[newhaul]

I currently have two 85 watt solar panels I intend to add 100 watts in two panels would it be overkill to install a 30 amp mppt controller or would I be fine with a 20 amp unit can I justify the extra cost?

 

[Don L]

well 2X85+100=270W, 270w/12.5V=21.6amps

so no a 30amp controller isn't an overkill and a 20amp controller wouldn't be enough

 

[newhaul]

Thanks don for some reason my math was off and I came up with 18 amps. 270 watts with 15 watts per amp output

 

[Omatako]

For me the calcs are not that simple.

It would be a very rare day that a solar array performed at 100% so firstly I would rate the output of the said array at 80% so about 220W. So 220w x 12.5v = 17.5A

And also if the battery bank is being constantly charged it will probably be better than 12,5V, maybe 12.8 or 12.9. So 220W x 12.9V = 17A

FWIW this pessimistic approach to solar output appears at least on my boat to be right on the money - I have 2 x 140W panels and a 20A MPPT controller and for a couple of years now it works just fine. My calculation looks like this: 280W x 80% = 224, 224W / 12.9V = 17.37A. I very seldom see the output above 15A even on a hot summer's day.

But if you have some money burning a hole in your pocket, go for the bigger controller.

 

[newhaul]

For me the calcs are not that simple.

It would be a very rare day that a solar array performed at 100% so firstly I would rate the output of the said array at 80% so about 220W. So 220w x 12.5v = 17.5A

And also if the battery bank is being constantly charged it will probably be better than 12,5V, maybe 12.8 or 12.9. So 220W x 12.9V = 17A

FWIW this pessimistic approach to solar output appears at least on my boat to be right on the money - I have 2 x 140W panels and a 20A MPPT controller and for a couple of years now it works just fine. My calculation looks like this: 280W x 80% = 224, 224W / 12.9V = 17.37A. I very seldom see the output above 15A even on a hot summer's day.

But if you have some money burning a hole in your pocket, go for the bigger controller.

This is actually for a customer that really doesn't need the extra power he does just fine as is his needs are about 45 ahr per day on average. They are however going to a 3 to 5 year cruise ( he just retired ) and I want to make sure they have an enjoyable time. As is I'm getting him to change from a shunt controller to mppt. Spending the money anyway-

 

[Don L]

Guess it comes down to whether you are willing to risk an overload. I can easily see a few days of no sun followed by a sunny day when the panels can put out, and the batteries accept, full amps.

Besides what is the cost difference between a 20 & 30 amp controller compared to the work to install etc.

 

[Maine Sail]

This is actually for a customer that really doesn't need the extra power he does just fine as is his needs are about 45 ahr per day on average. They are however going to a 3 to 5 year cruise ( he just retired ) and I want to make sure they have an enjoyable time. As is I'm getting him to change from a shunt controller to mppt. Spending the money anyway-

Be aware that if the panels are not identical panels the MPPT will not perform as well at tracking the maximum power point.

 

[newhaul]

Guess it comes down to whether you are willing to risk an overload. I can easily see a few days of no sun followed by a sunny day when the panels can put out, and the batteries accept, full amps.

Besides what is the cost difference between a 20 & 30 amp controller compared to the work to install etc.

Cost difference is a boat buck. The difference in cost and labor of install is negligible.

 

[newhaul]

Be aware that if the panels are not identical panels the MPPT will not perform as well at tracking the maximum power point.

This will seem like a dummy question but would I be better off having him install another 85 watt unit or using two independent controllers. Cost is minor concern for him.

 

[Omatako]

Guess it comes down to whether you are willing to risk an overload. I can easily see a few days of no sun followed by a sunny day when the panels can put out, and the batteries accept, full amps.

Besides what is the cost difference between a 20 & 30 amp controller compared to the work to install etc.

In my experience the panels will never put out "full amps". I have never seen much better than 80% on mine even in blazing sun directly overhead and 50 amps of draw off the battery bank.

Perhaps the question that should be asked is how big (or small) is the house bank? Is it going to be depleted easily enough to result in a 12V bank that will push the controller past its threshold? If yes, get the bigger controller. My house bank is 650Ah and doesn't deplete that easily.

Given that the OP had two 85W panels to begin with I made the assumption that he already had the 20A controller. In which case the cost of a 30A controller is the additional cost.

If that assumption was wrong then I guess there is no real difference.

 

[Don L]

Cost difference is a boat buck. The difference in cost and labor of install is negligible.

Really?????????????

The 50 amp Outback MPPT controller I got last year is pretty nice with a lot of features and it cost $588. I even paid extra for it to get the features I wanted. How can the difference between a 20 & 30 amp controller cost a boat buck?

BTW- I have seen my 290W single panel put out over 20 amps lots of times.

 

[newhaul]

Really?????????????

