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PV system - installer says dont overspec the Inverter?

  • 12-05-2021 5:21pm
    #1
    Registered Users Posts: 583 ✭✭✭ NobodyImportant


    Hi all,

    Had a conversation with a PV installer today and asked him about over sizing the invertor.

    I want to do this as I can only afford a 2.5kw array for now on the Southern roof, but will probably put up another 4 panels on the western roof next year to take into account the evening sun and when we use the most electricity and stretch that curve out.

    He said not to over spec the invertor, and to just get another invertor next year? Something about the startup voltage on the other string?

    Does this sound right?

    And can all invertors handle multiple strings?

    Thanks!


Comments

  • Registered Users Posts: 2,820 ✭✭✭ air


    It does sound right, 4 panels won't be enough to run the second string input on most inverters.
    Ask if you could select a Solis Ginlong inverter instead perhaps as they tend to have a lower minimum input voltage than most and 4 panels may be enough to run it successfully.


  • Moderators, Sports Moderators Posts: 16,039 Mod ✭✭✭✭ slave1


    Not sure about that, where I live the sun rises in the east so any west facing string will never have to power on an inverter and there will be plenty of hours in the day where both strings will be producing.
    Worst case scenario you could use a pair of micro inverters for the West facing rather than a full blown inverter unit.


  • Registered Users Posts: 2,820 ✭✭✭ air


    slave1 wrote: »
    Not sure about that, where I live the sun rises in the east so any west facing string will never have to power on an inverter and there will be plenty of hours in the day where both strings will be producing.

    A smart answer that only highlights your lack of understanding of the issue at hand, the issue is not "Powering an inverter".

    The issue is that inverters can only operate over a certain DC voltage range, and usually have a narrower range still within which they can do MPP.
    In a dual tracking inverter, each input is effectively a separate inverter.

    So you could well have your inverter happily running on the South array but the 4 panels connected to the other input producing nothing, because the combined output voltage of those 4 panels is not sufficient to start that input / tracker.
    slave1 wrote: »
    Worst case scenario you could use a pair of micro inverters for the West facing rather than a full blown inverter unit.
    He wants 4 panels, so he would need 4 micro inverters, not 2.
    He can avoid this by buying an inverter that is suited to the task, like I already advised.


  • Moderators, Sports Moderators Posts: 16,039 Mod ✭✭✭✭ slave1


    air wrote: »
    A smart answer that only highlights your lack of understanding of the issue at hand, the issue is not "Powering an inverter".

    The issue is that inverters can only operate over a certain DC voltage range, and usually have a narrower range still within which they can do MPP.
    In a dual tracking inverter, each input is effectively a separate inverter.

    So you could well have your inverter happily running on the South array but the 4 panels connected to the other input producing nothing, because the combined output voltage of those 4 panels is not sufficient to start that input / tracker.


    He wants 4 panels, so he would need 4 micro inverters, not 2.
    He can avoid this by buying an inverter that is suited to the task, like I already advised.


    Just so you know, the OP question was about "startup voltage" so my West facing string comment remains.
    In all reality 4 west facing panels would be plenty to power on most dual MPPT inverters and also provide enough voltage to generate, for a typical Solis the minimum is 90v to operate.
    If you want to provide data supporting 4 west facing panels not being able to generate 90v by all means add to the discussion.

    Regarding Micro inverters only two would be required, not four, as there are dual micro inverters that would suffice.
    They would prove more cost effective over another separate inverter and also eliminate the voltage vague possibility.


  • Registered Users Posts: 2,820 ✭✭✭ air


    slave1 wrote: »
    Just so you know, the OP question was about "startup voltage" so my West facing string comment remains.
    In all reality 4 west facing panels would be plenty to power on most dual MPPT inverters and also provide enough voltage to generate, for a typical Solis the minimum is 90v to operate.
    If you want to provide data supporting 4 west facing panels not being able to generate 90v by all means add to the discussion.

    You're still wrong, if you notice, it was I that suggested a Solis inverter precisely because of it's low startup input voltage.
    4 panels is not enough to start the vast majority of inverters, feel free to have a look at their minimum startup voltages to confirm this.

