over overpanneling?

[xcentric]

The maximum I can feed my 8kW inverter based on the Voc is 2 strings of 12 panels (405w panels). However, roof would actually take 32 panels. Given I will be going up there to fit rails and panels, is there any point in adding in the extra ones?

House uses typically 3kW/hour near peak, 25kWh/day. I have 15.5kWh of battery and an array I'm currently putting in of 10 450W panels feeding a different inverter. I can export up to 7kW to the grid but no more.

It seems to me that as I'm more or less maxing out both inverters, with spare to charge the battery and export, there's little point in the extra panels. Maybe more battery bank is where finds should be spent? Or am I missing anything?

(limit is from the G98 from the DNO. I did ask for more).
ETA - grid-tied system

 

[hwy17]

12 panels sounds risky unless it's a 1000v system. But I don't know your exact math, it's possible it's ok.

How about 3 strings of 10?

The purpose of overpaneling is to get better production in winter and other off peak generating times.

 

[xcentric]

12 panels sounds risky unless it's a 1000v system. But I don't know your exact math, it's possible it's ok.

How about 3 strings of 10?

The purpose of overpaneling is to get better production in winter and other off peak generating times.

PV Input Voltage 370V (125V~500V)

Open Circuit Voltage - Voc(V)

37.23

12 x Voc = 446.76V

Inverter only has 2 MPPTs.

But that's missing my main q - is it worth putting the extra panels up there, given I'd have to add another inverter?

I guess I could just add a charge controller and pipe it directly to the battery as an additional topup, but I suspect the battery will be pretty topped up anyway. Is there anything else I can usefully do with the power if I did put them up? As it is I think I'll have hot water, battery charged and export.....

 

[pollenface]

But that's missing my main q - is it worth putting the extra panels up there, given I'd have to add another inverter?

Absolutely,

 

[xcentric]

Absolutely/

and use them for what though? it's this bit I'm missing - seems I'm fully powered..... and can't add them to the inverter as is to cope with non-perfect soar days as I'm at the max of the inverters already.... so would you add a charge controller, or what?

 

[pollenface]

and use them for what though? it's this bit I'm missing - seems I'm fully powered..... and can't add them to the inverter as is to cope with non-perfect soar days as I'm at the max of the inverters already.... so would you add a charge controller, or what?

is it worth putting the extra panels up there, given I'd have to add another inverter?

I guess I could just add a charge controller and pipe it directly to the battery as an additional topup

You already said it. If that's you're goal.

The purpose of over-paneling is to increase your nominal output during overcast/rainy days and to generate a bit more when the sun is lower in the sky at morning and afternoon.

I'd suggest if you're worried about being maxxed out, leave it all setup idly by and switch it on during winter when you're unlikely to see maximum outputs from your other arrays.

I believe the panels help cool the roof in summer, providing little shady spots (with an air gap present), thus not going totally to waste

 

[Hedges]

PV Input Voltage 370V (125V~500V)

Open Circuit Voltage - Voc(V)

37.23

12 x Voc = 446.76V

What is temperature coefficient of Voc (on PV panel data sheet)?
What is record coldest temperature ever seen at your location?
You've only got 12% headroom, may be living on the edge.

Inverter only has 2 MPPTs.

So put two strings into each MPPT, maybe 8s2p x 2 = 32 panels.
Ideally, 8s facing one orientation like SE, 8s facing another like SW.

What is Isc, and what is max input current of MPPT? What is max allowed short circuit current?

I guess I could just add a charge controller and pipe it directly to the battery as an additional topup, but I suspect the battery will be pretty topped up anyway. Is there anything else I can usefully do with the power if I did put them up? As it is I think I'll have hot water, battery charged and export.....

Could be useful. But what is battery's preferred charging current, and how does inverter's rating compare?

 

[JRH]

and use them for what though? it's this bit I'm missing - seems I'm fully powered..... and can't add them to the inverter as is to cope with non-perfect soar days as I'm at the max of the inverters already.... so would you add a charge controller, or what?

Absolutely ….I agree… rare is a man involved with a hobby/ prodject and can foresee at first where it will lead ultimately ….

It is almost universal that once you learn up to the next level that you see new ways to do more and other paths to persue…that you didn’t know/think about earlier ….im going through that now…we almost all do…
Consider the extra unused panels as “cash” in a separate bank acct …..just waiting for a new reason to surface that will require it…. Sooner later you will be glad it’s there….haaa…

This stuff is addictive..

 

[wdwtx2.0]

Over-paneling is also a good use of spare panels.
They are already where you need them if any fail.

