Checking the math on 1.65 kwatt off grid solar system

[Uncle Bob]

Originally Posted by Kolchac

Yes, It is possible. You need a device, which has a generator, which creates alternating current 115V, then electrical transformer, which lowers 115V to 24V, then electric rectifier. You also keep in mind the size of the transformer because one of your coils should be able to pass current of 13A, another - 40A, and the system efficiency. Such device will cost $ several hundreds. And for what purpose? To correct an obvious mistake? Most of $20-50 systems just cut voltage.


By the way, I would not advise connecting 115V 13A directly to a transformer.

This is a joke, right? Couldn't possibly be anything but.

[Kolchac]

Originally Posted by StuM

Apparently, you are one of those "many people".

Why would he "lose 115V" if he is using a suitable MPPT controller. It will convert the total incoming Watts at whatever voltage to almost the same number of Watts at the required output voltage. That's what they are designed for. Why would he "cook" the controller if he uses an appropriately rated one?

In the middle of summer at 30° latitude, an appropriately angled solar array will give him a lot more than 50% of rated power.

His battery needs are a totally different issue and should be based on a number of factors other than the solar panel output.

This reply to all gentlemen who disagree with me:
I have Master’s degree in Electrical/Electronic engineering, developed many embedded systems for Boeing and Lockheed for my more than 30 years of experience. It is not my purpose arguing with you. I just give my professional opinion. I'm surprised that you took it so close to your hart. If you do not like it, do not take it, it is not my job to convince you, I have enough at my office. I'm very grateful to this forum for helping me to solve some problems with my boat, so I have obligations to pay back, no more.

[StuM]

Originally Posted by Kolchac

This reply to all gentlemen who disagree with me:
I have Master’s degree in Electrical/Electronic engineering, developed many embedded systems for Boeing and Lockheed for my more than 30 years of experience. It is not my purpose arguing with you. I just give my professional opinion.

Instead of spouting off alleged qualifications, how about providing answers to the questions asked.


1. Why would he "lose 115V" if he is using a suitable MPPT controller.
2. Why would he "cook" the controller if he uses an appropriately rated one?


And while you're at it, you could try to explain why solar panels need " a generator [WTF!], which creates alternating current 115V, then electrical transformer, which lowers 115V to 24V, then electric rectifier." when a standard MPPT controller consisting of a buck converter and some smart electronics does the job. ( or was that a clumsy attempt to describe the way the MOSFET, inductor and switch in a simple buck converter work in essence?)

[Adelie]

Originally Posted by sailor4life7777

Hi guys, I'm about to purchase the components for my off grid solar system but I wanted to check my math with all you amazing people first! I plan to buy 3 550 watt solar panels (Vmp=41.96, Voc=49.90, A=13.11) and wire them in series. I plan to buy a 24V, 100AH lithium ion phosphate battery. I plan to buy a 100 amp MPPT solar charge controller (1650watts/24V *1.25=86) and a 3000 watt inverter. Am I making any mistakes anywhere?



Thanks in advance guys!

3-49v panels in series would be 150v. Check that whatever controller you get can handle that voltage.

Better would be to get a smaller controller for each panel, individually controlling each panel will deal with shading better, ie more total output. Added benefit if a controller fails you’ve only lost 1/3 of you capacity, not all of it. If the controllers are sized for 2 panels then you can parallel 2 panels and still have almost the same average output.

100Ahr at 24v doesn’t sound like much battery capacity.

You need to find an inverter to run off of 24v.
For that matter you need to find 24v equipment to run or to get a set down converter.

[Dirk01]

Originally Posted by donradcliffe

Look at maximum discharge current for your battery BMS. If its 100 amps at 24 volts, thats about 2500 watts, and your inverter could trip the BMS. I agree that the battery is undersized for the solar array, and rwo 100 amphour 24 voltt batteries in parallel would solve both those issues.

Definitively 3 times undersized!
Take the 300AH eve or similar.
That is about 6,5 kWh net capacity and roughly your daily! production.

So, if you want to live an electric lifestyle (electric oven & cooktop and mikrowave included, but BBQ still gas for flavor), you'll need a storage capacity equivalent to 3x your AVARAGE daily consumption to skip the cloudy days until you get the juice back over the next two days. BTW, 4 times is not a mistake at all.
Maybe you should have a 48V system for the inverters (3x400V if European) to lower the current, the size of the wiring and to mitigate the inverters IGBTs?
Your 24V system is only for navigation, lighting, pumps, windlass, winches, watermaker, fridge & freezer, and so on and is connected via isolated DC/DC converter. Although you've the convert efficiency loss of about 4-6% - I think it's worth it. You'll then have an "always on" - system with backup energy.

Use the victron calculator to get the right configuration (including temperature and wiring consideration). This isn't an "I throw a few panels in a row and I get power" thing. Not rocket science, but something you should think about before acting.
5 P rule --> proper preparation prevents poor performance (typical German engineers thinking)

Only my 5 cts.

Cheers
Dirk

[Dirk01]

Originally Posted by mitiempo

True, about as dangerous as the 120 AC coming out of the inverter.

Done properly there should not be a problem.

WRONG!!! TOTALLY WRONG!!!
DC is MUCH MUCH more dangerous than AC!!!

Don't talk about things you have no idea about!

You're confusing your 9V DC children's toy with a 120V DC power supply system! That's like comparing a bobby car with a Porsche Cayenne 580kW!

Sorry for being loud, but that's really a PITA for me to read such things. This might lead to a sudden death!!!

