrote: »

    As far as I can tell so far, the gadget needs to recognise the phases of both V and I waveforms... and perform a calculation to work out whether Power/Energy is being sucked into the building or injected out into the grid. I imagine the equipment needs to be very accurate... :roll:

    I don't believe that the relative phases of the voltage and current waveforms will have any relationship to which way the current is flowing, but it seems to me that if you had a meter on the output of your inverter and a meter on your loads, then subtracting the load current from the inverter current would give you the flow into the grid. A zero or a negative number would mean you are not feeding the grid anything. Current is always flowing out of the inverter and into the loads; subtract the absolute value of the one from that of the other and the sign of the difference tells you the direction of the flow in the utility meter.

    To implement this you'd have to move all your loads to a subpanel and leave the main populated only with the backfed breaker from the inverter and the breaker feeding the subpanel. How you would use the information is a judgment call. For example, if you happen to be feeding just a little current into the grid, that means your loads and inverter current are just about equal. Would you want to shut your inverter down? I wouldn't think so. What amount of feeding the grid is too much?
  • lazzalazza Posts: 330Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction

    Mmm now you've confused me- posts from inetdog (#24) and boB (#16) suggest otherwise- that the relative phases of V and I waveforms DO represent power flow direction. But you're a PV engineer... arrrrghh :confused:

    I appreciate the other solution which would involve measuring in two places (at the inverter and at the loads) but it does complicate the wiring.. and would rather have one point of measurement for simplicity's sake.
  • RCinFLARCinFLA Posts: 1,280Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    ggunn wrote: »
    Now, if you cut that resistance in half by increasing the conductor size or paralleling conductors, you cut that voltage drop in half, but the current from the inverter remains the same. Keep cutting the resistance in half, the current stays the same. Take that resistance all the way to zero (OK, not realistic but imaginable); the voltage differential becomes zero but the current still flows at the same rate.

    Not precisely, for a given illumination level there is a certain amount of power to be pushed to grid. If GT inverter sees a lower grid voltage it will deliver more current to push the same amount of power into the grid. You obvious have to be within the grid voltage tolerance acceptance range.

    Some GTI will oscillate on and off grid if there is too much wiring resistance loss to inverter. It sees normal grid voltage before connecting. It connects and start to push power to grid. With too much wire resistance the current push raises the voltage the inverter sees. If it raises too high, out of high end limit, it disconnects from grid. Grid voltage seen by inverter then drops to normal grid voltage and after several minutes it tries to reconnects. A good grid testing algorythm will detect too much voltage fluxuation and disconnect shortly after a connection attempt to grid made.
  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    lazza wrote: »
    Mmm now you've confused me- posts from inetdog (#24) and boB (#16) suggest otherwise- that the relative phases of V and I waveforms DO represent power flow direction. But you're a PV engineer... arrrrghh :confused:
    I appreciate the other solution which would involve measuring in two places (at the inverter and at the loads) but it does complicate the wiring.. and would rather have one point of measurement for simplicity's sake.

    Disclaimer: I am a Physicist, so I will (almost) always be right about the theory, whether I manage to get something to work or not. An Engineer, on the other hand, will give you something that works, but may have trouble explaining it. :-)

    "In theory there is no difference between theory and practice. In practice, however..." :-)

    What ever explanation you follow, you will have to have two measurements to be able to tell which way the power is flowing. The combination of a DC clip-on Ammeter and a clamp-on capacitive voltage sensor will do the job. Measuring the current (including direction) at the same time you measure the voltage will do the job. It does not matter whether you measure the current with an Ammeter which senses the instantaneous direction of the current and correlates that with the voltage or you measure the current by looking at the voltages at two points separated by a known resistance. The result of that measurement will be the same. But a meter that just tells you the magnitude of the AC current and another meter that just tells you the magnitude of the AC voltage will not be able to tell you what you need to know.
    If the current is flowing towards the grid when the voltage is + and away from the grid when the voltage is minus you are selling. If the current is flowing away from the grid when the voltage is + and towards the grid when the voltage is -, you are buying.

