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Old 03-22-2015, 03:41 PM   #1
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Solar panel recomendations

I am looking to to install a 12v solar system. I would like to run the fridge(4.5a), house lights, 12 volt tv and water. I think two 140 watt panels should do the trick w two 75ah agm batteries. At dock we have no hook ups and would like to power the fridge during the week while we are at work. Any ideas or pointers would be great.

I have been on emarine.com
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Old 03-22-2015, 04:20 PM   #2
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Your electrical appliances are going to consume 75-150 amp hours each 24 hour period. Two 140 watt panels will produce about 100 amp hours on a sunny day. So you may need more battery capacity to stay on the hook more than a day or two. I would suggest 4 golf cart batteries wired series/parallel which will give you 440 amp hours of capacity or at least a pair for 220 AHs. Depending on what your actual usage is and sunshine available you may be able to stay on the hook for almost a week without any other charging with 440 AH of batteries.

During the week since you are not opening your fridge it will run on a very reduced duty cycle and consume maybe 30 amphours daily. Your 2-140 watt panels should be able to keep up with this easily even with several overcast days.

I don't think that there is much difference among solar panel manufacturers. I had a Kyocera 135 watt panel that is nice because it has a junction box that you can easily wire to. But MC-4 connectors now make it fairly easy to wire up.

You will need a controller. A decent PWM controller will cost $100 or so but you will lose 25% of the rated output. A MPPT controller will only lose about 5% but will cost double. Blue Sky and Morningstar are good brands. Don't buy Chinese controllers.

The controller should be rated for 20 amps output and 24 volts maximum input if you buy 12V nominal panels. If you buy 24 volt panels the max voltage spec will double.

Wire it with 12 gauge from each panel to a junction box or MC-4 splitter then 8 gauge (or even #6 if it is a long run) to the controller and #8 (or #6 if a long run) to the batteries. Put a 20 amp fuse near the batteries.

David
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Old 03-22-2015, 04:58 PM   #3
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Thanks for the reply David, I will take your advice on the battery banks. I build elevators for a living and do alot of wiring. The larger gauge wire wont be a problem to acquire from the "combiner box" to the controller. The MC4 connectors look expensive, I see the benefit in them just to parallel the two panels.
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Old 03-22-2015, 05:48 PM   #4
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Excellent advice from David.

I would highly recommend an MPPT charge controller. On a boat the limiting factory for solar is pretty much always space. So the challenge is to get the most power from the available physical space. On land, the goal is usually getting the lowest cost per watt, so people look at things differently. Using glass panels rather than flexible, and an MPPT charge controller is the best way to maximized power per square foot.

That said, another member here just completed an outstanding installation of flexible panels integrated into his Bimini. In that case using flexible panels allowed him to use space that would otherwise have been unsuitable for rigid panels, so it was the right tradeoff.

So I guess the message is that the "right" thing will depend very much on your boat. Just keep in mind the goal of getting the most power out of the available space.
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Old 03-22-2015, 06:16 PM   #5
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Do the 12v dc fridge's have a low voltage cutoff so the compressor doesnt smoke?
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Old 03-22-2015, 07:38 PM   #6
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Do the 12v dc fridge's have a low voltage cutoff so the compressor doesnt smoke?
Apparently some do based on prior posts, but you can easily add one if you can't confirm..
I plan to wire this into my fridge circuit..

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Old 03-22-2015, 07:38 PM   #7
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Do the 12v dc fridge's have a low voltage cutoff so the compressor doesnt smoke?
Danfoss compressors (BD50F and BD35F) have a low voltage shutoff of 10.6 VDC at the module. Once it shuts down, it won't start again till it sees 11.7 volts.
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Old 03-22-2015, 07:40 PM   #8
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This is a interesting thread. I have done this with my boat and have 4x 235 watt panels with a Maverick MPPT controller. I also have 10x T105 plus 6v batteries. (1125amps 20hr rate). At times in the Caribbean I am producing 50-60 amps. However the reality is I produce 11/2-2kw a day. This runs comfortably my inverter a large household type fridge 110v and a small wine fridge. My wife also cooks lunchs etc in 110vac skillets to save on starting the genny. It also runs all lights 12vdc stuff , music extensive lights in engine room. Most of the considerations have been covered by previous members. However my guess is your right on the limit for 24 hr fridge running.
Whereas you do not need sun for a panel to work , you need a very bright sky. With all respect to New york its never impressed me with the right light to effectively run panels particularly in winter. On a previuos boat in Bermuda where I had a similar setup 4x 185 watt panels and MPPT controller. I installed a doc watson. This meter did incredibly well and allowed me to monitor my amp production. However That 1000 miles north hampered KW output to 2-4kw per week, really suffered with wx.
New panels are more effective than 3-5 years ago, and MMPT controllers area necessity for reasonable output.
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Old 03-23-2015, 06:43 PM   #9
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We added two 80W monocrystaline panels about 9 years ago and they have been amazing. We use a Steca charge controller.

In summer, we run two DC fridges and a DC freezer and only need the genset on very cloudy days. Obviously, they are not so good in winter but we don't stay out for so long then.

I was told then that monocrystaline panels are more expensive but much more effective than the polycrystaline. Not sure if this is still the case as technology has improved and costs have halved.
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Old 03-23-2015, 06:49 PM   #10
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Uh...they should have spell checked the label before hitting print
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Old 03-24-2015, 05:40 AM   #11
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You will need a controller. A decent PWM controller will cost $100 or so but you will lose 25% of the rated output. A MPPT controller will only lose about 5% but will cost double. Blue Sky and Morningstar are good brands.

