# How to figure out what size controller to get



## budgetprepp-n (Apr 7, 2013)

I been trying to figure out how many panels I could run on a 40 amp controller I thought that 
the sticker on the back of the panel showed the amps that I would be working with.
"WRONG" ouch I called two different manufactures of controllers and the math goes like this, 
4 - 245 watt panels 4x245=980 watts ,, 980 watts -divided by 12 volts = 81.6 amps

If I switched to a 24 volt set up 980 watts -divided by 24 volts= 40.8 amps and a 40 amp
controller would be fine.

These 81.6 amps aren't real amps but that's how it's figured

The sticker on the back says 7.96 amps rated current because they figure 30.8 rated voltage so,,
245 watts - divided by 30.8 volts= 7.96 amps

I learned something today. And now my head hurts

before you say this is wrong go look at what size controller you get when you buy a solar kit
a 400 watt kit 12 volt will have controller that is about 40 amps 
http://www.ebay.com/itm/Complete-ki...70725088&tpos=top&ttype=price&talgo=undefined


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## Prepadoodle (May 28, 2013)

I'm curious why you say these aren't real amps?

P=IE, so power (in watts) is equal to current (in amps) times voltage (in volts)

Since P=IE, then P/I = E and P/E = I

Another handy equation to know is Ohm's law, which says E=IR, or voltage (in volts) is equal to current (in amps) times resistance (in Ohms)

You can mash these together too. Since P=IE and E=IR, we can also substitute "IR" for "E" and get P=I(IR), which is sometimes called the "I square R power law." It tells us that the amount of power (in watts) lost in a conductor is equal to the square of the current times the resistance.

For example, if your system is running at 40 amps and the resistance of the wires is 0.01 Ohm, we would get P=40 X 40 X 0.01 so P=1600 X 00.1 thus P=16 watts. The easiest way to minimize this power loss is to run at higher voltage (which lowers the current). You can also use bigger wire (which lowers the resistance), but this gets expensive fast. This is also why you want to keep your wire runs as short as possible with DC for the lowest possible resistance.

It seems complicated when you write it out, but all you need to know is that P=IE and E=IR and you can figure the others out pretty easily. I just remember the words "pie" and "ear" to help me remember.

If anyone would rather have a chart...


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## Medic33 (Mar 29, 2015)

so get a bigger controller -and this is a perfect reason why a person should stick to topics they actually understand and know a little about points finger at self. cause I don't know crud about this topic


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## budgetprepp-n (Apr 7, 2013)

Prepadoodle said:


> I'm curious why you say these aren't real amps?
> 
> P=IE, so power (in watts) is equal to current (in amps) times voltage (in volts)
> 
> ...


I say this because I have 2 - 235 watt panels on one controller and 3 - 100 watt panels on another. And I have a remote
information panel and I keep a close eye on them and very seldom do I ever see over 15 amps on ether one.

I experiment quite a bit and a lot of the time what I come up with ( real world ) is not the same answer as what comes up 
on paper.

And you are right it does seem complicated on paper. My panels are only 20 feet away from
the controller and I used 10 gauge solar wire.

If anyone would rather have a chart? No thanks I'll stick to a gauge

What size system do you have? And how many watts do you usually see?


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## graynomad (Nov 21, 2014)

budgetprepp-n said:


> ...
> These 81.6 amps aren't real amps but that's how it's figured
> ...


They are just as "real" as any other amps.

If those panels a have a Vmp of 30v then that are totally unsuitable for a 12v system unless you are using an MPPT regulator. They would be good for 24v though and as you say that halves the current therefor allowing a cheaper regulator.


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## budgetprepp-n (Apr 7, 2013)

Medic33 said:


> so get a bigger controller -and this is a perfect reason why a person should stick to topics they actually understand and know a little about
> 
> Yep I get into topics that I don't know much about and I don't klame to know much about them ether.
> But that's how I get to know a little about a subject
> ...


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## budgetprepp-n (Apr 7, 2013)

graynomad said:


> They are just as "real" as any other amps.
> 
> If those panels a have a Vmp of 30v then that are totally unsuitable for a 12v system unless you are using an MPPT regulator. They would be good for 24v though and as you say that halves the current therefor allowing a cheaper regulator.