    Now, just for fun, I will take a crack at ggunn's and boB's explanations directly:

    "I don't believe that the relative phases of the voltage and current waveforms will have any relationship to which way the current is flowing,..."

    If you follow the idea that the voltage out of the inverter has to be higher than the grid voltage to push power out, you need to recognize that the voltage from the inverter also has to be in phase with the voltage from the grid. That is the simple constant side of the formula.
    That also means that if the magnitude of the voltage from the inverter is slightly larger than the magnitude of the voltage from the grid and they are in phase, then the + peak from the inverters is slightly higher than the + peak from the grid. And the - peak of the inverter dips slightly lower than the - peak of the grid. (So far so good?)
    What that means, by Ohm's law, is that during the + cycle current is flowing toward the grid and during the - cycle current is flowing away from the grid.
    If you substitute a resistive load in place of the inverter, the voltage at the load will be slightly lower at the + peak than the grid voltage (otherwise no current would flow, goes the argument.)
    So if the current flows away from the grid during the + cycle and toward the grid during the - cycle, power is flowing into the load.
    The output of a simple AC ammeter is only going to tell you the magnitude of the current. A current transformer, as in a clamp-on ammeter or in some utility meters, knows exactly which direction the current is flowing at any moment of time, but the AC meter does not bother to tell you. :-) If you look at both waveforms on an oscilloscope, you will see that in one case the current and the voltage are in phase and in the other case current and voltage are 180 degrees out of phase. You can try the experiment yourself if you do not believe me.

    If you can measure the load power and the output power of the inverter (which can be done with two power meters which do not need to run backwards as both BB and ggunn stated) you will know whether you are buying or selling. If the utility will not pay you for excess you send them (or worse, charge you for it) you will need to make sure that you never overproduce, even if that means adding extra load. (You cannot throttle down a grid-tie inverter except by limiting the DC input to it.)
    SMA SB 3000, old BP panels.
  • stephendvstephendv Posts: 1,571Solar Expert
    Re: Measuring AC energy flow direction

    Lazza,

    Have a look at this discussion here: http://www.navitron.org.uk/forum/index.php/topic,13808.0.html
    The simplest solution seems to be to buy a standard electricity meter that only counts in one direction, and then wire it backwards. So when you're a net importer it doesn't move, but when you start exporting it'll start counting.
  • boBboB Posts: 951Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    ggunn wrote: »
    Well, consider this: Say you have a GT inverter connected to the grid. The current flowing through the resistance in the conductors between the inverter and the grid results in a voltage drop in the direction of the grid, in turn resulting in a higher voltage at the terminals of the inverter, true enough. Now, if you cut that resistance in half by increasing the conductor size or paralleling conductors, you cut that voltage drop in half, but the current from the inverter remains the same. Keep cutting the resistance in half, the current stays the same. Take that resistance all the way to zero (OK, not realistic but imaginable); the voltage differential becomes zero but the current still flows at the same rate.

    Superconductor ! Perfectly legal. Ya got me there GG !!

    OK, so imagine you are looking at the voltage outputs that WOULD be present and then connect the GT to the grid via superconducting wire.
    The GT's voltage would be slightly higher than the grid in order for current to flow into the grid when you finally connect them together.

    Otherwise, no current (or power/energy) would flow into or out of the grid from/to the GT inverter if they were exactly the same voltage.

    (Yes, I know, techincally with a current source the voltage when disconnected would be off the map for a true current source but voltage source
    would certainly behave this way. And, not all GT inverters are current sources anyway)

    This example along with the pictures I posted were only to help illustrate the concepts of direction of current that was
    asked and was not intended to get into the details of current sources and stuff like that. It usually works well as an illustrative aid.

    boB
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction

    Negative Voltage. Reverse current flow. Peak Voltage. RMS Voltage.

    Lions and tigers and bears! Oh my!

    Looks like another case of theoretical physics colliding with practical application.

    And that's what makes sparks* fly, right?:D

    (*And I don't mean Dave. :p)
  • boBboB Posts: 951Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    Negative Voltage. Reverse current flow. Peak Voltage. RMS Voltage.