David
Could you explain this? It seems very important and something I was not aware of.
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Old 03-24-2015, 10:27 AM   #12
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Could you explain this? It seems very important and something I was not aware of.
OK, here is a short tutorial on charge controllers:

You need a charge controller because solar panels are essentially a constant current producing device (with constant sun). They produce almost the same current from zero volts (the I short circuit value) up to about 22 volts for 12V panels. This is the Ioc value. If you were to just hook up a panel directly to the batteries the voltage would start near 12 and would rise until the electrolyte started boiling at about 14 volts.

Also solar panels are rated at the maximum power point (MPPT) which is about 17 volts. Battery charging can't efficiently use 17 volts and that is another place that controllers come in.

So a controller does a couple of things. It adjusts the voltage to keep from over charging batteries. And a MPPT transforms the voltage to more efficiently use the MPPT.

There are two types of charge controller in wide spread usage: pulse width modulation (PWM) controllers and MPPT controllers. The difference in the two is how they transform the input voltage.

The PWM controller takes the incoming voltage and pulses it to produce the output voltage that the battery needs- typically 12 to 14 volts. But since the solar panel isn't operating at its MPPT the extra power that it could be making at 17 volts is lost. Typically that loss is 1- 13/17 = 24%.

So the industry came out with MPPT controllers. These are much more expensive than PWM controllers. They let the solar panel operate at its MPPT of about 17 volts and converts it using an inverter/rectifier circuit to whatever the battery needs- 12 to 14 volts. They are about 95% efficient so you only lose about 5% of the MPPT power.

MPPT controllers are relatively expensive and solar panels have gotten relatively cheap. So if you have unlimited real estate a PWM controller can be cheaper overall, Just add solar panels to make up for the 25% loss. But if you have only so much real estate which is the case on a boat, a MPPT controller makes sense.

If you buy 24V panels (twice the number of cells) then you should use a MPPT controller if you have a 12V battery system. Othewise you will lose more than half of the solar panel's output.

There are two critical specs for charge controllers: the maximum current ouput that it can handle and the maximum input voltage it can handle. The maximum current value is approximately the panels power rating divided by 12. The maximum voltage is roughly 22 V for 12 panels and 44 for 24 volt panels. These values are given on spec sheets as Voc.

It is usually recommended to buy a controller with a 20% safety factor on the maximum current spec.

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Old 03-24-2015, 12:50 PM   #13
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Surperb info, David. Thanks to Marty for asking the question. I have panels now, but want to increase the system's output with new panels and controller. This info really helps! I cleared my 8.5 x 12.5 ft. roof for panels only.
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Old 03-27-2015, 12:58 PM   #14
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Solar panel recomendations

Yes, thanks David. Is the MPPT controller sized for the wattage of the panels based on 12 volts for the battery or at 17 volts for the output of the panels?

For example: 420 watts of panels @ 12v = 35 amps, @ 17 volts = 25 amps.

It makes difference in cost.


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Old 03-27-2015, 02:14 PM   #15
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In addition, I understand that some of the mppt controllers can cause VHF interference. Are there controllers that are certified to be free from VHF radio interference?


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Old 03-27-2015, 03:30 PM   #16
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Yes, thanks David. Is the MPPT controller sized for the wattage of the panels based on 12 volts for the battery or at 17 volts for the output of the panels?

For example: 420 watts of panels @ 12v = 35 amps, @ 17 volts = 25 amps.
Controllers, whether MPPT or PWM are rated as the maximum current output that they can handle, not input. So the first equation above gives you the current rating that you need. But add 20% for safety. For example if you are charging a dead battery starting at 10 volts, the current will be 20% higher. Doesn't happen often and should never happen, but sometimes it does.

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Old 03-28-2015, 04:49 PM   #17
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I have a 80" by 80" area. I think i am going w/ two of the 250 watt Kyocera panels. 8.39a each. A Blue sky 3000 mttp boost controller. and 2 125ah agm batteries in parallel. This mainship has one battery per engine. I am wondering if i should use the new agm batteries in place of the "engine" batteries or create a new bank and isolate them?
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Old 04-01-2015, 08:12 PM   #18
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I have a 80" by 80" area I think i am going w/ two of the 250 watt Kyocera panels. 8.39a each. A Blue sky 3000 mttp boost controller. and 2 125ah agm batteries in parallel. This mainship has one battery per engine. I am wondering if i should use the new agm batteries in place of the "engine" batteries or create a new bank and isolate them?
Depends on how you cruise. But I am assuming that since you are installing solar panels you want to hang out on the hook for several days at a time, right?

So install your two 125 AGMs as a separate bank dedicated to house use. Isolate them from the starting batteries so that you don't run them down and leave you stranded after several days on the hook. A $65 battery combiner is a cheap way to isolate the house batteries but combine them with the engine start battery when it is running. See yandina.com.

With that much solar you could run down both new AGMs each night to 50% (don't go lower for best life) and recharge them the next day to near 100% if you have good sun. If no sun then you are going to need some other charging source. The main engine alternator can do that, but not well.

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Old 04-01-2015, 08:35 PM   #19
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I have a 80" by 80" area. I think i am going w/ two of the 250 watt Kyocera panels. 8.39a each. A Blue sky 3000 mttp boost controller. and 2 125ah agm batteries in parallel. This mainship has one battery per engine. I am wondering if i should use the new agm batteries in place of the "engine" batteries or create a new bank and isolate them?
The 8.39 amps each must be at rated voltage which I assume is 27vdc or greater. You would get 19 amps at 250 watts (which you probably wont get)
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Old 04-01-2015, 09:16 PM   #20
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David Thanks for some great info.
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