I am using a MPPT controller. The 235 watt panels are like 30 volts but the 3 - 100 watt panels are 12 volt.
So why am I not seeing more amps? I'll see if I can get a picture of the gauge and what I'm talking about 
maybe I'm reading it wrong?

Ok here we go if I'm reading this right it shows 13.7 volts and 4.3 amps.
Am I reading this correctly? Or maybe I don't see a lot of amps because I keep the batteries at
12.3 or higher?


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## budgetprepp-n (Apr 7, 2013)

I should not have said that the amps are not real,,,,, I miss spoke

But it's like the watt rating on the panels. Do you ever see 100 watts from a 100 watt panel?

So are those ratings real?

I should have said the ratings are not realistic

budgetprepp-n hangs his head and kicks a stone


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## Prepadoodle (May 28, 2013)

Yeah I would wonder where it's taking its reading from. 15 amps @ 12 vdc is only 180 watts, which does seem low for your system.

I'm not being critical. I think your real world data is very interesting and adds value to the forum, keep it coming!

Panels are usually rated at 70 degrees under ideal conditions. Panels in full sun usually get much hotter, so produce less power. Manufacturers often seem a bit optimistic in their ratings, and the listed ratings are max power, not what you would normally see. Still, if you rarely see 15 amps, something might not be right.

You asked about my system... I don't have one yet, except for my tiny portable panels. I do have a degree in electrical engineering, and try to provide information to help those interested in understanding a little about the theory... where the numbers come from and how they relate to each other, which can help those looking to "go solar" get some sort of handle on what they will need. If you aren't interested in this info, that's fine too.

Anyway, I do find your posts to be quite interesting. Keep us posted!


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## graynomad (Nov 21, 2014)

> Do you ever see 100 watts from a 100 watt panel?


Sometimes you do, sometimes you don't, it depends on the honesty of the manufacturer I guess. Some of them multiply Voc by Isc to get the watts which is a bit naughty. I always look at the physical size, there are no free lunches so brand X 100W panel should be the same area as brand Y assuming the same chemistry.


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## Prepadoodle (May 28, 2013)

The display has icons for panels, battery, and load, and the indicator is under the battery. 13.7 VDC is a typical float voltage for 12 VDC batteries, so I would guess the display is saying your batteries are fully charged and drawing a small maintenance current to keep them that way. (13.7 VDC @ 4.3 A is less than 60 watts)


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## paraquack (Mar 1, 2013)

The meter is showing 13.7 volts. That would indicate to me the battery is pretty well charged so it could very well be a float voltage, but 4.3 Amps is pretty high for a float charge. The 4.3 Amps on the meter indicates the controller is trying to put 4.3 amps into the battery. I'd disconnect the battery from the system, connect a big bulb (for a minute) or somehow pull some power out of the battery to remove the "surface" charge, and then check the voltage of the battery. If the battery is actually fully charged. 13.2 VDC or higher, I would expect the controller to shut off. I the battery is around 12.8 VDC, I'd guess the controller is doing it's job. In water term, Amps is the equiv. to gallons per minute, and volts is equiv. to pressure. If the battery is empty, it will fill with less pressure (volts, because it is easy to push the amps) and take faster gal per min. (Amps). As the battery is filled, it is harder to push the Amps into the battery, so the volts goes up, and the battery doesn't need as much Amps so the Amps will go down. Have you really run the battery down (12.1 VDC) and then checked the meter on the controller while charging? I would expect to see higher amps. And yes, I'd go bigger on the controller if you have a lot of batteries. Try these links for more info.
Solar Charge Controller Basics
Solar Charge Controllers & Solar Panel Chargers | Free Help & Prices
Basic Tutorials: Charge Controllers for Solar Energy Systems


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## Kauboy (May 12, 2014)

I'm confused.
How do you get a 30vdc panel to work on a 12vdc charge controller?
Even if you only hooked up one, would you really bet getting max power from the panel with such a setup?


