New system-specific questions

[Bluedog225]

Good afternoon,

I’d appreciate some input on a basic (but large) off-grid setup.

I have not purchased anything. I’ve got a large (33’x33‘) flat, no-shade, south facing, metal roof on small cabin. Very difficult grid access.

Here is what I am thinking (all from Will’s site):

-31x (1 pallet)-440W Monocrystalline 72 Split-Cell Silver Solar Panel by Solarever-13,640 theoretical watts. $6,169 USD
-31x-IQ 7 microinverters (maybe wait for IQ8) ($187*31)-$5,797 USD
-1x-Sol-Ark 12k-$6,850
-1x-Energetech 13.4 KWh 48V 280Ah LiFePO4 lithium battery=$5,090 USD

Total (without installation): $23,900
Underground service is about the same for an 850 foot run.

This seems too simple (expensive but simple). A couple of specific questions:

1. Am I right in thinking that the Sol-Ark can accept AC the full input from the microinverters? I think it can accept about 12,000 watts which sounds about right.

2. Is there any reason to wait for the IQ8 with this off-grid setup? I suspect I will never be grid-tied.

3. With microinverters, do I simply tie the panel output into a ”regular” main panel junction box? Then feed the Sol-ark.

4. Can I expand my battery capacity with this setup? I have no critical loads, except for the 1000w mini-split air conditioner. The rest is manageable. It’s a cabin. But if needed, I’d like to be able to buy more battery capacity to keep the place cool longer.

Is this a workable set-up or am I missing the boat somewhere? I’m fairly new and still stumbling over some of the terminology.

Thanks

Tom

 

[FilterGuy]

IQ 7 microinverters

For the microinverters to turn on, they have to 'see' what they believe is grid power. I think that means they have to be tied to the output of the Sol-ark. I would have to go study the sol-ark again but I don't think the sol-ark would charge your batteries with that set-up.

I believe the Solark has MPPT inputs, but I don't know how many watts it can handle. Why not just use the sol-ark MPPT? (You could add an additional MPPT controller and split up your solar array if the Sol-ark can't handle all of it.

 

[Bluedog225]

I was afraid of that. I thought, perhaps, the battery could back feed the inverters and allow it to run in all situations except for a “black” start. Maybe not.

I’m not sure what is meant by MPPT inputs. I’m trying to get to ac from the panels on. Safer and simpler (for me). Additionally, the inverter will be about 40+ feet from the panels.

Thanks

 

[FilterGuy]

I’m not sure what is meant by MPPT inputs.

The sol-ark has solar controllers built in. The MPPT inputs are the solar inputs.

The following is from the Sol-ark 12K (Indoor) manual:

10. Solar Panel Install
1. Sol-Ark 12K has 2 separate pairs of solar panel inputs. (Dual MPPT)
3. Max PV input:
13,000W(+/- 5%) per system (6,500W per MPPT) PV = 500Voc Max
Max Isc input per MPPT: 25A (self-limiting to 20A @450Voc/300Vmp or 18A @ 500Voc/400Vmp)
Note: Damage will occur if PV Voc > 550V.

Note that the max solar input is 13,000W. As long as you don't exceed the 550V input you can probably over-panel to the 13,640 theoretical of your pallet of panels. (If you want to be completely safe, just don't use two of the panels)

Additionally, the inverter will be about 40+ feet from the panels.

I would do as many panels in series as you can. This means high voltage, low loss coming from the roof to the Sol-Ark. (Even less than if you had 220V coming from the micro-inverters)

Lets do some math.
I don't know your climate so I will assume pretty much a worst case for cold and de-rate the input voltage by 25%
That means we should use .75 * 500V = 375V for the max operating voltage and .75 * 550V = 412V for the 'no damage' voltage.
The panels you referenced have a Voc of 49.62V. Consequently the max number of panels for normal operation is 375/49.62=7.6 (Round down to 7) panels. The max number in series without damage is 412/49.62= 8.3 (round down to 8) panels. (Note: I am not sure what the inverter does if the input is between 500 and 550V.... It may shut off charging.)

You have 31 panels and 2 MPPT inputs.... Playing around with it, One possible arrangement is to put two parallel strings of 7 panels in series on each MPPT. (This would only use 28 of your 31 panels). Each input would have 6160W and the total watts would be 12,320.
There are a couple nice things about this arrangement.
- the operating Vmp would be 40.99V x 7 = 287V. Therefor the line-loss back to the inverter will be fairly low without going to super large wires.
- With only two strings in parallel, you do not have to fuse the strings. (but I would be tempted to anyway)

Note: I did not play with it a lot. There may be other arrangements that would use more of the panels without exceeding the Sol-Ark input spec.
As an example, 3 strings of 5 on each MPPT input would use 30 panels. This would not exceed the input voltage but would be more wattage than the SolArk spec calls for. Normally over-paneling like this is perfectly fine as long as you don't exceed the input voltage.....but I would check with Sol-ark to make sure it is OK. The working voltage at the input will be ~205V so line loss is going to be minimal. Also note that 3 strings in parallel means you must fuse each string.

You may want to review this:

Fusing & Wire Sizing guidelines for solar panels

This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what size the OCPDs need to be. There is also guidelines for wire sizing for the...

diysolarforum.com

 

[Bluedog225]

This is very helpful. Thanks. I will study this and make sure I understand.