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Panels and Inverter calculation


Hatish
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So I am very new to solar, and am trying to spec a system for my home. I will use an installer, but I am trying to learn for my own knowledge, and to determine if I am getting ripped off. If there are any guides out there to help with determining this, please share. Otherwise below are the details of my own exploration.

 

I am looking at the Sunsynk 8Kw Hybrid Inverter 48v IP65

Panels as follows: ARTsolar – 400 Watt Solar Panel – Mono Percium High Efficiency – Full Black (link)

 

Inverter specs for panel input:

Max power input – 8800W
Max V DC input – 500V DC
Max current input – 18A + 18A
MPPTs – 2

 

First question: The 500V DC input, is that total, or per MPPT?

 

Panel specs (NOCT):

Max wattage    - 302.00 
Max Voltage    - 39.60 
Max power current (amps) - 7.66 

 

So I am trying to figure out how many panels to get, and what is a good combination for them. (I hope I am using the term string below correctly)

If I go with wattage, then I can get 14(serial) x 2 panels. This gives me: 8456W, 554.4V (per string), 7.66A (per string). Voltage is too high, but amps are good.

To improve voltage, I can go 7(serial) x 2(parallel) x 2 panels. This gives me: 8456W, 277.2V (per string), 15.32A (per string). This looks better. But voltage may still be too high, if it is based on the entire system, and not per string.

 

So is my thinking on the panel sizing correct?

What are the hard limitations around voltage, amps, etc?

Can I over-volt the system, and then have some sort of limiter or overflow manager? Maybe that is done automatically by the inverter?

 

Any assistance is appreciated.

 

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42 minutes ago, Hatish said:

Panel specs (NOCT)

You will need to use the STD Voc value of 49.1V to calculate the Max Voltage. You can never "over-volt" the system. That is when the smoke comes out and you need to buy a new one. You can over-current it a bit (within reason <20%). For the current the NOCT value for those panels will be what you can expect to see under normal conditions. It can go a bit higher sometimes.

These inverters are designed to use a large string of panels. That is why you see the big max voltage value with a low current value. The 2 MPPT means you can have 2 strings and each string will be connected to it's own MPPT.

 

51 minutes ago, Hatish said:

14(serial) x 2 panels. This gives me: 8456W, 554.4V (per string), 7.66A (per string)

You can't do this. 

 

52 minutes ago, Hatish said:

7(serial) x 2(parallel) x 2 panels. This gives me: 8456W, 277.2V (per string), 15.32A (per string)

This should be fine

 

I'd go with 9s2p (9 series in 2 parallel strings). 49.1Vx9 = 441.9V with 8Ax2 = 16A for 7000W
This is with 18 panels vs 28 panels which will be much cheaper.

 

Having said all this. Are you sure you need 8kW inverter. I would first look at reducing my usage. This will be much cheaper and less wasteful. Change all your lights to LED, replace some appliances, or looks at optimizing any pumps usage and any heavy loads. This will save you much as you also will need a smaller battery. Read around this forum and the builds section. There are nice things people have shared there.

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Thanks for the feedback :)

Unfortunately, our usage is high, about 50kwh per day, and it really should not be. I am waiting for one of those power usage meters to arrive, the type you can just wrap around any cable to get readings and monitor. Then I can start the exercise of figuring out where my power is disappearing to. But otherwise, we have 2 pumps (pool and water pressure), 2 fridge, 2 freezers, solar geyser with power backup, 3 PCs (only during working hours), 1 LED TV, 1 Washing Machine, 1 Dishwasher, and the usual lights, etc

The 8kw inverter is more for future growth. Installation cost is a consideration, and so I would prefer to do this once, and not have to interfere for some time.

Edited by Hatish
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24 minutes ago, Hatish said:

Thanks for the feedback :)

Unfortunately, our usage is high, about 50kwh per day, and it really should not be. I am waiting for one of those power usage meters to arrive, the type you can just wrap around any cable to get readings and monitor. Then I can start the exercise of figuring out where my power is disappearing to. But otherwise, we have 2 pumps (pool and water pressure), 2 fridge, 2 freezers, solar geyser with power backup, 3 PCs (only during working hours), 1 LED TV, 1 Washing Machine, 1 Dishwasher, and the usual lights, etc

The 8kw inverter is more for future growth. Installation cost is a consideration, and so I would prefer to do this once, and not have to interfere for some time.

I have the 8kw sunsynk inverter and its brilliant for what I need. My usage is also about 45kwh per day and I'm currently generating on a normal day with no clouds in the region of 37kw.

Still have some tweaking to do on my system. 

Current panel configuration. 