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## paraquack (Mar 1, 2013)

The controller has a max input voltage spec. You can't exceed that voltage from your panels. You must have a battery connected to the output of the controller. No battery will give you erroneous readings, if any. Mixing panels with different voltage out into the right kind of controller will work if you don't exceed the max voltage but the higher voltage panels will not be putting out all the power they could because of the controller being connected to lower voltage panels.


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## 8301 (Nov 29, 2014)

Panels generally max out close to their rated wattage. Due to cloud effect they can even occasionally put out a bit more than rated power. My 490 watts worth of panels have put out as much as 548 watts (I have a DR Watts meter between my controller and the battery bank). Because of this occasional cloud effect they recommend that your charger controller be rated at least 10% higher than your panel rating.


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## 8301 (Nov 29, 2014)

Prepadoodle said:


> The display has icons for panels, battery, and load, and the indicator is under the battery. 13.7 VDC is a typical float voltage for 12 VDC batteries, so I would guess the display is saying your batteries are fully charged and drawing a small maintenance current to keep them that way. (13.7 VDC @ 4.3 A is less than 60 watts)


Agreed. When my batteries are full my controller sometimes put only a few watts into the batteries.


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## Kauboy (May 12, 2014)

I get all that, but what I don't get is how a charge controller and battery setup expect 12v, but the panel outputs at 30v.

Correct me if I am wrong, but the panel controls the output voltage, right? That panel, under full sun, will always put out ~30v.
The only way to change this would be to have an inline transformer that reduces the voltage to >12-14v for the batteries.
Does the controller do this?
Does it accept voltage as high as 30, and only put out half?

My only experience with panels and controllers so far has been sub 100w, ~17v panels and a 30amp controller.


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## slewfoot (Nov 6, 2013)

budgetprepp-n said:


> Medic33 said:
> 
> 
> > so get a bigger controller -and this is a perfect reason why a person should stick to topics they actually understand and know a little about
> ...


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## budgetprepp-n (Apr 7, 2013)

Kauboy said:


> I get all that, but what I don't get is how a charge controller and battery setup expect 12v, but the panel outputs at 30v.
> 
> Correct me if I am wrong, but the panel controls the output voltage, right? That panel, under full sun, will always put out ~30v.
> The only way to change this would be to have an inline transformer that reduces the voltage to >12-14v for the batteries.
> ...


Yes the controller takes care of it. When you hook up your batteries it knows if you need 12 volts or 24 
or what ever. Or at least a mppt controller will


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## 8301 (Nov 29, 2014)

all rough numbers here.
A depleted 12v battery that is fed 13v at 5 amps (65 watts) will absorb all 65 watts.
A depleted 12v battery that is fed 20v at 5 amps (100 watts) will only absorb about 65-70 watts and the remaining energy (watts) will be wasted.

A basic solar charge controller simply sends the power (watts) to the battery when it sees that the battery is under the set voltage (around 13.6v) This basic solar charge controller will turn off the power when it sees that the battery is fully charged as measured by the battery' s resting voltage. In reality most lower end solar charge controllers use Pulse Width Modulation (PWM) which means the controller gives the battery a zap of power and then cuts off for just an instant to measure the battery voltage. As the battery gets closer to full charge (around 13.6v) the pulses of energy become shorter and the breaks between pulses become longer effectively slowing and eventually stopping the charging process.

Basic battery charge controllers do a good job but waste the additional voltage and potential energy that voltage may carry.

A MPPT charge controller takes the additional voltage (roughly 7v in the example above.) and like a transformer convert it to about 14v and ups the amperage allowing you to harvest all of the energy your panels are making regardless panel voltage. Since most MPPT controllers can take over 150v and drop it to 14v you can wire your panels in higher voltage strings.

Since a MPPT charge controller can vary the amount of voltage drop and amperage increase it provides it can assure you get all the energy whether your panels are only putting out 14v in low light or 100 volts with several panels wired in series on a sunny day. Most panels are rated for a much higher pass through voltage than they actually produce.

In bright sunshine a panel may be rated at 22v output but like a string of batteries wired in series that panel can pass over 140v through allowing you to wire several panels in series reducing amperage and thus wiring size required.


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