Solar Controller Sizing, Please Help Me Out

janka · May 2019

Hi everybody. Please excuse my "newbie" mentality, but I am new to the word of solar power.

My question is if I have 2 x 110Amp 12v leisure batteries in parallel feeding into a controller, then what size controller (in Amps) do I need? I am just setting up a minimal solar system to run a laptop and a few lights in a building with no grid power connection. I have 2 x 100W (200 W in total) solar panels feeding into the 2 x 12v batteries connected in parallel. I also have a 3000W 12v inverter that I also want connected to the controller that will only ever be used for shaver/hairdryer/ hifi/ electric power tools -- eg drill/sander max 500w/ peak 1000w -- occasionally.

I guess that 2 x 110 Amp 12v leisure batteries in parallel make 220 Amps at 12v feeding into the controller. So, since I have 12V out and I want to use an inverter sometimes, I can not figure how to size the controller I need in Amps. Is there a particular type of make of controller that anybody here can recommend to me?

Thanks so much for reading - Janka

Photowhit · May 2019

The 'solar controller' is actually a 'charge controller'. It helps regular the voltage and charging of the battery bank.

So sizing should be based on the type and amount of solar power coming into the system.

There are 2 different types of charge controllers. Inexpensive ones are PWM charge controllers. They send the amperage available from the panel and pulse the charge to limit the line voltage taking advantage of batteries electro chemical properties.

With PWM a 12 volt deep cycle battery will charge at a max of about 14.5 - 15 volts. The voltage above this is lost. Some of this is expected and part of the losses in real NOCT values of the wattage from the panels.

The other type of charge controller, doesn't suffer these losses. It takes in the wattage and bucks the wattage and converting the incoming watts at 17.5 volts to watts at 14.5 volts (less some wattage lost in conversion, but typically about 95% efficient) So say a 100 watts coming in at 17 volts becomes 100 watts at 14.5 volts But they cost more money...

In general they gain an advantage of about 10% over properly sized solar panels. BUT, you can use less expensive higher wattage panels which are often considerably cheaper.

Without other sources of charging a 3000 watt inverter will be much too large for your system except very minimal use

mcgivor · May 2019

The controller size would be based on the current supplied by the panels to calculate this divide the wattage rating by the charging voltage, this will give a rough figure, 200W ÷ 14.8V = 13.5A so a controller of 15A would work but I would suggest a 20A one so as to be well above the maximum possible value. The actual output of the panels will be less than the rated output in most conditions as described in the above post,

The battery rating is in Ampere hours (Ah) not amps it is a calculation based on current drawn over a period of time and can differ widely depending on the hour rating used, this link describes in more detail https://www.upsbatterycenter.com/blog/ampere-hour-amp-hour-ah/

Remember you don't want to use more than 50% of the capacity of a lead acid battery, so in effect the usable capacity is 110Ah.

Based on the size of the system a PWM controller would probably be more cost effective, the panels you have would be wired in parallel, always connect the battery to the controller first.

Feel free to ask questions

janka · May 2019

Thanks so much photowhit and mcgivor and technodave:

I took phtowiht's advice and changed my inverter order to a 600W (1200W peak) inverter, so thanks for that advice. The inverter is only for a hairdryer/ shaver/ drill/ sander on occasional use. The main use of my solar systemette is to power a laptop and a few 12v lights and to charge some USB devices.

mcgivor you are right in that I prefer a PWM controller as I am not so rich. In my mind if you are learning something, then start simple.

The spec of one (1) of the leisure batteries I bought is:

Technical Specification
Voltage    12 Volts
Capacity (C5)    70 Ah (14A constant discharge for 70 AH max)
Capacity (C20)    90 Ah (4.5A constant discharge for 90 AH max)
Capacity (C100)    110 Ah (1.1A constant discharge for 110 AH max)
Length    353 mm
Width    175 mm
Height    190 mm

Q1: I guess I would double the above stats for two (2) leisure batteries wired in parallel?

I like to plan ahead and so I may (sooner or later) upgrade my solar system, and so I have four (4) more questions for photowhit and mcgivor and technodave if that is OK:

Q2: I found a cheap 400W wind turbine, so would I be right in saying if I get it that the wind turbine would be wired in parallel (as if it is a solar panel) along with the two solar panels that I already have wired into the same controller?

