Off Grid Inverter advice...

Spikeuk

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Nov 23, 2021
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Hi guys,

I'm trying to piece together a off grid solar system and have some questions so far I have:

I have 10 x 345W solar panels.
Spec:
panels spec 345w.png
Am I correct in thinking I multiply the VOC by number of panels - i.e. 410 VOC requirement for the inverter requirement - so an inverter with 450 VDC would be sufficient to run these pannels?


-I Also have a lifePo4 battery on its way - 48v.

I wanted to get the GroWatt SPF 5000 ES inverter as it looks perfect however unable to find any suppliers in UK - only China, dont want to buy as I can see some peoples feedback where unit has failed and no responses etc. anyone know where I can source in UK or EU?

....Alternatively I found this other 5k inverter with similar specs:


"Maximum PV Array Open Circuit Voltag : 450 VDC" - so this would run the above panels as they come to 410 VOC total?

-What is floating charge voltage? 54v - is this max battery voltage the unit will take?

- Any advice on how should I arrange my panel string - parallel or series ?

- I think this would work with my lifepo4 48v without issue.

- If this sun is shining and power is being produced, will my load draw power directly from the panels - before using the battery? or everything is drawn from battery regardless

- any other recommended inverters (sub £1000).

Thanks guys!!
 

Struc

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Oconomowoc / Land o Lakes WI
I'll comment on one part... How cold does it get where you are? You need to factor in the panel voltage at the coldest extreme for your area to be sure it will not surpass the max voltage of the solar input on the inverter.
 

wattmatters

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Apr 16, 2021
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Am I correct in thinking I multiply the VOC by number of panels
Only for panels in series.

i.e. 410 VOC requirement for the inverter requirement - so an inverter with 450 VDC would be sufficient to run these pannels?
If all 10 panels are wired in series then Voc at STC will be ~420V but Voc at lower temperatures could well exceed 450V.

You can of course create two parallel strings of 5 panels each. That halves the voltage but doubles the current. then it's a function of knowing the current limit of your inverter's MPPT input. It won't damage anything being over spec on the current side, the inverter will just clip production. It's MPPT voltage bounds you need to stay within.
 

MichaelK

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NO, this many panels is going to fry your controller on the first cold day. Use this calculator to determine what your winter low voltage will be.
Plugging in your specs, the 10S1P string will surpass 450V even before it drops to freezing. So, you have to wire your panels 5S2P.

1641690240342.png
1641690363017.png
 

wattmatters

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Plugging in your specs, the 10S1P string will surpass 450V even before it drops to freezing. So, you have to wire your panels 5S2P.
or drop a panel or two from the string if parallel for some reason isn't feasible
 

Spikeuk

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Nov 23, 2021
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Thanks guys thats a great help!

I will do 2 parallel strings of 5 panels in series. I saw a PV combiner box to help reduce cabling and also introduce a number of safety breakers lightening protection/surge etc. Are they worth getting?
There's a cheap Chinease one, AWG of wires might need upgrading but I think its good value.

I'm not sure if I could use it with my array setup?
Each string connection has 10amp fuse, 10 x 230v = 2300
5 x 345w panels = 1725w for each 5 panel string

is this correct, would the below box be suitable?

https://www.amazon.co.uk/ECO-WORTHY...ix=pv+combiner+48v,industrial,346&sr=1-8&th=1



Im in south UK so minimum temperature in uk recorded is -27.2c (-17F) so values in screenshot are valid.

Anyone know if these statements are true:

- If this sun is shining and power is being produced, will my load draw power directly from the panels - before using the battery? or everything is drawn via battery regardless

- Can you run an inverter without batteries?
 

upnorthandpersonal

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- If this sun is shining and power is being produced, will my load draw power directly from the panels - before using the battery? or everything is drawn via battery regardless

Yes, it will draw from solar.

- Can you run an inverter without batteries?

There are some that have that functionality (I think Will reviewed one some time ago), but in general, no.
 

wattmatters

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I saw a PV combiner box to help reduce cabling and also introduce a number of safety breakers lightening protection/surge etc. Are they worth getting?
For two strings in parallel it's not absolutely necessary (for 3 or more it is definitely a good idea) but there are some convenience advantages as it does provide isolation switching at/near the array (for the entire array and also for individual strings) as well as convenient surge protection.

I have the same brand of cheap combiner box, but designed for 4 parallel arrays, not 6 like that box. You only have two arrays so the small box is enough. Mine is 3p2s with 370W Longi panels. There are better combiner boxes but this one was good enough for my purposes. Just check all the screw terminals are tight. Mine were all secure.

Not sure the rating/gauge of wires inside your linked box but I'd be surprised if you could not change or specify the fuses to be 12A if you needed a little headroom for your panel's Imp rating. They are cheap fuses to replace, just those small glass cylinder fuses used in many electrical devices. My one came with 12A fuses and the fuse holders are rated for use with 20A fuses:

IMG_3172.jpeg

IMG_3232.jpegIMG_3231.jpeg
 
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