    Your West facing comment is nonsense, simple as that.
    The fact that the overall inverter is already powered up from the Southerly string won't help the Westerly input operate if it's voltage is below the minimum input voltage for the inverter.
    slave1 wrote: »
    Regarding Micro inverters only two would be required, not four, as there are dual micro inverters that would suffice.
    They would prove more cost effective over another separate inverter and also eliminate the voltage vague possibility.
    Cheaper than just selecting the correct inverter for the job in the first place? I doubt it.


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  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    air wrote: »
    You're still wrong, if you notice, it was I that suggested a Solis inverter precisely because of it's low startup input voltage.
    4 panels is not enough to start the vast majority of inverters, feel free to have a look at their minimum startup voltages to confirm this.

    Your West facing comment is nonsense, simple as that.
    The fact that the overall inverter is already powered up from the Southerly string won't help the Westerly input operate if it's voltage is below the minimum input voltage for the inverter.


    Cheaper than just selecting the correct inverter for the job in the first place? I doubt it.

    Can you provide some figures to back up your assertion? I'm finding min. voltage for inverters ranges 90-125v.
    Panels at MPPT are ~35v. so 35v*4= 140v.


  • Registered Users Posts: 2,820 ✭✭✭ air


    Deagol wrote: »
    Can you provide some figures to back up your assertion? I'm finding min. voltage for inverters ranges 90-125v.
    Panels at MPPT are ~35v. so 35v*4= 140v.

    Well first of all, you're mistaken here in your interpretation of the MPPT voltage as it pertains to this discussion.

    A typical panel might have it's Maximum Power Voltage at ~35V (in reality it's less than this for most panels
    33.2V for these common ones on solartricity)
    More importantly however, I suspect you are misunderstanding maximum power voltage.

    This isn't the voltage at which the panels will create maximum power in all conditions - if it was MPP tracking would not exist.
    Indeed in poorer conditions (95% of the time in Ireland for example and even more so at startup) the maximum power voltage (for the prevailing conditions) will be lower than this.

    For the Amerisolar panels linked, I wouldn't budget for more than 30V per panel at startup, so you would need an inverter with a startup voltage of under 120V.
    In reality you would want some margin on this also in my experience.

    Moving on to the second aspect, all of inverters I have used in the past have a higher input voltage than 90V with the exception of the Solis.

    Fronius IG Range (MPP-voltage range 150 - 400 V)
    ^ I tried running one of these on 5 panels and it did not start up at all on dull days.

    Tranergy Range (125 to 450V)


    I haven't bought an inverter in a few years, so if you have links for newer locally available dual MPPT inverters with better voltage ranges, I'm sure they would be of use to the OP as a possible alternative to a dual string Solis.

    I wouldn't consider anything with minimum input voltage above 110V for the application.


  • Registered Users Posts: 407 ✭✭ phester28


    I have 5 east 3 west ameri solar 300W panels.

    The West (900w) starts at 94 volt and stays modulating between 81 and 98V (you get a VOC peak at the turn on state of 94V and that kicks the Mppt on that string into life

    East (1500W) starts at 152V and stays producing until current falls to zero 135V

    Solis on some of its range have the lowest Mppt voltages compared to most other inverters


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    air wrote: »
    Well first of all, you're mistaken here in your interpretation of the MPPT voltage as it pertains to this discussion.

    A typical panel might have it's Maximum Power Voltage at ~35V (in reality it's less than this for most panels
    33.2V for these common ones on solartricity)
    More importantly however, I suspect you are misunderstanding maximum power voltage.

    This isn't the voltage at which the panels will create maximum power in all conditions - if it was MPP tracking would not exist.
    Indeed in poorer conditions (95% of the time in Ireland for example and even more so at startup) the maximum power voltage (for the prevailing conditions) will be lower than this.

    For the Amerisolar panels linked, I wouldn't budget for more than 30V per panel at startup, so you would need an inverter with a startup voltage of under 120V.
    In reality you would want some margin on this also in my experience.

    Moving on to the second aspect, all of inverters I have used in the past have a higher input voltage than 90V with the exception of the Solis.