Thanks technodave for answering, I am now abandoning the turbine slant for now. It's probably too complex for a beginner like myself. Also the turbine controllers cost way more than the turbines and so are out of my price range!

Q3: If I decide against the wind turbine and either double or triple my existing panels would my new controller calculations be:

(double my panels) 400W / 14.8V = 27.03 Amp controller
(triple my panels) 600W / 14.8V = 40.54 Amp controller

Q4: If I add one (1) or two (2) more leisure batteries (also in parallel) to my system in the future would that affect my controller calculations in any way?

Q5: Would I be right in saying that a 40 Amp controller would allow for a little system expansion in the future?

Thanks for reading

Janos

Tecnodave · May 2019

Janos,

You will need a separate controller for the wind turbine, they can not be connected to one controller. The wind machine needs a controller designed for it. Solar controllers don't do so well controlling a turbine. Small wind machines fail from high winds and being not controlled.....please do post more details on the turbine...high wind control is usually done with a dump load to accept the generated power when the batteries are full. The dump load needs the wind controller to do its job. Some small turbines are a kit with controller and dump load. MidNite solar is selling Chinook wind generators cheap as they discontinued the product. $250-300 for the 12 volt. They will continue to have parts available.....check out http://midniteftp.com/forum/index.php?topic=4583.0 MidNite is one of the leading solar companies located in Everett, Washington. Very reputable company. The Chinook comes with the dump load, send an email to MidNite requesting details, the address is at the page I send you to. Good choice on down sizing the inverter. There are many on this forum willing to lend a bit of help....stay tuned David

Tecnodave · May 2019

Janka,

something out out of order just happened, I posted right after your post and the forum software popped mine ahead of yours.......unless I read your mind about getting a wind turbine....I got some information in there

david

Tecnodave · May 2019

Janka,

apparently i I read your mind about the inverter too.......we were editing at the same time and the software got a bit confused......I guess

janka · May 2019

Yep I think we both pressed "save" at the same nano-second, my edited reply got dumped (lost) and I had to write it all again and now your reply is before my question - lol! But hey, that's good telepathy technodave

Tecnodave · May 2019

My iPad crashed and I had to log off and clear out cache and cookies, system didn't know me and I had to reset password........
david

Tecnodave · May 2019

Funny, I read your entire post, I guess you went back and edited....oh well I hope you find my information useful.

Tecnodave · May 2019

I agree with your decision to abandon the wind project right now, you are in a learning curve here. I started small, bare survival mode, now I have huge system, really two side by side 2.4 kW into 96 amp MPPT controller and 400-500 amp Flooded lead acid. , 24 volt.,, 4 kW 120/240 inverter.....all American made, major names, took 6 years to get real proficient .

david

Tecnodave · May 2019

Type your comment
Rereading a bit, the questions, yes a 40 amp controller will leave you some room to grow the system , and adding more batteries will not affect your controller calculations but with more batteries you will need more panels. At this point a can stand off a storm of 10 days without seeing the sun and no grid no generator, 10 years now......I add a lot of panels into circuit and get at least several hundred watts through the storm. 7500 watt total array size total, sunny daze only 4 kW or so.

david

mcgivor · May 2019

When planning a system with future expansion it's often better to consider a higher nominal voltage, 12V has limitations before conductor, controller, over current protection sizing begins to drive the costs up, it would be wise to consider before hand, if in the planning process.

Regards question 4, it's generally not a good idea to add new batteries in parallel with older, as the older tend to drag all to the lowest common denominator. Having 4 × 225Ah 6V golf cart batteries in series for 24V nominal would be better than multiple parallel 12V, GC batteries also tend to be less expensive due to supply and demand

Question 5, This relates to moving up in nominal voltage, at 24V a controller can handle double the array output that it could at 12V, so again it'ss cost effective. Budget allowing get a robust controller such as Morningstar TS series PWM, they have excellent tech support, can connect to a PC with free software MS View and are extremely reliable, Midnight is another good source.

The above are thoughts which may not fit in well with items purchased already and or budget.

