Sizing attempt #2 w/ string config

[chrisjx]

I started my sizing project with a 30kWh system. Now I'm doubling it to about 60kWh. I'll be in the countryside but will have access to the internet via t-moble. I'm a software devops person so I run a somewhat substantial homelab that'll draw some load. BTW, location is in Washington County, Texas. Open field, no shade, ground mount system and about 30' away, a covered/shaded 20' container to hold the equipment.

Something I can't quite understand is STC or NMOT? I've based the calcs on NMOT to be conservative, but in some of my calcs it runs the panel count up over 100. So some guidance there would be helpful.

High Level Summary​

1. Solar Panels: 60 REC420AA-PURE-2 panels
   • Power Output: 19.2 kW (60 panels * 320W)
2. String Configuration: 2 panels per string
   • Voltage per String (VMPP): 94.2V
   • Current per String (IMPP): 6.88A
   • Total Number of Strings: 30 strings
3. Combiner Boxes: 6 combiner boxes, each handling 5 strings
   • Voltage: 94.2V
   • Current: 34.4A
4. Inverters: 3 EG4 6000XP inverters
   • Each Inverter:
     • MPPT 1: 94.2V, 34.4A (Combiner Box 1, 3, or 5)
     • MPPT 2: 94.2V, 34.4A (Combiner Box 2, 4, or 6)
   • Total Input Power per Inverter: 3.24kW per MPPT * 2 = 6.48kW per inverter (well within the 8kW MPPT capacity)
   • Total AC Output Capacity: 18kW (6kW per inverter * 3 inverters)
5. Battery Bank: 13 x 48V 100Ah batteries
   • Total Capacity: 62.4kWh (48V * 1300Ah)

In this setup I plan to use the new (coming soon) REC420AA-PURE-2. I was working with the specs of the REC420AA-PURE-R but they seem to have been bought up.

Finally, spoiler alert, I used chatgpt to run some of these numbers, but garbage in, garbage, out. Don't blame chatgpt if these numbers are crazy, because I gave it the info.

Thanks,
Chris.

REC420AA-PURE-2 Panel Specifications​

NMOT Values:

• Power at MPP (PMPP): 320W
• Short Circuit Current (ISC): 8.68A
• Open Circuit Voltage (VOC): 46.0V
• Current at MPP (IMPP): 6.88A
• Voltage at MPP (VMPP): 47.1V
• Panel Efficiency: 21.7%

Rated Array Value​

Total Array Power = 60 panels * 320W = 19,200W

So, the rated array value would be: 19.2 kW.

Energy Generation​

Using the average daily irradiance, worst case, for December (3.1 kWh/m²/day):

1. Daily Output per Panel in December:

Daily Output per Panel = 320W * 3.1 / 1000 = 0.992 kWh/day per panel

1. Total Daily Output for 60 Panels in December:

Total Daily Output = 60 * 0.992 = 59.52 kWh/day

Since the battery bank capacity is about 62.4 kWh, this configuration will cover most of the required energy even in December, the worst month for irradiance. The generator would be used to cover what's missing.

Optimal String Count and Combiner Boxes​

String Configuration:

• Panels per String: 2
• Voltage per String (VMPP): 47.1V * 2 = 94.2V
• Current per String (IMPP): 6.88A
• Total Number of Strings: 60 panels / 2 panels per string = 30 strings

Combiner Box Configuration:

• Combiner Boxes: To evenly distribute the strings and optimize for the 3 inverters, 6 combiner boxes with 5 strings each can be used.

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Inverter Configuration with 3x EG4 6000XP Inverters​

Each EG4 6000XP inverter has 2 MPPT inputs. Each MPPT input will be connected to one combiner box, ensuring that each inverter handles PV input from 2 combiner boxes.

Inverter Specifications:

• AC Input Voltage: 120/240VAC
• Max. AC Input Power: 9000W
• AC Output Voltage: 120/240VAC
• Nominal Power Output: 6000W
• Surge Capacity: 12,000W for approximately 3.5 seconds, 11,000W for approximately 5 seconds
• PV Input Data:
  • Number of MPPTs: 2
  • Max Usable Input Current: 17A per MPPT
  • Max Short Circuit Input Current: 25A per MPPT
  • DC Input Voltage Range: 100-480V
  • MPP Operating Voltage Range: 120-385V
  • Max Utilized Solar Power: 8000W (4000W per MPPT)
• Battery Data:
  • Max Discharge Current: 140A
  • Max Charge Current: 125A
  • Nominal Voltage: 48V

Inverter Configuration:

• Inverter 1:
  • MPPT 1: Combiner Box 1 (5 strings, 34.4A)
  • MPPT 2: Combiner Box 2 (5 strings, 34.4A)
• Inverter 2:
  • MPPT 1: Combiner Box 3 (5 strings, 34.4A)
  • MPPT 2: Combiner Box 4 (5 strings, 34.4A)
• Inverter 3:
  • MPPT 1: Combiner Box 5 (5 strings, 34.4A)
  • MPPT 2: Combiner Box 6 (5 strings, 34.4A)

Safety Margins and Cold Weather Implications​

Temperature Coefficient of VOC: -0.29%/°C

To account for colder weather, check that the VOC of the panels does not exceed the maximum input voltage of the MPPT charge controllers. Calculating the maximum VOC for each configuration at -10°C (14°F):

