Greetings!

I'm finalising my PV strings and due to roof size and design, will likely use 2x MPPTs with 5-6 panels per MPPT (i.e. on one string each). Was hoping to also have a second smaller string on one of the MPPTS (4 panels, and reduce other string a bit) to catch more northerly sun, but shading from neighbouring trees may not make this feasible after-all. 

The plan is to use +- 550W panels (probably JA) which have VOC just short of 50V and ISC of 14A.

Since the 8KW SunSynk has MPPT of 22A+22A, does that not mean that in effect with ISC of 14A or thereabouts, one is effectively unable to use two strings on one MPPT? I realise there's clipping, and VOC upper limits may be more important. 

The JA panel specs provide electrical parameters at STC (standard test conditions) as well as at NOCT (Nominal Operating Cell Temperature), with the latter apparently closer to real world operating conditions. The amp spec for the 550W panel is ISC 14A at STC, and just over 11A at NOCT. 2x roughly 11A on one MPPT (at NOCT) seems to be more in spec with the inverter's 22A rating per MPPT. 

Am I missing the point somewhere or is one realistically limited to 1 string per MPPT with these larger panels that have ISC of 13-14 amps? Or is it all moot, as amp clipping will sort this out, and one only worries about not exceeding the VOC spec?

 

1 hour ago, Kalahari Cruiser said:

Am I missing the point somewhere or is one realistically limited to 1 string per MPPT with these larger panels that have ISC of 13-14 amps? Or is it all moot, as amp clipping will sort this out, and one only worries about not exceeding the VOC spec?

I think the point you're missing is that the input current (/operating current) is not the same as the short circuit current (which is the maximum that the current can put out under fault conditions or if you short-circuit the ends so the voltage difference between input and output ends is zero. Your inverter should have a spec for each that it can handle, and it should clip down by about 10% to reach the I

 

max.

 

2 hours ago, Kalahari Cruiser said:

The JA panel specs provide electrical parameters at STC (standard test conditions) as well as at NOCT (Nominal Operating Cell Temperature), with the latter apparently closer to real world operating conditions.

Meanwhile, back in the real world in sunny SA, an irradiance at NOCT at 800W/m2 and 20 deg C seems not normal enough.

 

Personal layman's 2c is to use Imp at STC for matching the panel operating current to the inverter's current spec.

At your own risk, for consultation with an expert, and/or for your installer to confirm.

 

 

3 hours ago, Kalahari Cruiser said:

will likely use 2x MPPTs with 5-6 panels per MPPT

I would suggest you put more panels in series on 1 string. You want to get closer to your 425V max range (without overshooting) to maximize your production day. I would put 7, probably even 8 panels in series on 1 string

2 hours ago, GreenFields said:

I think the point you're missing is that the input current (/operating current) is not the same as the short circuit current (which is the maximum that the current can put out under fault conditions or if you short-circuit the ends so the voltage difference between input and output ends is zero. Your inverter should have a spec for each that it can handle

Thanks. The SunSynk inverter spec (which seems different to the relevant Deye spec) has max PV current of 22A (and Isc of 28A) - Deye seems to have 26 and 44 respectively. If I look at the panel specs, and Imp (rather than Isc), this is 13.11A.  So two strings would add up to 26, meaning the inverter (22A) would clip amps down to 22? That doesn't sound like an ideal scenario surely? I'm still a bit unclear whether 2 strings with 13A (Imp) on a MPPT is consistent with a Sunsynk that has 22A max PV input current. 

 

2 hours ago, GreenFields said:

Personal layman's 2c is to use Imp at STC for matching the panel operating current to the inverter's current spec.

Yes, per above then, at 13A (STC) it would push two strings using 550W panels over the limit, not? 

At NOCT the Imp would be 10.55 ie two strings would be within the 22A spec, if it's a realistic standard (I'm in the Cape Town area). 

1 hour ago, wolfandy said:

I would put 7, probably even 8 panels in series on 1 string

I would probably, but am a bit limited by roof space and/or orientation, unfortunately. May be able to push my panel number slightly to get closer to that voltage sweet spot. 

14 minutes ago, Kalahari Cruiser said:

Thanks. The SunSynk inverter spec (which seems different to the relevant Deye spec) has max PV current of 22A (and Isc of 28A) - Deye seems to have 26 and 44 respectively. If I look at the panel specs, and Imp (rather than Isc), this is 13.11A.  So two strings would add up to 26, meaning the inverter (22A) would clip amps down to 22? That doesn't sound like an ideal scenario surely? I'm still a bit unclear whether 2 strings with 13A (Imp) on a MPPT is consistent with a Sunsynk that has 22A max PV input current. 

 

Yes, per above then, at 13A (STC) it would push two strings using 550W panels over the limit, not? 

At NOCT the Imp would be 10.55 ie two strings would be within the 22A spec, if it's a realistic standard (I'm in the Cape Town area). 

I would probably, but am a bit limited by roof space and/or orientation, unfortunately. May be able to push my panel number slightly to get closer to that voltage sweet spot. 

Most of us use STC values while this is perhaps the value to use in the high irradiation areas in SA. For most of the country the NOCT value would be closer to the real world. Some coastal areas would be even lower. 