BB. · May 2019

Looks like Janka's "lost post" got in the spam queue.

The system software looks for possible reasons that a post may not be "proper" (IP address, User Email, too many posts close together, too many links, key words, etc.)...

I don't know why Janka's post end in the holding area (for my check), but it was there. I have released the post and now you should see the proper posting order. It is possible that the message was posted and then pulled back by the spam checker (not sure, but I think this can happen). There does seem to be processing delays--And while I check the queue pretty often, it may not have been "in the queue" until later.

-Bill

janka · May 2019

OK, this post should be in the middle of the thread, but there was a double entry, so I deleted one entry and the other entry vanished as well :-/

Thanks so much photowhit and mcgivor and technodave:

I took phtowiht's advice and changed my inverter order to a 600W (1200W peak) inverter, so thanks for that advice. The inverter is only for a hairdryer/ shaver/ drill/ sander on occasional use. The main use of my solar systemette is to power a laptop and a few 12v lights and to charge some USB devices.

mcgivor you are right in that I prefer a PWM controller as I am not so rich. In my mind if you are learning something, then start simple.

The spec of one (1) of the leisure batteries I bought is:

Technical Specification
Voltage    12 Volts
Capacity (C5)    70 Ah (14A constant discharge for 70 AH max)
Capacity (C20)    90 Ah (4.5A constant discharge for 90 AH max)
Capacity (C100)    110 Ah (1.1A constant discharge for 110 AH max)
Length    353 mm
Width    175 mm
Height    190 mm

Q1: I guess I would double the above stats for two (2) leisure batteries wired in parallel?

I like to plan ahead and so I may (sooner or later) upgrade my solar system, and so I have four (4) more questions for photowhit and mcgivor and technodave if that is OK:

Q2: I found a cheap 400W wind turbine, so would I be right in saying if I get it that the wind turbine would be wired in parallel (as if it is a solar panel) along with the two solar panels that I already have wired into the same controller?

Thanks technodave for answering, I am now abandoning the turbine slant for now. It's probably too complex for a beginner like myself. Also the turbine controllers cost way more than the turbines and so are out of my price range. For example you can get a Hong Kong 400W 5 prop turbine for $20.00, but the controller will cost you 10 times (or more) as the turbine :-/

Q3: So deciding against the wind turbine, I can either double or triple my existing panels would my new controller calculations be:

(double my panels) 400W / 14.8V = 27.03 Amp controller
(triple my panels) 600W / 14.8V = 40.54 Amp controller

Q4: If I add one (1) or two (2) more leisure batteries (also in parallel) to my system in the future would that affect my controller calculations in any way?

Q5: Would I be right in saying that a 40 Amp controller would allow for a little system expansion in the future?

Thanks for reading

Janos

BB. · May 2019

Hmmm, Janos, I just found another one of your posts in the spam queue...

I have released it, but it can take a few minutes (it seems) before it reappears.

It is possible that a couple of your posts are "near duplicates" that is causing this behaviour.

-Bill "magic" B.

janka · July 2019

OK, the next three questions I have are about cable sizing, controller type and junction box:

Panels::

The best spot for the panels is not on the roof or walls of the house-- but in the garden. In the garden is one spot that gets the sun all day long.

The 12v cable run from the panels in the garden to the controller panel inside of the house is 15 meters. So my question is what is the correct cable gauge for me to use?

I have 300 Watt of solar panels wired in parallel going into this 12V cable via a junction box (connector).

******************************

Junction Box::

Based on the correct cable size if any body here can suggest a good junction box connector type please do so. I mean the part that connects all of the individual panels to the cable that heads into the controller.

******************************

Controller::

I want to use dry cell batteries (fiberglass type) as they can be sited anywhere, but I notice that many controllers do not support this type of battery.

I do not have a back up battery bank or enough batteries to run a 24v system (at this time) so a simple 12v PWM controller is fine-- I am only using two 110A leisure batteries wired in parallel at 12V wired into the controller.

So my questions here are (i) do i need a MTTP controller or a PWM controller?, and (ii) what kind of controller can handle dry cell fiberglass batteries?

*****************************

Thanks - John

Estragon · July 2019

For 15m, 10awg (typical mc4 extension size) should be fine.

