smallsolar avatar image
smallsolar asked

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:

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


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.


smart solar set-up help
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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).

0 Likes 0 ·

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.

0 Likes 0 ·

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.

0 Likes 0 ·
I am aware we would not just be able to add panels. If we decide in the future to add more panels we would have to go a bigger controller then.

I did check the forums prior to purchasing the charge controller and found that if you have different sunlight characteristics hitting each panel, two charge controllers can be both advantageous and cost effective. (As answered below) We also bought them because the retailer recommended this config, though I am questioning their expertise, hence why the sanity check now.

We go Lifepo4, but the cost and our cold winter climate make them less attractive at the moment.

I am aware you can only use 50% of lead acid normally.

I can't really change the charge controllers at this point, nor does it seem I absolutely need to. I am looking for pros/ cons of the two panels for these charge controllers.

0 Likes 0 ·

It's not just the useable battery capacity. It's also how much current you can draw from the battery and how fast and efficient you can recharge it. Also the need to recharge the lead-acid battery to 100% daily, if possible (to keep it in good shape). Cold environment is a big factor, but lead-acid are not that happy to be frozen, either.

The chargers are not bad, but appear to be undersized for your needs. For a system with a 1300W inverter, I would want at least a 1300W PV array. But maybe the inverter is oversized for your needs and you will not use it to the maximum.

Is it so difficult to install at least two panels in the same plane (facing the same way) in that location?

You can't skimp on PV array size and expect to have plenty of power/energy. Especially in winter (short and maybe cloudy days).

0 Likes 0 ·
Yeah I hear you. But cost is a big factor in non-lead acid batteries and the lead acid are easily attainable locally, plus they handle the cold better.

It is a seasonal/occasional use cabin that power is not mission critical. It is just a bonus that we are not trying to spend an arm and a leg on to get started. If we use it and decide we want more panels then we can add more accordingly with charge controllers if needed.

We could install the panels in the same plane, but I would like to take advantage of the sun as it moves because of our geographic location. Winter is not critical as the cabin will be sparsly used in winter.

0 Likes 0 ·

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

That would mean you can get 2x370W=740W for $636.

Or 3x320W=960W for $682.

0 Likes 0 ·
Ideally, yes. But in this case the retailer only had the two 320w which were mistakenly sent to us(again, don't ask lol) and are a brand I can't find much about. So my choice at this point is to keep the two 320w panels or get the 370 watt panels instead.
0 Likes 0 ·
Keep those two 320W panels (wired in series) for your 100/15 and get one 370Wpanel for the 75/15.
0 Likes 0 ·

Is there no concern about the unknown brand for the 320w panels as well as their lower efficiency? Also wiring them in series I would be leaving about 200w total capacity on the table which doesn't seem cost efficient unless there is significant benefits to having higher voltage from the two panels (better low light performance). Ie I am only going from 740 watts to 810 total watts capacity at an increased cost of an extra $320, so the benefits of the two panels in series would have to be quite significant to warrant that.

0 Likes 0 ·

Is your weather mostly sunny? If not, then those "wasted" 200W will be very useful in cloudy weather (and also when the Sun is not in the best position relative to the panels).

0 Likes 0 ·
During the summer, usually quite hot and sunny. Fall and spring less so, so I suppose that it would be useful at those times. Wish there was a graph to estimate so I could have a better idea if the added cost was worth it.
0 Likes 0 ·
Show more comments
2 Answers
Sarah avatar image
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.

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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?

0 Likes 0 ·
Yes, you are correct. You will however find that the panels get up to operating voltage with minimal levels of sunlight.
0 Likes 0 ·
@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.
0 Likes 0 ·
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.
1 Like 1 ·
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. :)

1 comment
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Haha, yeah my bad, meant west
0 Likes 0 ·