    For AC, think negative thoughts !!
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction
    boB wrote: »
    For AC, think negative thoughts !!

    I always think negative thoughts, Bob; I'm a dreadful cynic! :p

    But, when it's negative on one wire it's positive on the other wire so the net effect must be zero current flow! That means inverters don't actually back feed the grid at all and none of this stuff really works!

    It's all in our heads! :p

    And before Bill or Niel or Tony jumps on me I promise to stop razzing everyone about this now. :blush:
  • nsaspooknsaspook Posts: 396Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    lazza wrote: »
    Mmm now you've confused me- posts from inetdog (#24) and boB (#16) suggest otherwise- that the relative phases of V and I waveforms DO represent power flow direction. But you're a PV engineer... arrrrghh :confused:

    I appreciate the other solution which would involve measuring in two places (at the inverter and at the loads) but it does complicate the wiring.. and would rather have one point of measurement for simplicity's sake.

    Knowing the relative phases of V and I waveforms can't show power direction unless there is a point of reference "ie.. the Poynting vector".

    Here is what you need to establish the direction of power flow(at a single point):

    A reference direction
    The angle between the voltage and current

    Attachment not found.

    P is real power
    Q is imaginary power
    S is the complex power

    The direction of P can be left or right depending on the point of reference. In this case the grid is the point of reference.

    http://en.wikipedia.org/wiki/Electric_power
  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    boB wrote: »
    For AC, think negative thoughts !!

    I think we can all agree that if you can buy power for $.06 per KwH, that is an *absolute value*. :-)
    SMA SB 3000, old BP panels.
  • Ken MarshKen Marsh Posts: 114Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction

    Good Grief Guys, lets give lassa something he can work with.
    Here is a circuit for an indicator which seems like it should work.
    That is if I can figure how to post a .jpg
    I looks like we have a lot of electrical / electronic types on this board.Attachment not found.
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    boB wrote: »
    Superconductor ! Perfectly legal. Ya got me there GG !!

    OK, so imagine you are looking at the voltage outputs that WOULD be present and then connect the GT to the grid via superconducting wire.
    The GT's voltage would be slightly higher than the grid in order for current to flow into the grid when you finally connect them together.

    Otherwise, no current (or power/energy) would flow into or out of the grid from/to the GT inverter if they were exactly the same voltage.

    Not so. The voltage at both ends of a superconductor is by definition the same, no matter how much current is flowing and in which direction.

    My point is that with a current source like a PV powered inverter*, even in the real world, the magnitude of the current output does not depend on the magnitude of the voltage drop in the transmission lines. You can rewire to cut the voltage drop/rise in half and the current output will remain the same. You could rewire with a superconductor (if you can find one) and there would be no voltage drop/rise, and the inverter would still put out the same amount of current. The voltage rise at the terminals of the inverter is an effect of current flow, not the cause.

    *Actually, as normally implemented a PV powered GT inverter is a power source, since its output voltage is clamped by the grid voltage.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Measuring AC energy flow direction
    ggunn wrote: »
    *Actually, as normally implemented a PV powered GT inverter is a power source, since its output voltage is clamped by the grid voltage.

    And there is the truth of it in a nutshell!
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction

    Consider the following:

    Attachment not found.

    The inverter (current source) and grid power (voltage source) both supply power to the household loads (Rh) and the grid loads (Rg). The conductors between the inverter and the household service are modeled by Rt and the utility meter is the ammeter A. If Rh goes to infinity (all the household loads are off), then the current through A is the output of the inverter going onto the grid. The magnitude of the current through the inverter is independent of the values of Rt, Rh, and Rg, as long as Rg remains low enough (it will). Rt can go to zero and the inverter does not care. The grid power generator is a voltage source; it is constantly adjusting its current output so that the voltage drop across Rg remains constant.

    The output current of the inverter is dependent only on the amount of PV connected to it and the amount of insolation it is getting. When the inverter current goes up, the current from the grid generator (theoretically) must go down by the same amount to keep the grid voltage constant. This is why large PV and wind generators can be a problem for utilities; they can start up and shut down unpredictably and the grid generators have to be nimble enough to respond quickly with adjustments in output in order to keep the grid voltage stable.