With 2 panels, an mc4 "Y" connector could be used. For 3 or more, I use these:
https://www.solar-electric.com/mnpv3.html

Most controllers worth using should have a generic "AGM" (Absorbed Glass Mat) setting, which is likely what you mean by "dry fibreglass" battery. You'd just need to confirm the generic setting is suitable for the specific battery you have in mind.

In your application, I'd use a pwm controller.

mike95490 · July 2019

janka said:

......
Controller::
I want to use dry cell batteries (fiberglass type) as they can be sited anywhere, but I notice that many controllers do not support this type of battery.
.........
So my questions here are (i) do i need a MTTP controller or a PWM controller?, and (ii) what kind of controller can handle dry cell fiberglass batteries?

Dry Cell batteries. You are going to have to be more specific. Dry cells are the AA, C & D size non-rechargeable

Leak resistant lead acid batteries are AGM (Absorbent Glass Mat) or GEL (electrolyte gelled with silica)

AGM are suitable for Solar, GEL are not..

The panel and battery voltage determine the type of controller. for a 12V battery, you can use a 19Vmp PV panel with a PWM controller

Once you get above 25Vmp a MPPT controller will harvest more power that a PWM leaves behind

BB. · July 2019

"Fiberglass" batteries sounds like AGM (GM=Glass Mat). If the controller does not have AGM, usually "Sealed" battery setting should work OK. As always, look at the actual charging voltages/setpoints/time on charge settings vs the manual to be sure (typically AGM have 14.4 volts as maximum charging voltage).

And, I would suggest that a (true or real) MPPT charge controller would be a better choice. Let's say you have 3x 100 Watt solar panels with Vmp~17.5 volts, 15 meters (~50 feet), and 10 AWG cable:

300 Watts / 17.5 volts Vmp array = 17.14 Amps Imp

Using a simple voltage drop calculator, the voltage drop for 17.24 amps over 50' (one way run) of 10 AWG cable (typical solar panel cable is around 12 or 10 AWG):
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=17.5&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=17.14&x=0&y=0

Result

Voltage drop: 1.71
Voltage drop percentage: 9.78%
Voltage at the end: 15.79

That is really too much drop.. You need to allow for ~2+ volts drop at the charge controller, drop in Vmp-array as the solar panels get hot in full sun, and a battery charging at ~14.4 volts...

Typically, we aim for closer to 1-3% voltage drop (better operation, less losses in system). Playing around with AGM numbers, we get:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=17.5&phase=dc&noofconduforctor=1&distance=50&distanceunit=feet&amperes=17.14&x=59&y=20

4 AWG cable:
Voltage drop: 0.43
Voltage drop percentage: 2.43%
Voltage at the end: 17.07

To:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.3224&voltage=17.5&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=17.14&x=56&y=17

1/0 cable:
Voltage drop: 0.17
Voltage drop percentage: 0.96%
Voltage at the end: 17.33

You can compare prices of a "cheap" MPPT controller vs a "not cheap" MPPT controller and the cost of copper cable... An MPPT system with the 3 panels in series (52.5 volts Vmp-array 5.71 amps Imp-array for the Imp of one panel), and 50 feet), and a 3% drop would look like:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=52.5&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=5.71&x=64&y=19

14 AWG cable:
Voltage drop: 1.44
Voltage drop percentage: 2.75%
Voltage at the end: 51.06

14 AWG cable is much cheaper than 4 or 1/0 cable... This is assuming you have 3x 100 Watt "12 volt" panels... If you have a 1x 300 Watt panel with Vmp~30 volts--That is a different set of calculations (and you would need a MPPT controller for "efficient" operation on a 12 volt battery bank.

-Bill

janka · July 2019

I meant the absorbed glass mat type batteries, such as this one:

https://www.ebay.co.uk/itm/12V-110-AH-Xtreme-AGM-Deep-Cycle-Leisure-Battery-5-Year-Warranty/122337027359?hash=item1c7bdae51f:g:rD0AAOSw0itZ5NKJ

So two of these AGM type batteries in parallel then into the controller at 12V.

Absorbed Glass Mat technology

Solar Controller Sizing, Please Help Me Out

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