 

[xcentric]

What is temperature coefficient of Voc (on PV panel data sheet)?
What is record coldest temperature ever seen at your location?
You've only got 12% headroom, may be living on the edge.

yeah realised that: have to add 3.6V to that, which makes it a bit too close for comfort!

So put two strings into each MPPT, maybe 8s2p x 2 = 32 panels.

I assume it's an issue that the spec says
No. of MPPT / Strings Per MPPT 2/1+1
or can I use an external combiner? *(see ETA 2)

Ideally, 8s facing one orientation like SE, 8s facing another like SW.

Single roof span so all same direction I'm afraid. Separate system faces SW.

What is Isc, and what is max input current of MPPT? What is max allowed short circuit current?

PV input current îs 26+26. Max PV Isc of MPPT is 34 + 34. Panel says 13.88 short circuit current.

Could be useful. But what is battery's preferred charging current, and how does inverter's rating compare?

Max charging current from inverter is 135A. Fogstar 15.5kWh battery, can't find preferred charging current but it uses the EVE Grade A LF304 cells.

ETA - panels are likely

https://www.bimblesolar.com/docs/Panther%20405w%20datasheet.pdf

inverter is likely

https://www.bimblesolar.com/docs/8.8kW%20ECCO%20Data%20Sheet.pdf

* ETA 2 - so I assume I can have 2 paralleled strings of 8 cos the current will be within limits and the voltage of the 8 is much more within tolerance? So essentially put them all up there and wire them in as 2 x 8s2p?

 

[Hedges]

Two strings could be paralleled with 8 awg MC4 Wye cables. Always use matching brand or UL listed together, e.g. Stabuli Wye if PV panels have Stabuli. The combined current is rather high for most MC4 (although Stabuli has some high current ones.) Consider cutting MC4 off combined output of Wye, use it for a 2-headed pigtail coming out of a box.

Or use a combiner box.

Current sounds like a great fit for two strings paralleled.

135A/304Ah = 0.44C, probably fine but check specs.
If battery temperature gets cold, that is too high. So if operating near 32F, find recommended max current for that cell and consider dialing down charge current to that level. Some equipment may allow current setting to vary with temperature. But unless you need to recharge in less than 3 hours, consider something like 0.15C that is OK at colder temperatures and stick with that year-round. (If running a generator, you may want to pick an optimum current.

Adding extra SCC might mean excessive charge current. Nice thing about an AIO is it should process extra PV for loads while keeping charge current at a target rate.

Yes, it looks to me like if 32 panels available for this inverter, 2x 8s2p could be a way to go.
Any shadows? Consider them when planning which panels in each string. Keep pairs of paralleled strings partially shaded by similar amounts. Make sure minimum MPPT voltage reached, even when high temperatures reduce Vmp.

 

[xcentric]

Two strings could be paralleled with 8 awg MC4 Wye cables. Always use matching brand or UL listed together, e.g. Stabuli Wye if PV panels have Stabuli. The combined current is rather high for most MC4 (although Stabuli has some high current ones.) Consider cutting MC4 off combined output of Wye, use it for a 2-headed pigtail coming out of a box.

Or use a combiner box.

was planning to use the cables on the panels and a combiner box. Is there a preference for combining near the inverter or near the panels? I guess running less current through more cable is lower loss and less heat issues, so nearer inverter. Also means if it has surge protection built in that it can be easily reset, too. Is there one that can provide monitoring of strings (even into something running on home assistant or similar) at a low price? Essentially reporting voltage and current from each string would be interesting - bluetooth, radio or wifi all fine.

135A/304Ah = 0.44C, probably fine but check specs.
If battery temperature gets cold, that is too high.

Battery has built in heater to help with this. Plan will be relatively slow charge anyway.

Adding extra SCC might mean excessive charge current. Nice thing about an AIO is it should process extra PV for loads while keeping charge current at a target rate.

Yes, it looks to me like if 32 panels available for this inverter, 2x 8s2p could be a way to go.
Any shadows? Consider them when planning which panels in each string. Keep pairs of paralleled strings partially shaded by similar amounts. Make sure minimum MPPT voltage reached, even when high temperatures reduce Vmp.

No shadows which makes things easy.

 

[Hedges]

More wires in a conduit, derate ampacity.
Individual wires per string gives flexibility to connect other inverters or separate MPPT.
When strings have multiple orientations, parallel at array could make better use of copper (one pair of wires at lower current longer, vs. two pairs each peaking at different time.

Both approaches work.

I've used clamp ammeter to check current of individual strings, when I suspected a problem (one turned out to be open circuit. Another time, there were degraded panels which took additional effort to track down.)