2 strings of 7 panels on MPPT 1 

1 string of 11 panels on MPPT 2 

Giving me a total of 7.6kw of panels.

Edited by Achmat
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1 hour ago, Louisvdw said:

You will need to use the STD Voc value of 49.1V to calculate the Max Voltage. You can never "over-volt" the system. That is when the smoke comes out and you need to buy a new one. You can over-current it a bit (within reason <20%). For the current the NOCT value for those panels will be what you can expect to see under normal conditions. It can go a bit higher sometimes.

These inverters are designed to use a large string of panels. That is why you see the big max voltage value with a low current value. The 2 MPPT means you can have 2 strings and each string will be connected to it's own MPPT.

 

You can't do this. 

 

This should be fine

 

I'd go with 9s2p (9 series in 2 parallel strings). 49.1Vx9 = 441.9V with 8Ax2 = 16A for 7000W
This is with 18 panels vs 28 panels which will be much cheaper.

 

Having said all this. Are you sure you need 8kW inverter. I would first look at reducing my usage. This will be much cheaper and less wasteful. Change all your lights to LED, replace some appliances, or looks at optimizing any pumps usage and any heavy loads. This will save you much as you also will need a smaller battery. Read around this forum and the builds section. There are nice things people have shared there.

I'd go with 9s2p (9 series in 2 parallel strings). 49.1Vx9 = 441.9V with 8Ax2 = 16A for 7000W
 

Do you mean 9 in series on each MPPT, since the inverter has 2? Also, where do you get the 8A from in the calculation? The spec says 18A + 18A, which I assume to mean 18A per MPPT. Am I missing something?

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Apologies, I may be making this thread a bit messy now, but lets keep it all in one place.

The plan is to also have battery backup, using 2 of these (48v 120ah 6.1kwh LiFePO4 Wall Mount Solar UPS Battery). Will these work well with this Inverter?

The operating mode of the system (hopefully) is to use the grid as my primary battery, push generated power into the grid as long as batteries are full, and have use from the batteries when grid is off-line, eg load-shedding. Does this make sense? What should I be concerned about?

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2 minutes ago, Hatish said:

Do you mean 9 in series on each MPPT

Yes

 

2 minutes ago, Hatish said:

where do you get the 8A from

When you join 2 power sources (batteries, panels, etc and let's assume they have the same specs) in series the voltage is doubled while the current stays the same.
When you join 2 power sources in parallel, then the voltage stay the same and the current is doubled.

8A is the current you could expect for those ArtSolar panels you are looking at (avg. between the Imp and Isc (NOCT values)).

 

4 minutes ago, Hatish said:

48v 120ah 6.1kwh LiFePO4

These will work, but they do not have communication capability. So the State Of Charge is an estimate from the inverter. Many use these batteries and other without these comms without problems, but you do loose some efficiencies.

 

8 minutes ago, Hatish said:

push generated power into the grid

This is another can of worms. Make sure your local electric authority allow this and get the approved list of inverters from them. 
The list I have from Mosselbay (I think it is the same list from Cape Town) does not list any SunSync inverters as approved (My list is from last year so this might have changed). 

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2 minutes ago, Louisvdw said:

8A is the current you could expect for those ArtSolar panels you are looking at (avg. between the Imp and Isc (NOCT values)).

 

Ahhh, ok cool. I didn't see 8A anywhere, so I was wondering. But now I know :)

 

3 minutes ago, Louisvdw said:

This is another can of worms. Make sure your local electric authority allow this and get the approved list of inverters from them. 
The list I have from Mosselbay (I think it is the same list from Cape Town) does not list any SunSync inverters as approved (My list is from last year so this might have changed). 

Hmmm. I have had two installers tell me that this is not a problem, that my meter is bi-directional, and so it will simply roll back the kwh count. I am in Durban (Ethekwini municipality). Anyone have any more info on this area, do share :)

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2 hours ago, Hatish said:

Hmmm. I have had two installers tell me that this is not a problem, that my meter is bi-directional, and so it will simply roll back the kwh count. I am in Durban (Ethekwini municipality). Anyone have any more info on this area, do share :)

Does this mean you have an old analogue disk meter that runs backwards? Or do you have a more modern electronic bi-directional meter? Either way, maybe ask Ethekwini municipality if they think rolling back your meter is a problem or not.

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11 minutes ago, GreenFields said:

Does this mean you have an old analogue disk meter that runs backwards? Or do you have a more modern electronic bi-directional meter? Either way, maybe ask Ethekwini municipality if they think rolling back your meter is a problem or not.

Yes, its the old analogue meter that spins. I didn't think that pushing power into the grid was an issue, based on what was told to me before. But I am definitely looking into it further.

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