    This also explains why you cannot run a GT inverter off grid. Without the connection to the grid to stabilize the voltage and sink and source current as needed, the voltage across the household loads would vary with the magnitude of the lumped load because the power from the inverter cannot change.
  • boBboB Posts: 951Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    ggunn wrote: »
    Not so. The voltage at both ends of a superconductor is by definition the same, no matter how much current is flowing and in which direction.

    My point is that with a current source like a PV powered inverter*, even in the real world, the magnitude of the current output does not depend on the magnitude of the voltage drop in the transmission lines. You can rewire to cut the voltage drop/rise in half and the current output will remain the same. You could rewire with a superconductor (if you can find one) and there would be no voltage drop/rise, and the inverter would still put out the same amount of current. The voltage rise at the terminals of the inverter is an effect of current flow, not the cause.

    *Actually, as normally implemented a PV powered GT inverter is a power source, since its output voltage is clamped by the grid voltage.


    Doesn't matter anyway. You won't be finding any superconducting wire in a typical installation any time soon and arguing this is silly and not going to help
    answer the question on how to tell the direction of flow of anything. Also, you (and others) did not look at or understand the simulation that I posted.
    And when the forum automatically changed my GIF file to a JPG file of lower resolution, that did not help.

    The circuit that was posted by Ken is most likely way more on topic and pertinent to answering the question. Your current source to voltage source schematic is nice too but
    it doesn't show how to tell direction of current or power flow relative to AC voltage. It adds confusion actually. Really there are several
    different topics in this thread, including power factors less than 1.0 which doesn't have anything to do with the question if we were talking
    strictly about resistive sources and loads.

    I understand how this works and I assume that deep down inside you do too, but it is pointless to the original discussion to waste time on
    who killed who.... This is supposed to be a happy occasion !!

    boB


    P.S. Another way of expressing what I was talking about in our off topic discussion is associated with "Compliance" voltage of
    the current source.

    I like the description from the keithley site.

    "Compliance voltage is the maximum voltage a current source will go to in its attempt to source the programmed current."

    In other words, the current source (the grid tie inverter) must have some compliance voltage that is higher
    than the positive grid voltage, otherwise it will not source any current and sell to the grid.

    (more negative V than the minus AC voltage half cycle too of course)
  • ggunnggunn Posts: 1,973Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction
    boB wrote: »
    Doesn't matter anyway. You won't be finding any superconducting wire in a typical installation any time soon and arguing this is silly and not going to help
    answer the question on how to tell the direction of flow of anything. Also, you (and others) did not look at or understand the simulation that I posted.
    And when the forum automatically changed my GIF file to a JPG file of lower resolution, that did not help.

    The circuit that was posted by Ken is most likely way more on topic and pertinent to answering the question. Your current source to voltage source schematic is nice too but
    it doesn't show how to tell direction of current or power flow relative to AC voltage. It adds confusion actually. Really there are several
    different topics in this thread, including power factors less than 1.0 which doesn't have anything to do with the question if we were talking
    strictly about resistive sources and loads.

    I believe that the simplest solution to his problem is the one I proposed, which is to move his loads to a subpanel and meter the flow into it. That way he doesn't have to worry about trying to figure out which way the current is flowing through either meter. It's always flowing into the subpanel and out of the inverter, and the utility meter gets the difference between them. If he subtracts the subpanel current from the inverter current and gets a positive number, he is feeding the grid with that much current. Whether or not analyzing current and voltage waveforms would tell him anything useful is a moot point; getting the gear to do that would be problematic.
  • MarkusMarkus Posts: 1Registered Users
    Re: Measuring AC energy flow direction
    lazza wrote: »
    HI Forum

    I am looking at a solution to the fact that surplus energy from grid-tie systems in Spain is not compensated for (or worse is charged for as if it were consumption). We are therefore looking at an electronic gadget that recognises when the energy flow is going in or out of the house in question.

    I have brought this up before, but my specific question is how direction of energy flow can be measure with clamp type sensors. My clamp meter recognises current flow direction in DC but not in AC.

