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Old 07-08-2017, 05:07   #1
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Solar design - 12 Volt or 24 Volt?

I am currently considering buying 250 watt panels that put out 24 volts instead of 12. Will I have added expenses for a 24 volt inverter and battery storage system?

I have room for 12 panels (39" X 65") or perhaps 3000 watts.

If I'm interested in pushing everything over to electric. This will include future conversion to an electric auxiliary. Does 48 Volt have an advantage for propulsion? Can the house bank be the propulsion bank? I mean reuse start bank area for bigger house bank.

I am willing to buy a new controller etc as part of the upgrade. I noticed the Costco golf cart battery suggestion in another post. I hear good things about the Outback controller.

Do these household Rules of Thumb also apply onboard?

12V 1,000W max
24V 2,000W max
48V 4,000W max

Finally if you could design a 12 panel system from the battery box up, what components would you select?
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Old 07-08-2017, 05:26   #2
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Re: Solar design - 12 Volt or 24 Volt?

I think you'll find that most solar panels are rated way above the intended use voltage. For example, panels to be used on a 12v system can often put out 24v or much higher depending on their size. It is your MPPT controller that takes all the voltage it can get and then reduces it to 12v, or thereabouts, for charging purposes. It is more efficient for the controller to receive maximum voltage.
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Old 07-08-2017, 05:35   #3
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Re: Solar design - 12 Volt or 24 Volt?

I'm not familiar with your rules of thumb. In fact I can't think of how to apply them at all!

If your considering electric propulsion, 48 volts trumps lower voltages any day of the week.
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Old 07-08-2017, 05:59   #4
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Re: Solar design - 12 Volt or 24 Volt?

"Do these household Rules of Thumb also apply onboard?

12V 1,000W max
24V 2,000W max
48V 4,000W max"

Systems over 48 volts are quite rare I believe. A 3000 watt system would clearly realize large gains from a 48 volt system. How can you do 3000 watts on a boat? That would be highly unusual.

Personally, I am inclined to consider 24 volts upon reaching 600 watts or so. It doubles the capacity of wiring and the charge controller output capacity. Useful if more panels are added. Also tends to lower parallel battery connections which is desirable.

24 volts can be advantageous over 48 volt when the battery system is composed of largish individual 2 volt cells. It is also difficult to find quality smaller 48 volt inverters.
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Old 07-08-2017, 07:34   #5
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Re: Solar design - 12 Volt or 24 Volt?

These days of MPPT controllers, the "nominal voltage" concept becomes less relevant. FYI 12V panels are 18-22V, 24V get up into the 40's.

Some of the best panel deals were originally designed for home grid-tie usage and get much higher.

The Victron controller line are great quality and give a lot of flexibility, even the smaller 75/15 (under $100!) will handle up to 75V total Voc - do not exceed that spec, may damage the controller.

The second number is maximum **output** 15 amps. You can go right up to 20A on the "lsc" input side, and such "over dimensioning" will increase the total real-life power output per controller, especially good for getting more amps into a bank earlier in the morning when its acceptance rate is higher.

But any output over 15A, in peak conditions, will be discarded.

The ideal config against partial shading is one SC per panel, and at least with the Victron line that is not crazy cost-wise, since the next step up, 100/30 cost over twice as much.

But once you get up to the highest units, they do get the option of fitting external temp sensing wires, which makes compensation more accurate.

They all handle 12 vs 24V banks. Going higher than that increases shock danger and increases infrastructure cost with the exception of wire gauge.
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Old 07-08-2017, 07:53   #6
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Re: Solar design - 12 Volt or 24 Volt?

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Originally Posted by SecondBase View Post
Does 48 Volt have an advantage for propulsion? Can the house bank be the propulsion bank? I mean reuse start bank area for bigger house bank.
Propulsion electric means no fossil fuel driving the boat. That would be IMO the only reason to go over 24V.

Yes one design I like is a Main (house) bank, and a smaller Reserve, capable of running the critical engine nav bilge and safety loads if the Main bank goes down.