VOC Increase Calculation​

• Standard Test Condition (STC) Temperature: 25°C
• Temperature Difference: -10°C - 25°C = -35°C
• VOC Increase: VOC * (1 + (Temperature Coefficient * Temperature Difference))
• Panel VOC: 46.0V

VOC Increase = 46.0V * (1 + (-0.0029 * -35)) = 46.0V * 1.1015 ≈ 50.67V

Maximum VOC per String Configuration​

• 2 Panels in Series:
  • VOC per String at -10°C = 50.67V * 2 = 101.34V

MPPT Capacity of Each Inverter​

Each EG4 6000XP inverter's MPPT can handle:

• Maximum DC Voltage: 480V
• Maximum Current: 17A per MPPT (usable input current)
• Maximum Power Input: 8kW per MPPT

In this configuration:

• Input Voltage per MPPT: 94.2V (VMPP)
• Maximum Input Voltage per MPPT (at -10°C): 101.34V
• Input Current per MPPT: 34.4A
• Power Input per MPPT: 94.2V * 34.4A ≈ 3.24kW (well within the 8kW capacity)

Battery Configuration​

• Batteries: 13 SOK 48V 100Ah PRO Server Rack Batteries
  • Total Battery Capacity: 48V * 1300Ah = 62.4kWh

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​

 

[ricardocello]

Set aside the difference between NMOT and STC for the moment.

Your 92V VOC per string at 25 degC will never generate any power on the 6000XP MPPTs!
You should first maximize the number of panels per string to never exceed 480V at your coldest temperature.
Keeping in mind that everything over 385V will be lost power (see table below).

With 60 panels, I would guess you could do something like 8 panels per string.
String VOC 368V at 25 degC, 405V at the -10C you specified (8*50.67).
No damage occurs until 480V, so you could even push it to 9 panels per string. But not 10.
Someone will chime in with a better analysis I'm sure. There are online calculators as well.

Now once you've done that, you realize that 8 or 9 do not divide into 60.
Either keep spares, or buy more to get to a good divisor.

So 7 strings of 8 is 56 panels, and we call that 8s7p.

Or 6 strings of 9 is 54 panels. That means no combiner boxes, because each 6000XP has two MPPTs, and you said you want 3 inverters.
So each inverter MPPT would handle one string. 9s per MPPT.


EG4 6000XP

 

[BentleyJ]

Didn't read the whole thing, stopped at the first section where the proposal is 2 panels per string. This will not satisfy the minimum start up requirement of 100V. The MPPT input range is 120V to 385V with a max of 480V. The only way to get into this range is with more panels in series.

Also the Amperage limit on each MPPT input is 25A with 17A usable. The proposed 34 A per input basically means 1/2 the available power at peak time of the day will be ignored and wasted assuming the inverter can handle the over paneling.

 

[chrisjx]

BentleyJ, ricardocello,​

Thanks for you help. Looking at my setup, I'm now looking at different ways to configure strings. Ultimately I'm trying to support about 60kWh per day. I know I should be thinking in terms of days of autonomy, but I think 60 panels is about the right size. Now to get the strings right for 3 inverters w/ 2 mppts each.

VOC Increase per panel?=49.1V×(1+(−0.0029×35))=49.1V×1.1015≈54.06V ?
Total Voltage per String (8 Panels): 8 * 54.06V = 432.48V
Total Voltage per String (9 Panels): 9 * 54.06V = 486.54 <<--- oops...

Next I have planned for 3 inverters with 2mppts each. So I need to end up with 6 strings to feed the 6 mppts, right? For the balanced symmetry?
8*6 = 48 panels = 6 strings into 6 mppt <-- too small
6*12 = 72 panels = 12 strings into 3x4 string combiner boxes into 6 mppts? <-- too big

Thanks for any advice, Chris.

 

[BentleyJ]

60 panels / 6 inputs = 10 panels per input. 5S2P configuration will work perfectly. Vmpp = 230V and Impp = 13.7A This fits well within the MPP voltage range of the inverter. 120V to 385V and up to 17A usable per input.

Would use a combiner box (or 2) with double pole breakers for each string of 5.

 

[ricardocello]

60 panels / 6 inputs = 10 panels per input. 5S2P configuration will work perfectly. Vmpp = 230V and Impp = 13.7A This fits well within the MPP voltage range of the inverter. 120V to 385V and up to 17A usable per input.

Would use a combiner box (or 2) with double pole breakers for each string of 5.

I like this math better, it’s a more natural fit for the inverter specs.

 

[chrisjx]

That sounds great. I love it.

You're suggesting 5S2P which means 5 panels in series and each 2 of them in parallel, making a 10 panel count which fits perfectly into 60 panels. I had not gotten into the parallel mindset. This feels very symmetrical and efficient.

Now I've got 6 sets of 10 panels with each set comprised of 2 series strings which are paralleled.

Would use a combiner box (or 2) with double pole breakers for each string of 5.

Finally, 2 combiner boxes with 3 double pole breakers for each set of 10 panels and each individual breaker is for a single 5 panel series string. Do I have that right?

Thanks so much.