7 hours ago, Kalahari Cruiser said:

So two strings would add up to 26, meaning the inverter (22A) would clip amps down to 22? That doesn't sound like an ideal scenario surely? I'm still a bit unclear whether 2 strings with 13A (Imp) on a MPPT is consistent with a Sunsynk that has 22A max PV input current. 

Again, a personal opinion, subject to lots of disclaimers:

On one level, looking at it as you have, the panel with 13A at STC is not perfectly suited to this inverter. Problem is, if you find a panel that has 11A or less at STC, it is likely to be a smaller panel, maybe 400W or less, and you need to begin considering a different configuration, costs, number of panels, and fittings, etc.

How often will you exceed 11A? Even in Cape town, here's the forecast for a day in this week with peak solar irradiation above 900w/m^2, and there may be more such days especially in summer where you may lose around 10% of potential performance at mid-day due to clipping.

But is that relevant? Is it worth it? Will your system be running flat out often enough at that time of day to justify using a different panel? If you were supplementing power for a business, or running a solar farm grid-tied, or charging a big battery bank, maybe?? For general daytime home usage maybe not?? Just some food for thought.

 

18 hours ago, Kalahari Cruiser said:

So two strings would add up to 26, meaning the inverter (22A) would clip amps down to 22?

Correct. In full sun conditions, your inverter would clip each panel at 11A - meaning your panels would only produce max 462W (11 x 41.96)

18 hours ago, Kalahari Cruiser said:

That doesn't sound like an ideal scenario surely?

Yes and no. It is called over-panneling. In summer-time during peak times, you will not be able to utilize the full potential of your panel and will 'lose' 90W of your peak production capability. However, the benefit of doing this is that both during summer-time non-peak times (morning, afternoon) as well as during winter (so basically whenever your panel never even gets to producing 11A / 462W), your panel will produce more power than a smaller panel (that would only produce 11A during peak times).

If the price difference between the 550W panel and a smaller panel is not too much, I would definitely go for the 550W panel.

18 hours ago, Kalahari Cruiser said:

I'm still a bit unclear whether 2 strings with 13A (Imp) on a MPPT is consistent with a Sunsynk that has 22A max PV input current. 

18 hours ago, Kalahari Cruiser said:

I would probably, but am a bit limited by roof space and/or orientation, unfortunately. May be able to push my panel number slightly to get closer to that voltage sweet spot. 

I have to admit that I do not fully understand those 2 statements together. On the one hand you are saying that you want to put 2 strings in parallel on 1 MPPT, on the other hand you are saying that you are limited by roof space / orientation.

Putting 2 strings in parallel only really makes sense if they are the same direction (unless you want to go really wild and put them in directly opposite directions). So if you can put 2 strings of 5 panels into the same direction, I would probably rather put 9 panels on 1 string.

Or what do others think?

2 hours ago, wolfandy said:

Correct. In full sun conditions, your inverter would clip each panel at 11A - meaning your panels would only produce max 462W (11 x 41.96)

Yes and no. It is called over-panneling. In summer-time during peak times, you will not be able to utilize the full potential of your panel and will 'lose' 90W of your peak production capability. However, the benefit of doing this is that both during summer-time non-peak times (morning, afternoon) as well as during winter (so basically whenever your panel never even gets to producing 11A / 462W), your panel will produce more power than a smaller panel (that would only produce 11A during peak times).

If the price difference between the 550W panel and a smaller panel is not too much, I would definitely go for the 550W panel.

I have to admit that I do not fully understand those 2 statements together. On the one hand you are saying that you want to put 2 strings in parallel on 1 MPPT, on the other hand you are saying that you are limited by roof space / orientation.

Putting 2 strings in parallel only really makes sense if they are the same direction (unless you want to go really wild and put them in directly opposite directions). So if you can put 2 strings of 5 panels into the same direction, I would probably rather put 9 panels on 1 string.

Or what do others think?

Very well explained. Unlike going to above or to close to Voc over panelling is good for winter and no harm to inverters when they clip the output on their maximum current. 

We will also see in the specs that a number of inverters will actually go over their maximum for short periods before clipping take place. On some it could be as high as 20% over the maximum. 

My 2c worth. 

Thanks @Scorp007@GreenFields @wolfandy

These are very useful and good inputs, much appreciated. 

15 hours ago, wolfandy said:

I have to admit that I do not fully understand those 2 statements together. On the one hand you are saying that you want to put 2 strings in parallel on 1 MPPT, on the other hand you are saying that you are limited by roof space / orientation.

OK - my roof situation is such that there is no problem having two MPPTs, each with one string (to get series voltage up), comprising 5 and 5-7 panels respectively. I have space for a more optimally facing string comprising 4 panels max across two places but on further investigation have found that one half of this will be partly shaded in winter, and the issue wont get better. So in a sense the point is moot, and a bit theoretical (ie having two strings of panels on an inverter, where the panels can theoretically provide 14amps, thereby hitting the inverters 22A max once doubled up. 

But as was pointed out later, this scenario may only be relevant in summer under optimal conditions for part of the day, a time when efficiency is probably less important than in winter. So while clipping may then occur, it's probably no biggie, and the benefits (per @wolfandy and @Scorp007) of having larger panels during less optimal times make up for this loss in a way. Those radiation stats for Cape Town look interesting @GreenFields. 

Thanks again for the valuable inputs and also some realism here and there - much appreciated. 

Edited January 12, 20232 yr by Kalahari Cruiser