Cheers
Dirk

[Adelie]

Originally Posted by mitiempo

True, about as dangerous as the 120 AC coming out of the inverter.

Done properly there should not be a problem.

The reason that most Electric Propulsion systems are 48v DC is because at 60v DC no longer qualifies for "low" voltage for regulatory purposes and more stringent safety measures are required. Charging voltage will be in the mid-50v range, just below the 60v cutoff.

If you go full DIY then you could have whatever voltage you heart desires. But you run the risk of problems with insurance for your vessel if the insurer finds out and you run the risk of issue with foreign customs deeming your vessel unsafe.

[Dirk01]

Originally Posted by Adelie

The reason that most Electric Propulsion systems are 48v DC is because at 60v DC no longer qualifies for "low" voltage for regulatory purposes and more stringent safety measures are required. Charging voltage will be in the mid-50v range, just below the 60v cutoff.

If you go full DIY then you could have whatever voltage you heart desires. But you run the risk of problems with insurance for your vessel if the insurer finds out and you run the risk of issue with foreign customs deeming your vessel unsafe.

Absolutely right. But I do not intended to point to insurance or customs.
What I wanted to say is, that many people (not only sailors) think, that DC is not dangerous. But the opposite is the case.
In conjunction with LiO (or the better to use LFP on board and as a house battery bank in the garage or the basement) and the nearly zero internal resistance (0.3mOhm I guess) there is, in a case of a shortcut, nearly nothing that limits the current from the battery. Only the external resistance can limit the juice. But who cares about a really good isolation against earth (or better to say the minus of the battery) to not close the loop?
So, even with lower voltage than the low voltage directive starts with (60V), IT IS DANGEROUS.

This has to get into the brain of everybody. Careless life is probably short life. It's not rocket science, but you have to know about what might happen if you don't care.

Would you agree on that?

Cheers
Dirk

[Dirk01]

Originally Posted by redneckrob

But basically the more parallel you are the more robust your system is and the more you can just install and forget it. The more series the more sensitive it is and the more you'll have to monitor and tend to it.

To top that absolutely correct statement:
The more MPPT-controller you've in parrallel to the charging point, the more you are going to harvest.
(yes, there is some precaution in setting charge voltages or better to get them talk to each other like Victron does, but that's another topic).

Cheers
Dirk

[Dirk01]

Originally Posted by donradcliffe

After investigating a solar panel fire on a friend's boat, I found that it was caused by having more than two panels in parallel on the same controller. A panel developed an internal short. The panels are designed to handle this kind of short with two panels in parallel, but not the 5 he had without additional fusing.

This is a VERY GOOD hint.
There's a thread about that configuration and the problems that might occur in case of a failed panel. (two weeks ago we've had a fire on the roof top of a gym nearby - same cause.)
The result was: Never put more than two in Parrallel unless you checked the back current situation. Normally (but that has to be checked as well!!!) the back current allowed is in the range of Isc..

It's not rocket science unless you know what you do...

Cheers
Dirk

[Dirk01]

Originally Posted by StuM

Bear in mind that he's got over 2 KWh (100Ah @ 24 V LiFePo4 (2.4KW - nearly all usable) That's the equivalent of at least 360AH of 12V FLA bank (50% usable).

Yes, but still only a third of his day production.

Cheers
Dirk

[Dirk01]

Originally Posted by Kolchac

Unfortunately many people, who are giving advice, do not know even the basics.
1. Parallel: voltage does not change, current - summary of the currents.
2. Series: current does not change, voltage - summary.
3. You are interested in power: VxI.
So, if your battery 24V, each solar is V=41V, series=123V, you are going to loose ~115v, or 115v x 13A=~ 1300W, so use parallel. Also you might cook your controller.

Also in the middle of summer, say 30 deg. Latitude your solar will give you only around 50% during ~from 11 AM to 3 PM max. if you have the Sun. Rest hours - from 0 to 50%. So you can calculate what battery you need.

Stop talking nonsense. You have absolutely no idea what you are talking about. Ever heard of a MPPT controller and how it works? Surely not...

Cheers
Dirk

[Adelie]

Rule of thumb is that average the panel nameplate capacity Watts will produce 1/3 of that in Amp-hr using an MPPT controller.

100W panel x 1/3 = about 33Ahr average daily output = about 400Whr

That’s for panels laid flat with no tracking.

[StuM]

Originally Posted by Adelie

Rule of thumb is that average the panel nameplate capacity Watts will produce 1/3 of that in Amp-hr using an MPPT controller.

100W panel x 1/3 = about 33Ahr average daily output = about 400Whr

That’s for panels laid flat with no tracking.

That assumes a 12V system, it's only 16.5Ah for a 24V system (which the OP has) .

I find it simpler to just say:
4 times the Rated Watts in Watt Hours per day.
(100W = 400 Wh per day.) Then you can divide by your voltage to get Ah for your installation.

Of course that factor of 4 is dependent on time of year, latitude and local climate and can vary widely.

[Adelie]

Originally Posted by StuM

That assumes a 12V system, it's only 16.5Ah for a 24V system (which the OP has) .

I find it simpler to just say:
4 times the Rated Watts in Watt Hours per day.
(100W = 400 Wh per day.) Then you can divide by your voltage to get Ah for your installation.

Of course that factor of 4 is dependent on time of year, latitude and local climate and can vary widely.

Yes, I should have indicated 12v.

I feel that Ahr works better than Whr because you don't need to do calcs for battery efficiency. Ahr in at 14.2v = approximately Ahr out at 12.5v or whatever.