    Fronius IG Range (MPP-voltage range 150 - 400 V)
    ^ I tried running one of these on 5 panels and it did not start up at all on dull days.

    Tranergy Range (125 to 450V)


    I haven't bought an inverter in a few years, so if you have links for newer locally available dual MPPT inverters with better voltage ranges, I'm sure they would be of use to the OP as a possible alternative to a dual string Solis.

    I wouldn't consider anything with minimum input voltage above 110V for the application.

    Not sure if you're out of date with current tech or I'm misunderstanding something but:

    If I use a solar calculator (https://gold-coast-solar-power-solutions.com.au/posts/solar-panel-minimum-voltage-calculator/

    It shows for instance for QCells cells, will operate at ~125vdc with 4 panels. Well above the Solis 90v MPPT min.

    I also was looking my own panels all day and dividing the numbers out (not sure if that's valid though) gave a minimum value of ~122vdc for 4 panels. Unfortunately, my background in engineering didn't include how solar works so I'm not really comfortable figuring it all out. At the same time, you are being a little vague so I'm struggling to follow the discussion. Can you throw an example out based on datasheets, say from midsummer.ie? Much obliged.


  • Registered Users Posts: 2,820 ✭✭✭ air


    Deagol wrote: »
    Not sure if you're out of date with current tech or I'm misunderstanding something but:

    If I use a solar calculator (https://gold-coast-solar-power-solutions.com.au/posts/solar-panel-minimum-voltage-calculator/

    I'm not sure what it is exactly you think I'm misunderstanding?
    I'm out of date with the very latest specs on inverters as I've stated, but that's about it.
    Deagol wrote: »
    It shows for instance for QCells cells, will operate at ~125vdc with 4 panels. Well above the Solis 90v MPPT min.
    I also was looking my own panels all day and dividing the numbers out (not sure if that's valid though) gave a minimum value of ~122vdc for 4 panels.

    I'm not sure what you are trying to say, I suggested the Solis precisely on the basis that it would work with 4 panels due to it's low startup voltage of 90V.

    I bought one myself for this exact reason as I have split one of my strings into 7 and 4 panels due to shading of the 4 impacting production of the remaining 7.
    I'm speaking from both a technical understanding of the subject at hand as well as direct first hand experience of the exact issue the OP has.

    I'm sure your value of 122v for 4 panels is accurate, but this is during operation and would be lower at startup - which is the issue here.
    Also, even 122V is still too low to run any of the inverters I've listed, apart from the Solis which I have recommended the OP purchase.



    Deagol wrote: »
    Unfortunately, my background in engineering didn't include how solar works so I'm not really comfortable figuring it all out.
    Then maybe hold off on contradicting people that do know how solar works until you've developed a better grasp of the area.
    Deagol wrote: »
    At the same time, you are being a little vague so I'm struggling to follow the discussion. Can you throw an example out based on datasheets, say from midsummer.ie? Much obliged.
    I'm not sure I can be any more clear than what I've outlined above to be honest, it's a fairly comprehensive explanation I provided and I already linked the data sheets for every piece of hardware I referenced in my example.

    Maybe have a read online about the general principal of operation.
    If there is anything in particular in my last post that you find "vague" then feel free to quote it and I will do my best to explain it in greater depth.

    In conclusion, the OP just needs to buy a Solis or other dual string inverter with a minimum operational tracking DC input voltage of 110V or under and he can happily install his Westerly string at any point in the future, and it will work - assuming relatively standard panels (~300W 60 cell) are used.


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  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    air wrote: »
    I'm not sure what it is exactly you think I'm misunderstanding?
    I'm out of date with the very latest specs on inverters as I've stated, but that's about it.



    I'm not sure what you are trying to say, I suggested the Solis precisely on the basis that it would work with 4 panels due to it's low startup voltage of 90V.

    I bought one myself for this exact reason as I have split one of my strings into 7 and 4 panels due to shading of the 4 impacting production of the remaining 7.
    I'm speaking from both a technical understanding of the subject at hand as well as direct first hand experience of the exact issue the OP has.

    I'm sure your value of 122v for 4 panels is accurate, but this is during operation and would be lower at startup - which is the issue here.
    Also, even 122V is still too low to run any of the inverters I've listed, apart from the Solis which I have recommended the OP purchase.





