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Another noob: adding panels to my inverter/battery


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I bought an Axpert-like 3kVA solar inverter from GeeWiz some time back, to use as a load shedding backup controller. It has saved us many times! The original 2x 12V 100Ah Royal 1150K batteries have been doubled up (one each old and new battery in each bank). This runs a fridge, two freezers, the garage door opener and the internet. Occasionally the TV and stereo.
But a recent 20-hour maintenance power outage has got me thinking that it is time to add some solar panels.
I'm a solar noob, but my understanding is that the PV-1800's MPPT is rated at 30-130V 80A. What is the best configuration of panels to make best use of the existing controller? Can I run two or more strings of three panels on one MPPT? Or will three panels in series exceed the voltage rating under no-load conditions?

I realise that at some point I'm going to want to upsize - better batteries, more capacity, more power out, and at least two MPPTs (house faces east-west). But for now I just want a starter-for-one investment that will help to keep us sane through the next round of load shedding.

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3 hours ago, moron51 said:

but my understanding is that the PV-1800's MPPT is rated at 30-130V 80A. What is the best configuration of panels to make best use of the existing controller? Can I run two or more strings of three panels on one MPPT?

Can you please post a picture of the sticker on the side of the inverter for us to see the specs. If I am not mistaken, these PV-1800 models were clones of the Axperts. If this is correct, I would suggest you go 2 series in all your strings. 

On 72 cell panels you will be over 130 volts with 3 in series. 

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Welcome to the forums!

I believe I had a similar setup to your from Geewiz, it was a Must inverter and definitely a clone. I wasn't too concerned about solar for mine but when I sold it I'm not sure it actually had an option for MPPT.

May be well off, but maybe post some pics of the bottom of the inverter too?

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21 hours ago, Jaco de Jongh said:

On 72 cell panels you will be over 130 volts with 3 in series. 

Agreed. However, 3S of 60-cell panels is OK, and indeed pretty much required. 2S of 60-cell panels would be too low in voltage outside of good solar conditions, which is at least half the time. With too-low panel voltage, you'll get poor to zero output, despite moderate power being available. [ Edit: Oops; the above is true for 48 V inverter-chargers, but I see that you have a 24 V model that works down to some 30 VDC (probably depending on battery voltage). So 2S of any panels would be OK for you, unless you might upgrade to a 48 V inverter-charger one day. But 3S is a lot easier to wire than 2S, saving some money and time, so it may be worth hunting down 60-cell panels so that you can wire 3S. ]

I note that 60-cell panels are less common (but Canadian Solar up to about 320 W or so often are). Also, you often have to dig to find out whether they are 60-cell, 72-cell, or other (I've seen 66-cell panels, 11 x 6 cells). Sometimes it's easier to work it out from a spec like Voc; 72-cell panels are often about 40 V Voc or higher; 60-cell panels usually have a Voc of 38 V or lower.

If the number of cells is over 100, it's likely to be a half-cell paralleled design. So a 144-cell (really 144 half-cell) panel is equivalent to a 72-cell panel, and so on.

Edited by Coulomb
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Thanks guys. So I'm hearing maybe six (or nine?) 60-cell panels wired in strings of three, to deliver a best-case 1.8 kWp (2.7 with nine panels). That sounds like it will charge my existing 4.8kW batteries while running average daytime loads. With some fat cables from the roof into the house.

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Thanks guys. So I'm hearing maybe six (or nine?) 60-cell panels wired in strings of three, to deliver a best-case 1.8 kWp (2.7 with nine panels). That sounds like it will charge my existing 4.8kW batteries while running average daytime loads. With some fat cables from the roof into the house.

Update - one more thing...

My east-facing roof is surrounded by trees, so wherever I put the panels, some of them will be in shadow for some of the day. Am I better off then wiring all my panels in parallel (it's a 24V system), to avoid one shaded panel shutting down an entire string? Fatter cables...

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On 2020/05/10 at 1:41 PM, moron51 said:

but my understanding is that the PV-1800's MPPT is rated at 30-130V 80A.

It says it can charge 80 A. Your battery voltage is 24 V so 80 x 24 = 1920 W. Nothing is said about what you can connect on the PV side in terms of W or A so you have to now what type of panels you want to use first.

6 hours ago, moron51 said:

My east-facing roof is surrounded by trees, so wherever I put the panels, some of them will be in shadow for some of the day. Am I better off then wiring all my panels in parallel (it's a 24V system), to avoid one shaded panel shutting down an entire string?

Well what you can do is find out what the best config is. 9 Panels in parallel is a lot and will give you appr. 90 A which is a lot and a (too?) low voltage, not desirable. So go 2 in series when you have a lot of shading. Again, find out what panels you want to use. Then, try to figure out how you can put them on the roof so 1 serie gets shade as a whole (both panels) that serie won't do much but it will not drag the other series down. If you have 1 panel in a series of 2 shaded it will give you the same result as the series with 2 shaded panels. Hope you understand what I mean.

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On 2020/05/15 at 10:48 PM, RikH said:

It says it can charge 80 A. Your battery voltage is 24 V so 80 x 24 = 1920 W. Nothing is said about what you can connect on the PV side in terms of W or A so you have to now what type of panels you want to use first.

Well what you can do is find out what the best config is. 9 Panels in parallel is a lot and will give you appr. 90 A which is a lot and a (too?) low voltage, not desirable. So go 2 in series when you have a lot of shading. Again, find out what panels you want to use. Then, try to figure out how you can put them on the roof so 1 serie gets shade as a whole (both panels) that serie won't do much but it will not drag the other series down. If you have 1 panel in a series of 2 shaded it will give you the same result as the series with 2 shaded panels. Hope you understand what I mean.

RikH, I do indeed understand! And thanks. This inverter is designed for 24V battery systems, so theoretically a parallel row of panels (maybe 8, since the rated max PV current is 80A) should deliver enough voltage, if the Vmp is around 30-35V. Yes, that's a lot of current, and I'd need to run some heavy cables. But it would mean that all panels that get sun would contribute to the total. As soon as I start arranging panels in strings, my risk of panels shutting down due to shade increases - if I understand correctly? One shaded panel shuts down two panels.

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