5 Questions to Ask Before Going Solar | MIT Federal Credit Union

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For more 36 cell solar panelinformation, please contact us. We will provide professional answers.

Hi all,

I'm building a variation on Will's recent "hand-cart generator" build.

• 8S 280ah EVE cells
• Growatt 24V SPF TL LVM
• Electrodaucs BMS (if they come back in stock)

• Single Phase, fully off-grid system
• To be charged by large gasoline generator when needed
• To be charged by grid-AC when available
• Panels to be ground-mounted only semi-permanently (I live in a rental house)
• My solar availability is fair, but not great. Southeast US.

My question is this: Which combination of 3 or 4 of the santansolar panels would be the best for my application and how should I connect them?

72 cell or 60 cell? and wired in what series/parallel combo?

Price is not the main concern, but of course cheaper is better.

Yes - I know this is not enough panels to support this generator capacity - It's the most I can fit right now, and I will supplement with the Gas Generator.

Thanks!

A BMS is mandatory.

Yes, absolutely. I just meant I want to get the Electrodacus but they are out of stock.

Panel and wiring doesn't matter as long as the OCV is between 36V and 119V (allowing for cold weather use). I'd buy as much wattage as you can afford in terms of $ and/or space.

Perfect, thanks!

Just for redundancy/failsafe use: Do you think any single 72 cell panel would charge the system if I had to. (I know it would be tiny.) They are all rated right around 46-48 VOC

What about a single 60 cell panel? They are all around 36-39 VOC. The user manual on the the Growatt claims "30-115Vdc PV Array Voltage Range"

IMHO, I would go for 4 72S panels in parallel as that will provide a nice Voc as well as making the system extremely shade tolerant if that's a concern.

Shade is definitely a concern so thank you for that.

In parallel the VOC would about 46-48V correct, pushing 36-40A? What constitutes a "nice VOC"? Meaning: Is lower or higher or sweet spot voltage better for my inverter?

Growatt is a light duty unit. It has a very poor surge capacity. Even in grid/generator mode, you will be limited to the 3kW rating, i.e., it will not pass more than 3kW of power through it regardless of the source.

Understood! I don't plan to use it in passthrough mode often, if ever. Just to charge and disconnect. 30-115V is the MPPT range where it can provide the most efficient charging. Can be up to 145V. I limited it to 119V to allow for voltage spikes from cold panels.

A 60S panel will struggle to stay in the MPPT range. It will work, but it will be less efficient.

A single 72S panel would be better.

Any suitable single panel should charge it provided the daily consumption is below what the panel can provide.

Shade destroys panel output and affects all panels in a series. This is because a shaded cells drops its current. In a circuit, the same current has to pass through all components. There are bypass diodes that lessen the impact, but it is still severe.

By putting the 4 panels in parallel, one panel could be shaded, but the others are unaffected.

Yes, your voltage/current numbers are correct. You would likely need 8awg wire instead of the typical 10awg AFTER all the panels are joined into a single pair of +/- wires, i.e., 10awg to connect all the panels in parallel and then 8awg for the single pair to the charge controller input. Since you're not likely to be pushing maximum current for very long during the day, you could get away with 10awg, but 8awg is better. Recommend a 50A breaker in the (+) line between the panels and the SCC.

Nice Voc = a Voc that should be in the MPPT sweet spot in operation. Optimal MPPT efficiency is about 50% higher than the battery charge voltage. Even in the MPPT range, the larger the disparity in voltage, the lower the efficiency. This is a small effect, like around 1% difference, but over many cycles, that could be a meaningful amount. As an example, the 30-115V MPPT range is best around 36V. It's still highly efficient, but it decreases as you increase the input voltage.

That said, I have my 72S panels in series pushing 141Voc even though it would be about 1.5% more efficient around 95V. For me the difference was fitting everything onto one charge controller or needing a second. I gave up the 1.5% for a reason.

Lastly, a SCC can pull the voltage of a higher voltage panel down into the MPPT range, so Voc above the MPPT optimal range isn't generally an issue as Vmp is typically IN the MPPT range.

Fantastic!

That answers all of my questions. The 72 Cell panels are all quite a bit more expensive so I think I'm going to start with 3 and then add on later.

That should give me - watts, at 48V / 30A and all operating independently in parallel - That's exactly what I need.

The add later portion: If you plan to add it to the same series/parallel string, it needs to the same type of panels. Or you can start a new string with similar panels to a different charge controller. Important to match them. So plan ahead and make sure you can get the same panels. Most of the solar panel vendors make different specs in a year or two after production. It gets harder to obtain the same panels.

Here is better explanation by @SolarQueen 's video for mismatched panels.

