Voc, Vmp, Inverter startup voltage

[Ranging6880]

Hi all,

I did a search and couldn't easily find clarity on this, so apologies if it's already addressed elsewhere.

I am designing a new system, including an inverter with (direct from spec sheet):
Min PPT Voltage: 125 V
Max PPT Voltage: 425 V
DC Startup Voltage: 150 V
DC Shutdown Voltage: 150 V

I only have space for four panels, and likely not behemoths. There will initially be between 1 and 3 Pylontech US5000 batteries associated with the system (4800 Wh, Depth of discharge: 95%, 48V)

What is the relationship between Vmin/mp/oc, Min PPT voltage, and DC Startup voltage? I'm trying to ensure I install an array that can always (bar night) produce enough DC voltage to start the inverter, or if that's not possible given the constraint on panel number and size, designing the array such that I understand when (times of day and year) the array is likely to be non-functional.

Happy to add an additional MPPT direct into the BMS if that solves the issue. Any guidance much appreciated

 

[LydMekk]

My first thought is that this will be very low in PV to charge those 3 batteries unless you mean to use the grid to charge them mainly to shift charge and use to times the grid are cheaper.
4 panels ain't much ^^.

 

[Ranging6880]

Timeshifting from grid is indeed, initially, the main goal. There'll be a second array going into the inverter's second MPPT eventually (with dual axis tracking), but I need to build the building first before I can put that array into service!

I've got the hardpoints in the new roof for these four panels (SW facing), no rails as yet. Right now want to make sure that I put panels up there that actually have a chance of doing something, even if it's not a vast amount!

 

[bt77]

What are the nominal voltages of the panels? You must clear the 150 V threshold with your panels so that the inverter can start using solar power input. If you have space for 4 panels only, you must have at least 37.5 V from each of them so you reach the 150 V. But that would be borderline.

By the how can the DC Shutdown Voltage be the same as DC Start Up voltage? I would expect 500 V or something. Could you check that?

 

[Ranging6880]

Which voltage is the nominal voltage? Voc? [E:] Panels are not bought yet, so there's complete flexibility there too. Currently looking at the highest wattage Jinko Tiger Neo I can fit)

If it's borderline, are there any MPPT-SCCs out there with lower start-up voltages? I could go direct from standalone SCC to batteries, and have the inverter solely dealing with grid-to-battery and battery-to-load

DC startup and shutdown are evidently typos on the vendor's site. Spec sheet says:
PV Input Voltage: 125-500V
MPPT Range: 140-425V
Start-up Voltage: 125V
Full Load DC Voltage Range: 300-425V

 

[Mattb4]

It is not at all unusual to have a higher startup voltage than the minimum voltage. This is because voltage from panels drop the second they become loaded. Ideally you need to have your Vmp above Vmin of the AIO. In practice it is better to have about a 10% cushion above Vmp to ensure adequate range and production. Yes, there are AIO's with lower Vmin requirements (and also lower Vmax) but you have not stated your other requirements for power.

So to sum up keep below Voc of AIO by an adequate amount based on cold weather operation. Keep above Vmp by 10% and you should be in the sweet spot.

 

[Ranging6880]

Bottom line on this all is that am seeking a system that can suck down as much AC as possible in a 4hr window of cheap electricity, while not being oversized etc., deriving some benefit from the roofspace currently available for panels while also being compatible with a larger additional string in future

With that in mind, does this set up make any sense:
4x Jinko Tiger Neo 445WP (JKM445N-54HL4R-V-BF) (Voc 39.59, Vmp 33.02, Vmpcoeff -0.25%)
1x Sunsynk Ecco 5kW inverter (SUN-5K-SG04LP1-EU) (G99 certificate, Zero export possible, 2x MPPTs for this string and another, 25A AC max current both in and out)
3x Pylontech US5000 batteries (95% DoD for 4560kWh useable capacity)

I'm in no way relying on the panels to fill the batteries - they're very much in the "nice to have" category. I'd just like to get a sense of when they're going to be of use and able to start the inverter vs. when they're just dead weight. Is it going to be operational for most of most days in winter? Or are we talking a spring/summer bonus but otherwise not much? Or is it more 1000-1400 in winter, probably, and fine the rest of the year?