    We are working with an electronic engineer here who says that the clamp meters will recognise direction of energy flow... but I am not sure. I dont have a very profound understanding of how AC works. But I understand that the current flows in both directions... which poses a quandary... how then, if the current goes in both directions, do you measure ENERGY flow in or out???

    Maybe this will be a solution
    http://files.panucatt.com/datasheets/cs50_100_200a_datasheet.pdf
  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    Markus wrote: »

    This product provides a way of measuring current, which will give you access to the actual phased waveform rather than just an amplitude. But that will still have to be properly combined with the voltage waveform measurement to determine how much power is flowing and in what direction.
    Any watt-hour meter needs both a current and a voltage input. The current input is usually provided by a current transformer or a shunt. The method used in these sensors gives you an alternative which works better in some situations, but does not solve any directionality problems on its own.
    SMA SB 3000, old BP panels.
  • lazzalazza Posts: 330Solar Expert ✭✭✭
    Re: Measuring AC energy flow direction

    Mmm yes, struggling with this project. The University researcher we were cooperating with appears to be less like Doc of Back to the Future, and more like his dog Einstein.

    Wish i'd studied electronics!
  • stephendvstephendv Posts: 1,571Solar Expert
    Re: Measuring AC energy flow direction

    Hi Lazza,

    Don't know if this project is still ongoing or not... but I stumbled across a product which could be of interest: Nedap Power Router. It's an all in one, inverter/charger/MPPT tracker and has a "dynamic feed in limiter" which apparently lets you limit how much you're selling to the grid.

    It needs a battery, but it can be a very small one, they mention minimum size is 200Ah, even on a 5kW solar array - and you don't need any other components, the solar panels and the battery all connect up to this one device. The panels can be wired in typical grid-tie voltages up to 600V.

    Docs and specs here: http://powerrouter.com/ and it's available in Spain.
  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: Measuring AC energy flow direction
    stephendv wrote: »
    Hi Lazza,

    Don't know if this project is still ongoing or not... but I stumbled across a product which could be of interest: Nedap Power Router. It's an all in one, inverter/charger/MPPT tracker and has a "dynamic feed in limiter" which apparently lets you limit how much you're selling to the grid.

    It needs a battery, but it can be a very small one, they mention minimum size is 200Ah, even on a 5kW solar array - and you don't need any other components, the solar panels and the battery all connect up to this one device. The panels can be wired in typical grid-tie voltages up to 600V.

    Docs and specs here: http://powerrouter.com/ and it's available in Spain.

    From the specs, which are minimal, it sounds like an all-in-one hybrid system, but one which promotes the use of the batteries for short term energy storage for taking any peak of the solar production above the current local demand, using it to charge the batteries and then using that energy to generate AC again locally when the solar PV is below the local load power. Unless there are some really big improvements in the batteries which are available and practical for the small (residential) user, the time-shifted power will not even come close to paying for itself in savings in your power bill unless you have the really bizarre FIT found in Great Britain. The cost of regular replacement of the batteries will more than offset the savings on your power bill.

    The small minimum size battery described is totally incompatible with storing a meaningful amount of energy across a daily cycle. If the built-in inverter(s) can use the small battery to stabilize an inverter which is running primarily off PV during a power outage during solar hours, that might make sense, but not for daily use.

    The fact that the web site prominently seeks distributors and investors raises some suspicion too.
    SMA SB 3000, old BP panels.
  • stephendvstephendv Posts: 1,571Solar Expert
    Re: Measuring AC energy flow direction
    inetdog wrote: »
    The small minimum size battery described is totally incompatible with storing a meaningful amount of energy across a daily cycle. If the built-in inverter(s) can use the small battery to stabilize an inverter which is running primarily off PV during a power outage during solar hours, that might make sense, but not for daily use.

    Yes, they suggest a bigger battery for time shifting. But for lazza's application, he doesn't want time shifting at all, just no feeding into the grid with minimal costs. The price of the 3kW version of this system is just slightly more than a 3kW GTI. And much less than a 3kW battery based inverter + charge controller. So it could be an economical option.
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