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RCT Axpert MKSII 5KVA/5KW Inverter - minimum PV array?


MKRandburg
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Hi,

I'm pretty new to this solar thing.

I'm thinking of getting this inverter: RCT Axpert MKSII 5KVA/5KW Inverter 48VDC 4500W 450VDC MPPT

But I'm on a budget, so what is the minimum solar array I can hook up to it? And how do I figure that out.

I'm under the impression that these types of inverters have a minimum that they need to operate.

I won't be using batteries but will by grid tied. 

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Rated Power 5000VA / 5000W 
Maximum PV Array Power 4500W
MPPT Range @ Operating Voltage 120VDC~430VDC
Maximum PV Array Open Circuit Voltage 450VDC
Maximum Solar Charge Current 80A
Maximum AC Charge Current 80A

You could do 6-8 300-400 watt panels.

 

I started with 6 and the inverter would just go into bypass mode with loads higher that what the panels could supply and on cloudy days that's around 300-400 watts with 6 panels. It really depends on what you are trying to do. If you are not buying batteries then I would buy as many panels as is reasonable to avoid paying for installation on panels again. If you are doing the installation yourself than it doesn't matter, you buy and install as funds allow.

80A

 

 

 

                                                                                                                                                                                                                                                                                                                                                                   

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1 hour ago, Tariq said:

what about the VMIII models, they do a sort of blending of pv and grid AND don't need batteries

I was thinking about a second hand one, so wanted to check that I can do what I want to do on that one. 

1 hour ago, Buyeye said:

to avoid paying for installation on panels again

But @Buyeye has a good point. My plan was to slowly buy more panels as my budget allowed (but I didn't consider installation costs - although I'm pretty handy, I might be able to do subsequent ones myself). Which is why I want to go without batteries. I have a generator for load shedding but would prefer to use solar for day time load shedding (and save some Eskom pennies on non-loadshedding days). It's a 2.2kw genny, so I know what I can manage on that. 

Edited by MKRandburg
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On 2021/04/26 at 12:19 PM, Buyeye said:

You could do 6-8 300-400 watt panels.

How do you work this out? Which numbers should I be worrying about? 

Let me check I understand you correctly, the number I need to worry about is the number of watts my panels are putting out vs whatever load I have hooked up to the inverter. So if the load on the inverter is higher then the panel, this inverter will just switch over to grid power and I lose whatever my panels are producing? It won't blend the 2 supplies. It's either or.

So is this the same as what I was initially worried about, that if the panels don't produce enough watts the inverter doesn't work? Just the above is relative to the panels?

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@MKRandburg I recommended the number of panels based on mppt operating voltage range and power that the panels would be producing. The panels are the cheapest part of the install so it makes no sense to underspec  on panels.

on the specific inverter you want you cannot blend pv and grid so you will lose the pv production. 

Please have a look at this inverter's features https://www.solarwaysuppliers.co.za/product/sorotec-5-5kw-revo-ii-on-off-grid-hybrid-inverter-demo/

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2 minutes ago, ThatGuy said:

The solar panels will have a power rating (in Watts) as well as an open-circuit voltage rating (in Volts). You need to put solar panels in series (sometimes called strings) so that the voltages add up to a number between 120V - 430V (the MPPT range of your inverter). Safest is to aim for the middle of this number, so around 300V. Most of the solar panels I've seen in the 300-400W range have a voltage a little lower than 50V, which is why it was recommended to get 6 to 8:        6x50V = 300V, Or 8x50V = 400V.

After the panels have been chosen, you can then add the power together (example 350W panels): 6x350W = 2100W, or 8x350W = 2800W. Generally it's good practice to add a little bit (~7%) headway onto this number, because it is possible that in very cold clear weather you could develop more than the rated power of the panels (2800W x 1.07 = 2996W). This number must be lower than the max PV array power of your inverter (in this case 4500W).

If you want more power you have to add additional panels in parallel so that the voltage doesn't increase. You will need to keep the voltages of the strings equal, so if you have 8 panels in the first string then you need 8 in the second string. However this might mean that your power rating is then too high for the inverter (2x2996W = 5992W > 4500W). Therefore when considering multiple strings, you're better of "working backwards" to first determine how many panels you need and then divide by the number of strings to tell you the voltage (EG 350W panels):          4500W / (350Wx1.07) ~ 12 Panels -> 2 Strings of 6 panels

To "get up and running" you'd need minimum 3 panels, but as others have suggested you'll probably need (or want!) more almost immediately. Although the panels are rated at EG 350W, you'll probably only get 280W out of them most of the time and almost nothing when a cloud passes over. I'd recommend at least 6, but size so you can add a second string later if you want. Just bear in mind if you want to add the 2nd string yourself: You'll need to be comfortable working at heights with a 2m x 1m wind catcher that's live as soon as the sun hits it - fair warning, this is the only part of the install that I would personally refuse to do myself :)

Question: I've got a different inverter than the one you've specified (but also made by Voltronic as yours is). Mine is a lower voltage version, but simply doesn't work without a battery (the whole unit shuts down). Can someone who owns this particular model confirm that batteryless operation on this inverter is even possible?

Thank you @ThatGuy what a great explanation! Much appreciated. 
I hope your question gets answered.

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4 hours ago, ThatGuy said:

Question: I've got a different inverter than the one you've specified (but also made by Voltronic as yours is). Mine is a lower voltage version, but simply doesn't work without a battery (the whole unit shuts down). Can someone who owns this particular model confirm that batteryless operation on this inverter is even possible?

I can confirm that batteryless operation is not possible on models with a 145 V max solar charge controller.

If you are asking whether it really is possible to run the higher voltage SCC models batteryless, yes, several users have reported this. It's much better with AC-in available, so it can still power loads when PV is insufficient (by blending AC-in power with PV power). It almost becomes like a grid-feed inverter in these conditions. At least, that's how I assume it works, without owning one, and without studying the firmware in great detail to confirm it.

 

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I have a generator for load shedding. I wanted to start with solar to reduce costs and eventually once the budget allows replace the generator with batteries for a more environmentally friendly load shedding option. 

So you reckon this invertor won't do what I want because it does not blend the grid power with solar... because it sounds like I need to be able to blend. Or get a small battery to buffer when a cloud goes over? 

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