The add later portion: If you plan to add it to the same series/parallel string, it needs to the same type of panels. Or you can start a new string with similar panels to a different charge controller. Important to match them. So plan ahead and make sure you can get the same panels. Most of the solar panel vendors make different specs in a year or two after production. It gets harder to obtain the same panels.

That's a good point. Plus, if he's buying from Santan, the chance he can buy additional panels of the same thing - a year later - is close to zero.

For a ground based system, I would be purchasing the panels with the biggest bang for the buck. Large panels work well there. On an RV, where you're limited on space, large often doesn't work.

The add later portion: If you plan to add it to the same series/parallel string, it needs to the same type of panels. Or you can start a new string with similar panels to a different charge controller. Important to match them. So plan ahead and make sure you can get the same panels. Most of the solar panel vendors make different specs in a year or two after production. It gets harder to obtain the same panels.

Thank you for the video! I'll watch the whole series later, but I'll ask: When you say "exactly" what do you mean?

Her videos leads me to believe using wildly different panels will cause big losses, which makes sense.

But just using the santan solar example:

If I buy one of their:
72 C / 365W / 48V / 9.24A panels today.

and next year I add one and they have:
72 C / 335W / 46.1V / 8.85 panels

That is not a big deal right? Yes there will be slightly less efficient but I imagine still function fine, right?

There is no safety concern there correct?

It's hard to tell on forums what people mean when you "must" do something.

In my mind:

I must do certain things for safety.
I should do certain things for efficiency.

That's a good point. Plus, if he's buying from Santan, the chance he can buy additional panels of the same thing - a year later - is close to zero.

For a ground based system, I would be purchasing the panels with the biggest bang for the buck. Large panels work well there. On an RV, where you're limited on space, large often doesn't work.

I was just throwing a year out there as "random date in the future", but I take your point.

my question from my last post is same: Wouldn't most 72 Cell, 350W-ish, 48V-ish, 8-10A-ish commercial panels all function pretty much the same when I string them together.

Again - This is a backup, "portable" generator. No grid-tie. No mission-critical loads, etc. Hi there. Yes, that example was showing wildly different volts and amps. I have some other videos showing same amps different volts, and visa versa. The system will work best with all the same, and worst with all different. It'll work ok with just one difference, the closer you can get them the better. If the 60 cell panels are much less per watt, so you can get 4 instead of 3, I'd recommend doing 2 parallel strings of 2 in series. Just know that you need to keep the strings even, so if you buy more, you'll need to buy in sets of 2.

Thank you for the video! I'll watch the whole series later, but I'll ask: When you say "exactly" what do you mean?

Her videos leads me to believe using wildly different panels will cause big losses, which makes sense.

But just using the santan solar example:

If I buy one of their:
72 C / 365W / 48V / 9.24A panels today.

and next year I add one and they have:
72 C / 335W / 46.1V / 8.85 panels

That is not a big deal right? Yes there will be slightly less efficient but I imagine still function fine, right?

There is no safety concern there correct?

It's hard to tell on forums what people mean when you "must" do something.

In my mind:

I must do certain things for safety.
I should do certain things for efficiency.

Per the other replies, it best to be matched or close to the other numbers and not far off will be okay.

The video is the extreme example of mismatched panels.

For the portable generator, the 60/72 cells are heavy 40+ lbs and takes up space. Most people opt for few 100W or under 200W compact panels. Easier to move around.

Hi all,

I'm building a variation on Will's recent "hand-cart generator" build.

• 8S 280ah EVE cells
• Growatt 24V SPF TL LVM
• Electrodaucs BMS (if they come back in stock)

• Single Phase, fully off-grid system
• To be charged by large gasoline generator when needed
• To be charged by grid-AC when available
• Panels to be ground-mounted only semi-permanently (I live in a rental house)
• My solar availability is fair, but not great. Southeast US.

My question is this: Which combination of 3 or 4 of the santansolar panels would be the best for my application and how should I connect them?

72 cell or 60 cell? and wired in what series/parallel combo?

Price is not the main concern, but of course cheaper is better.

Yes - I know this is not enough panels to support this generator capacity - It's the most I can fit right now, and I will supplement with the Gas Generator.

Thanks!

Look at Hitech Solar. They are a panel manufacture in Indiana and build a good panel.

Hi there. Yes, that example was showing wildly different volts and amps. I have some other videos showing same amps different volts, and visa versa. The system will work best with all the same, and worst with all different. It'll work ok with just one difference, the closer you can get them the better. If the 60 cell panels are much less per watt, so you can get 4 instead of 3, I'd recommend doing 2 parallel strings of 2 in series. Just know that you need to keep the strings even, so if you buy more, you'll need to buy in sets of 2.

Makes sense - and thank you for the excellent videos you all do!