Located in SE England; Gatwick is normally the best spot for relevant climate. High temps of ~32C, low temps of ~7C (extremes clipped off)

 

[bt77]

Which voltage is the nominal voltage? Voc? [E:] Panels are not bought yet, so there's complete flexibility there too. Currently looking at the highest wattage Jinko Tiger Neo I can fit)

If it's borderline, are there any MPPT-SCCs out there with lower start-up voltages? I could go direct from standalone SCC to batteries, and have the inverter solely dealing with grid-to-battery and battery-to-load

DC startup and shutdown are evidently typos on the vendor's site. Spec sheet says:
PV Input Voltage: 125-500V
MPPT Range: 140-425V
Start-up Voltage: 125V
Full Load DC Voltage Range: 300-425V

Voc is open circuit voltage. That will be high. There is usually a mention of nominal voltage or maximum power voltage in the data sheet. You should use one of those for your calculations. Attached is a sample data sheet. This one from JA Solar. Just check those specs for the product you wish to buy.
Yes, there will be MPPTs which start with lower voltages. You must just match your panel setup to meet the minimum start up voltage of the Inverter/MPPT. You can do that by adding more panels in series or using panels with higher voltages.

 

[donb108]

I have the same question; putting panels on a shed roof and only have space for four panels. I wanted to use EG4 3000EHV-48, but it's operating range is 120VDC - 450VDC. Typical used, 300-350 watt panels are 30-35 Vmp. So, four panels (4 @ 30Vmp) barely starts the inverter.

Higher watt panels are generally higher voltage. Unfortunately, they're also larger and won't fit onto the roof.

MPP has a similar AIO (3048LV-MK), but it has the opposite issue; max VOC is 145V. Doesn't look like it could handle four panels.

 

[Mattb4]

I have the same question; putting panels on a shed roof and only have space for four panels. I wanted to use EG4 3000EHV-48, but it's operating range is 120VDC - 450VDC. Typical used, 300-350 watt panels are 30-35 Vmp. So, four panels (4 @ 30Vmp) barely starts the inverter.

Higher watt panels are generally higher voltage. Unfortunately, they're also larger and won't fit onto the roof.

MPP has a similar AIO (3048LV-MK), but it has the opposite issue; max VOC is 145V. Doesn't look like it could handle four panels.

Although it might be a higher cost per watt there is advantages in smaller watt panels when it comes to setting voltage. Take for instance a 200w 12v panel of about 1sqm in size. It has a Voc of 21.6v and Vmp of 18v. So to stay below the AIO's rating of 145Vmax you can place 6 in a series string (6 X 21.6v = 129.6Voc) The Vmp is 6 X 18v = 108Vmp. The max amperage is 10.83a. You have an area covered of 6sqm if flat.

 

[donb108]

Although it might be a higher cost per watt there is advantages in smaller watt panels when it comes to setting voltage.

Good idea, but I was trying to stay with mono/half-cut/perc, which are generally 300 watts and higher. I'll keep the smaller panels as plan-b if I can't solve it another way. I could mount a fifth panel on the south facing wall of the shed, but that's kinda lame, and, I'm not sure it would work.

 

[bt77]

Good idea, but I was trying to stay with mono/half-cut/perc, which are generally 300 watts and higher. I'll keep the smaller panels as plan-b if I can't solve it another way. I could mount a fifth panel on the south facing wall of the shed, but that's kinda lame, and, I'm not sure it would work.

That would work to clear the start-up voltage threshold but would create another problem.

Since that 90-degree panel will be inferior in terms of current, its current will dictate the current of the entire system as the panels will be in series. So your entire system will underperform (in terms of current, and then accordingly power) because of that panel on the wall.

So you shouldn't do that.