panel strings and Victron SCCs

[Maple]

Hi DIY Solar Folks,

Thanks to your many helpful posts, I am considering adding solar power to my rural home near Madison, WI.

Could you please help me understand panels and SCCs (for an off-grid system for use during a power outage and to reduce electricity bills)?
Suppose I bought this panel:

NE Solar 370W Mono Solar Panel NESW 370MH-M6:
VOC: 41 V
ISC: 11.42 A
Temperature coefficient of VOC -0.29% / degree C
IMP 10.82 A
VMP 34.2 V

The record low in my area is -37 degrees F = -38 degrees C, so I think the VOC increase from 25 degrees C is
(41 V) * (25 - (-38) degrees C) * (0.29% / degree C) = 7.5 V
and the VOC at -38 degrees C is 41 + 7.5 = 48.5 V.

I have a SW-facing standing seam metal gambrel roof on which I could mount panels with S-5-N clamps. (I installed the roof, leaving ridge anchors, and can safely install panels. I'm not excited about leaves, rodents, snow (which slides off the metal and therefore also off panels in reasonable time), and maintenance, but a roof install would save money, install work, and aesthetic cost compared to ground mounts.)

On the top 14-degree roof, I could install 15 panels with these Victron SCCs:
- $184 MPPT 150/35 3s2p 6 panels, VOC=48.5 * 3 = 145.5, ISC=22.84
- $184 MPPT 150/35 3s2p 6 panels, as above
- $184 MPPT 150/35 3s1p 3 panels, VOC=48.5 * 3 = 145.5, ISC=11.42

On the side 70-degree roof, I could install 10 panels:
- $479 MPPT 250/70 5s2p 10 panels, VOC=48.5 * 5 = 242.5, ISC=22.84

On the garage 14-degree roof, which is shaded by the house 2nd floor after late afternoon, I could install 9 panels:
- $184 MPPT 150/35 3s2p 6 panels, as above
- $184 MPPT 150/35 3s1p 3 panels, as above

Have I done these calculations correctly?

Is there a better arrangement of strings and SCCs than what I've proposed?

Thank you.

 

[sunshine_eggo]

Hi DIY Solar Folks,

Thanks to your many helpful posts, I am considering adding solar power to my rural home near Madison, WI.

Could you please help me understand panels and SCCs (for an off-grid system for use during a power outage and to reduce electricity bills)?
Suppose I bought this panel:

NE Solar 370W Mono Solar Panel NESW 370MH-M6:
VOC: 41 V
ISC: 11.42 A
Temperature coefficient of VOC -0.29% / degree C
IMP 10.82 A
VMP 34.2 V

The record low in my area is -37 degrees F = -38 degrees C, so I think the VOC increase from 25 degrees C is
(41 V) * (25 - (-38) degrees C) * (0.29% / degree C) = 7.5 V
and the VOC at -38 degrees C is 41 + 7.5 = 48.5 V.

Yes

I have a SW-facing standing seam metal gambrel roof on which I could mount panels with S-5-N clamps. (I installed the roof, leaving ridge anchors, and can safely install panels. I'm not excited about leaves, rodents, snow (which slides off the metal and therefore also off panels in reasonable time), and maintenance, but a roof install would save money, install work, and aesthetic cost compared to ground mounts.)

Roof installs may be subject to additional requirements including rapid shutdown. If inspections of any kind aren't an issue, and if there is zero risk of insurance not covering a PV started fire, you're probably fine.

On the top 14-degree roof, I could install 15 panels with these Victron SCCs:
- $184 MPPT 150/35 3s2p 6 panels, VOC=48.5 * 3 = 145.5, ISC=22.84
- $184 MPPT 150/35 3s2p 6 panels, as above
- $184 MPPT 150/35 3s1p 3 panels, VOC=48.5 * 3 = 145.5, ISC=11.42

Voltage is right, but the units are rated in OUTPUT current, not input. 35A * 58V = 2030W assuming you have a 48V system (I can't see that you specified).

Since they're all 3S, there is no compelling reason to run them each their own MPPT. A single 150/100 should cover it:

15 * 370W = 5550W

5550/58V = 96A

And your 15 panels are under the 70A VP input current limit.

On the side 70-degree roof, I could install 10 panels:
- $479 MPPT 250/70 5s2p 10 panels, VOC=48.5 * 5 = 242.5, ISC=22.84

250/70 with 10 panels, you're slightly over-paneled, but well within allowances.

On the garage 14-degree roof, which is shaded by the house 2nd floor after late afternoon, I could install 9 panels:
- $184 MPPT 150/35 3s2p 6 panels, as above
- $184 MPPT 150/35 3s1p 3 panels, as above

No reason for separate controllers. A single 150/60 could handle 3S3P

Have I done these calculations correctly?

Voc-wise, yes, but battery voltage is missing.

 

[Maple]

Thanks very much for your reply. I am excited to be figuring it out. I will follow up with questions soon.