    Then maybe hold off on contradicting people that do know how solar works until you've developed a better grasp of the area.


    I'm not sure I can be any more clear than what I've outlined above to be honest, it's a fairly comprehensive explanation I provided and I already linked the data sheets for every piece of hardware I referenced in my example.

    Maybe have a read online about the general principal of operation.
    If there is anything in particular in my last post that you find "vague" then feel free to quote it and I will do my best to explain it in greater depth.

    In conclusion, the OP just needs to buy a Solis or other dual string inverter with a minimum operational tracking DC input voltage of 110V or under and he can happily install his Westerly string at any point in the future, and it will work - assuming relatively standard panels (~300W 60 cell) are used.

    No need to be so rude mate. I missed the links on the post earlier as mobile's are not easy to see. Adjust the attitude and you might actually come across as helpful not something else.


  • Registered Users Posts: 2,820 ✭✭✭ air


    Think you'll find it was yourself that suggested that I was "out of date with current tech" without bothering to check if you'd read the preceding post apparently.

    You also said that my response was vague which is complete nonsense, it couldn't have been more comprehensive, I explained the full technical background.
    You just dismissed this in its entirety as vague without highlighting any particular element you had issue with to give me the opportunity to clarify.

    So perhaps look in the mirror before you start casting any accusations of people being impolite.


  • Registered Users Posts: 1,584 ✭✭✭ Deagol


    air wrote: »
    Think you'll find it was yourself that suggested that I was "out of date with current tech" without bothering to check if you'd read the preceding post apparently.

    You also said that my response was vague which is complete nonsense, it couldn't have been more comprehensive, I explained the full technical background.
    You just dismissed this in its entirety as vague without highlighting any particular element you had issue with to give me the opportunity to clarify.

    So perhaps look in the mirror before you start casting any accusations of people being impolite.

    Nope, your first answer to Slave1 was rude "A smart answer that only highlights your lack of understanding of the issue at hand, the issue is not "Powering an inverter"."

    I'm not the problem here, you obviously think you're the expert who can't be questioned so I'll leave you alone in your bubble of knowledge. I hope you don't ever have a question that I can help with because I certainly wouldn't be interested in engaging with you as you don't like having anyone question you. I was interested in learning something but I've only learnt one thing on this thread. /unfollow.


  • Registered Users Posts: 2,820 ✭✭✭ air


    Deagol wrote: »
    Nope, your first answer to Slave1 was rude "A smart answer that only highlights your lack of understanding of the issue at hand, the issue is not "Powering an inverter"."

    I'm not the problem here, you obviously think you're the expert who can't be questioned so I'll leave you alone in your bubble of knowledge. I hope you don't ever have a question that I can help with because I certainly wouldn't be interested in engaging with you as you don't like having anyone question you. I was interested in learning something but I've only learnt one thing on this thread. /unfollow.

    I stand by that comment, it was a smart answer by slave1 that did nothing to help the OP and he clearly did not understand the problem the OP had.

    I'm happy to be questioned on anything I post, you're the one that came on here with limited (by your own admission) knowledge of the topic, yet tried to undermine everything I had posted for the OP (with references), none of which you have been able to dispute whatsoever.

    I don't see a single shred of useful information that you have added to the thread yourself.


  • Registered Users Posts: 1,386 ✭✭✭ MAULBROOK


    Hi all,

    Had a conversation with a PV installer today and asked him about over sizing the invertor.

    I want to do this as I can only afford a 2.5kw array for now on the Southern roof, but will probably put up another 4 panels on the western roof next year to take into account the evening sun and when we use the most electricity and stretch that curve out.

    He said not to over spec the invertor, and to just get another invertor next year? Something about the startup voltage on the other string?

    Does this sound right?

    And can all invertors handle multiple strings?

    Thanks!

    You asked a good question and all of a sudden we have handbags a dawn :D:D.
    In answer to your question, go for a Solis 5kw Hybrid inverter and get the highest wattage panels you can get when doing the west side.
    To be honest panels are fairly inexpensive.

    PM sent.


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