The 50 amp Outback MPPT controller I got last year is pretty nice with a lot of features and it cost $588. I even paid extra for it to get the features I wanted. How can the difference between a 20 & 30 amp controller cost a boat buck?

BTW- I have seen my 290W single panel put out over 20 amps lots of times.

Here in the pnw a boat buck is a c note or a hundred bucks.

 

[vtsailguy]

Seeing as we are talking controllers

I have two identical 90W panels, solid ones. I currently have a BZ Products MPPT250 Solar Controller.
BZ Products MPPT250 - 25 Amp 12/24 Volts MPPT Charge Controller

I just added 4 flexible panels, two 30W and two 50W:
Review of Cheap Chinese Alternative to Solbian Flexible Solar Panels ? Sailing with Kids - Caribbean cruising with children

Should I get a second controller? If so, any recommendations? I dread to bring up the series vs parallel wars , but any suggestions on wiring?

 

[Maine Sail]

Seeing as we are talking controllers

I have two identical 90W panels, solid ones. I currently have a BZ Products MPPT250 Solar Controller.
BZ Products MPPT250 - 25 Amp 12/24 Volts MPPT Charge Controller

I just added 4 flexible panels, two 30W and two 50W:
Review of Cheap Chinese Alternative to Solbian Flexible Solar Panels ? Sailing with Kids - Caribbean cruising with children

Should I get a second controller? If so, any recommendations? I dread to bring up the series vs parallel wars , but any suggestions on wiring?

If you want the best performance then the two 30W would get one controller and the two 50W would get one controller. The Genasun GV-10 is really one of the best values for this and is a significantly better MPPT than the BZ... I have had to remove more BZ controllers than any other due to massive amounts of noise at radio frequency. They also are a very slow and antiquated MPPT algorithm, unless this recently changed, and pretty power hungry in sleep mode. I have measured 0.14A in sleep mode on a BZ and in contrast a Genasun draws 0.0009A. If the Genasun sleeps for 12 hours it uses 0.011Ah's. The BZ I measured would use 1.68Ah's..... The very sophisticated Rogue 3048 30A controller uses just 0.009A or approx 0.108 Ah per 12 hours in sleep mode....

There is really no real debate on boats about parallel vs. series. I have only had ONE case where series beat parallel and this was a massive power boat with a 100% unobstructed cabin top. I have yet to have series beat parallel on a sailboat and I suspect much of that is due to shading issues. Yes they do kick in slightly earlier in series, and run slightly later, but the shading aspect really does a number on them when compared to parallel...

Will your existing controller work with all panels in parallel? Yes, but it will not give you all the performance it should or could......

 

[vtsailguy]

Great info MS, thanks.

So you are saying that I should have three controllers total? Each on controlling an identical pair in parallel?

How do I go from those three controllers to the batteries?

 

[Omatako]

BTW- I have seen my 290W single panel put out over 20 amps lots of times.

And so it should. At 290W running into a 12.5v battery bank, the 100% rating is 23.2A

So with a battery bank at 12v and the solars running at 83% the input would be 20A. But at 100% it would be putting in about 24A. If you're getting that then you have an exceptional solar panel, you're very lucky.

Or your battery voltage is lower than you think.

 

[Don L]

And so it should. At 290W running into a 12.5v battery bank, the 100% rating is 23.2A

So with a battery bank at 12v and the solars running at 83% the input would be 20A. But at 100% it would be putting in about 24A. If you're getting that then you have an exceptional solar panel, you're very lucky.

Or your battery voltage is lower than you think.

Guess I'm lucky then! And I'm 110% positive about my battery voltage as I've spent so much time testing those batteries I'm sick of it.

But the comment as about those that seemed to suggest that you didn't have to plan on 100% output because the system never does it, because it does.

Maybe I'm getting some extra because I sized all the wiring etc for 2x290W panels in case I wanted to expand the system later.

 

[Maine Sail]

Great info MS, thanks.

So you are saying that I should have three controllers total? Each on controlling an identical pair in parallel?

How do I go from those three controllers to the batteries?

In all honesty I would suggest using what you have. The outlay in expense for a few % gain in solar performance is really tail chasing at this point..

If you were starting from scratch then we could design a nice system than eek every little bit out of the panels. For now feed them into the BZ and call it a day. As I said it will work, just won't work at its highest potential, which in the grand scheme is not a huge loss...

 

[vtsailguy]

In all honesty I would suggest using what you have. The outlay in expense for a few % gain in solar performance is really tail chasing at this point.

Oh, so just stick all six panels into the BZ MPPT250?

I think I am losing the plot

 

[Maine Sail]

Oh, so just stick all six panels into the BZ MPPT250?

I think I am losing the plot

So the two 90W panels did not have a controller? I thought the BZ was for the new panels and the 90W already had one?