Starters are then wired with switches so they can easily run off either bank. This saves a lot of dead weight carried by boats with dedicated starter banks. Windlass, bilges, etc also don't get any dedicated banks, all consolidated into Main.

Both banks could be joined into one big one in normal high SoC conditions, very efficient (see Peukert's Law), and the batts will last longer due to shallower draw down. There should be an accurate SoC monitor or LVD isolating Reserve at a pretty high setting to ensure it never gets drawn down too far by the non-essential House loads.

Note the above paragraph is an unorthodox idea, and an experienced marine electrician should at least sign off on the design and inspect the final implementation.

Finally, note that is not ideal for members of the same bank to be separated by different wiring lengths/gauges.
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Old 07-08-2017, 07:56   #7
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Re: Solar design - 12 Volt or 24 Volt?

Quote:
Originally Posted by SecondBase View Post
I have room for 12 panels (39" X 65") or perhaps 3000 watts.
Panels come in a huge variety of shapes and sizes. Can you please link to the detailed specs of the panels you're talking about?
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Old 07-08-2017, 08:51   #8
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Re: Solar design - 12 Volt or 24 Volt?

The Outback MPPT controller only works when the input voltage is over 15 volts. Therefore, the higher the voltage of the panels, like 24 volts, you can have the panels connected individually to the Outback. If you use 12 volt panels they need to be in series in order to use during low output. When placed in series the shading of one of the panels will reduce the output of the individual series. Parallel connections are much better
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Old 07-08-2017, 09:15   #9
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Re: Solar design - 12 Volt or 24 Volt?

Great ideas
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Old 07-08-2017, 09:18   #10
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Re: Solar design - 12 Volt or 24 Volt?

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Originally Posted by Henk Koornstra View Post
The Outback MPPT controller only works when the input voltage is over 15 volts.
True also for all other mainstream SCs, including PWM.

There are no quality "12V" panels with that low a voltage. Most are over 18V. That's what "nominal 12V" means.

The cheap little trickle ones that connect to a starter batt with no controller don't count.


That is why this
Quote:
Originally Posted by Henk Koornstra View Post
If you use 12 volt panels they need to be in series in order to use during low output.
is false

Quote:
Originally Posted by Henk Koornstra View Post
Parallel connections are much better
Yes, but best is one controller per panel
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Old 07-08-2017, 09:58   #11
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Re: Solar design - 12 Volt or 24 Volt?

Look at " SUN POWER " solar panels one of the strongest watt per inch !
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Old 07-08-2017, 10:15   #12
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Re: Solar design - 12 Volt or 24 Volt?

Quote:
Originally Posted by john61ct View Post
Panels come in a huge variety of shapes and sizes. Can you please link to the detailed specs of the panels you're talking about?
I am curious what boat can support such a large solar array.

60' catamaran possibly?
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Old 07-08-2017, 13:31   #13
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Re: Solar design - 12 Volt or 24 Volt?

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Originally Posted by Symphony View Post
Great ideas


Indeed. Great Ideas. Thanks for the suggestions and frankly education for me here.

Does this calculator make sense based on sailor's experiences? Or is there a better calculator to use?

https://www.batterystuff.com/kb/tool...alculator.html
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Old 07-08-2017, 13:48   #14
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Re: Solar design - 12 Volt or 24 Volt?

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Originally Posted by Sailmonkey View Post
I'm not familiar with your rules of thumb. In fact I can't think of how to apply them at all!

If your considering electric propulsion, 48 volts trumps lower voltages any day of the week.

I didn't understand them either. There was a point about increasing wire gauge but that may not apply as much on the water.

Thanks - leaning towards 48V
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Old 07-08-2017, 14:56   #15
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Re: Solar design - 12 Volt or 24 Volt?

I would not consider to mix house bank and propulsion system together .
12 volts on boats is common for many reasons, safe voltage in salty enviroment is one of them. Future propulsion system would be considered based on required range and required voltage for motor invertor. That would be quite separate design.
Perhaps avoiding hundreds of kg led acid batteries , LiFe batteries might be considered.
It isn't too difficult to design, brushless motors have range invertors to choose from.
You select what is more suitable considering bat. storage and charging .
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