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Panel choice for Smart solar 75/15 or 100/15

I am setting up a small off grid setup for a cabin. I have two Smart solar controllers, one 100/15 and one 75/15 (don't ask) hooked up to a 24 v system that will each control one panel. I think I am over-thinking panel choices and am looking for a sanity check. I have a choice currently of these two panels:

http://www.solarland.com/upload/file/2018/11/07/10064036489.pdf

320 W, 44.97 V VoC, 37.20 Vimp, 9.11 A Isc, 72 cell

or

https://hespv.ca/hesproductspecs/LONGi/Longi-LR4-60HPH-355-385.pdf

370 W, 38.3 Voc (NOCT), 32.0 Vimp (NOCT), 9.25 A Isc (NOCT), 60 cell

The 370W longi cells are a little more expensive but a newer and more well known brand, but I am worried that the voltages are too low for a 24v system with one controller per panel. Some help and the pros/cons of using each panel would be appreciated. I used the Victron MPPT sizing calculator and it shows both these panels as compatible with my controllers but would like a sanity check.

TIA

smart solar set-up help
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seb71 avatar image seb71 commented ·

The 370W longi cells are a little more expensive

More expensive in price per panel or price per Watt?

You are right that higher PV voltage (but still under the charger limit) would be better.

What type of battery (lead-acid, LiFePO4)?


Can you sell the two small chargers and get a bigger one instead? That would be the best option. And more than two PV panels. 650W-700W PV is not that much, even for a small cabin (unless you only want a few LED lights and similar small loads, without an inverter).

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smallsolar avatar image smallsolar seb71 commented ·

More expensive per panel and per watt. The 370 watt panels will probably be around $0.86 per watt and the 320 watt panels will be $0.71 per watt.

They will be lead acid, at least to start off.

It is quite a small cabin, and will only have a few led lights and other small draws, total estimated daily load would probably never go higher than 1 kWh. We have a Phoenix 1200w inverter coming for the system.

We went with the dual charge controllers to increase flexibility of the system (scalable if we need more panels), budget, better production for different light conditions, and the smaller controllers have a load output. It was my understanding that the smart solar controllers communicate so it should be doable if not ideal.

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seb71 avatar image seb71 smallsolar commented ·

With a 24V (nominal) battery, each of those two chargers can output about 430W at maximum. So there is not a lot of room to grow just by adding panels.


For the 100/15 charger, you might consider buying two of those 60-cell panels (and wire them in series, both placed in the same plane) from the start. The PV array would be slightly oversized in good weather for that 100/15, but in cloudly weather might be worth it. Not ideal, but you should have asked before purchasing anything.


Also consider going for LiFePO4. You can get a smaller capacity than lead acid, for the same performance. For instance 50Ah/24V (and limit the loads to sensible values). Of course, bigger capacity would be better, if you can afford it. But make sure the battery are kept above at least 5 Celsius.

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Sarah answered ·

The panel voltage needs to be 5v above the battery voltage so either panel would be fine. Given that limited power of the panels I would opt for the more efficient 370 w panels. Having two seperate controllers can be advantageous if the panels are mounted in different locations with differing levels of sunlight.

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smallsolar avatar image smallsolar commented ·

Yes we will have one panel south facing to catch morning sun and the other more *west to catch the later sun better.


I was just concerned about the longI panels being only 32 Vimp which is only about 7 volts above battery voltage. Am I correct that the only downside of the lower voltage is that they will just require more sun before starting the controller and producing power than the 320 w panels but will then produce more power once they actually start?

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Sarah avatar image Sarah smallsolar commented ·
Yes, you are correct. You will however find that the panels get up to operating voltage with minimal levels of sunlight.
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Sarah avatar image Sarah Sarah commented ·
@SmallSolar - I have been giving this some more thought. Unfortunately the manual doesn’t specify what voltage we need to exceed - a possible range of 24 to 28 volts. Early in the morning you will be in bulk charge when the battery is at its lowest, as the day proceeds the voltage will typically increase up to the absorption voltage (likely 28-29) volts. At this point you will likely no longer have the five volt margin. To be on the safe side I would defer to the lower output - higher voltage panel. Sorry for misleading you I was thinking of a 12 volt system which has half the voltage range.
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smallsolar avatar image smallsolar Sarah commented ·
Yeah I am going with the smaller panels to maximize the use of one controller. I will have two panels on the one controller, not sure yet whether series or parallel (see above comments about VoC when temperatures decrease) No worries. The +5 volts is only required to start charging/the controller once charging has begun you only need +1 volt over battery voltage.
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seb71 avatar image
seb71 answered ·

Yes we will have one panel south facing to catch morning sun and the other more east to catch the later sun better.

That must be a strange place if the Sun is rising form the South and is moving East. :)

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smallsolar avatar image smallsolar commented ·
Haha, yeah my bad, meant west
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