Solar - Matching panels to controller question.

[Kaos116]

I am trying to match a controller to panels. I thought I understood this solar stuff, but one thing has me a bit confused. First, here are the links to what I am looking at.

Grape Solar 250-Watt Monocrystalline Solar Panel-GS-S-250-Fab5 at The Home Depot

direct link to controller spec pdf

Notice on the spec sheet it has a maximum of a 270W panel for 60 cell panel which the one linked above is.

Let's start with the math (lets forget any losses or efficiency for the moment)

I * E = P So, for the panel 8.15A * 30.7V = 250.205W This makes sense.

Now with the MPPT controller and it's specs (max 270W panel and 20A output)

I assume it's going to convert the 30.7 Voc to a usable voltage for charging a 12V system (~14v)

Math again. So, the controller is dealing with 250W. And trying to maintain ~14v. 250W/~14v = ~17.85A This makes sense, sorta fits the specs and is what an MPPT controller is supposed to do.

Now after all this, The question is this...

Can I connect only ONE of those panels to that controller?

Mathematically, that is what I am seeing, but that doesn't click in my head. When I see the panel putting out 8A and the controller being 25A I figure I could connect 3 panels.


An additional question that I just want to confirm my thinking.

I have to watch that the Voc of the panel does not exceed the Voc rating of the controller.

And finally, any recommendations for a controller that can handle 4 of those panels (1000w)?

 

[rommel]

There is lot of info to cover and I am on my phone so I will be brief. First, you want a mppt controller. If you do not use mppt controller then you have to match the solar panel voltage output with your battery bank. 30 volt panels to 24 volt battery bank. You can use 30 volt panel with 12 volt battery bank if you use mppt controller. Yes you can use just one panl and that mppt controller. To size the controller correctly you need to add the maximum output in amps for each solar panel then add 25%. So if you have 5 panels at 8 amps that is 40 amps then add 25% which would equate to 50 amp controller. The reason for the extra 25% is panels can go into a state of overcharge and exceed the 8 amps.

Without getting real technical the mppt controller actually calculates the amperage generated by the panels and calculates the state of charge of the battery bank and the most efficiently possible regulates the charge to the batteries. They are much more efficient and if you can afford one you should get one. I would also suggest getting a bigger one in case you add more solar panels.

 

[ninemilshooter]

First, I'm not a solar expert, but I can talk about your power question.

Mathematically, that is what I am seeing, but that doesn't click in my head. When I see the panel putting out 8A and the controller being 25A I figure I could connect 3 panels.

What you are missing is that current by itself does not tell you anything. Power is the key. Your input power should be roughly equal to the output power (plus any loss in the controller). So if you have a 30V panel capable of 8A = 240watts. That is equivalent to the output power of 12V * 20A = 240W.

 

[rommel]

I got a chance to check out the mppt controller specs. That controller could handle one or two of those panels. To make it simpler just deal with amps. To do 4 of those solar paneLs you would need minimum 40 amp controller. i think the spec sheet was trying to simplify the compatibility for people by stating how many watts the controller could handle because solar panels are sold by wattage. Basically all panels in that watt range (250) are going to put out 30 volts and 8 amps. Remember to do everything in amps and add 25% and you should be ok.

The solar panels at maximum sunlight will put out the 8 amps they claim, reduced sunlight and that will reduce the watts out and thus lower the output amperage because th output voltage will always be 30 volts. Your math equation above about the controller is wrong. The controller is not dealing with watts from the panel, but 30 volts output of the panel and the corresponing amps depending on how many watts the panels are generating.

 

[ninemilshooter]

If what you are saying is that you can wire more panels in parallel and maintain up to 8 amps even in low light (up to a point I would imagine) then that makes sense. But you would be leaving a lot of peak power unused because the controller is only going to use 8 amps on the input even if all of them are capable of sourcing that current at the same time.

Your math equation above about the controller is wrong.

My electrical engineering degree and 25 years of designing power supplies says it is not.

The controller is not dealing with watts

Yes it is. The controller is nothing more than a switching power supply and lead acid battery charger. You cannot equate current on the input at a set voltage to current on the output at a different voltage.

IF you can wire panels in parallel (and I'm not sure you can, normally for circuits like this I would use a diode to prevent current from flowing from one source to another) then you might get more output power because you have more current available. But in direct sunlight each panel would be capable of 30V*8A (240W) and the controller won't do anything more than the spec says it will:

SPECIFICATIONS Solar Boost 2512i-HV
Output Current Limit 25 amp maximum 36 cell modules 20 amp maximum 60 cell modules

So all you can possibly get out of the controller is 20A @12V. 240W (for the 60 cell).

 

[Palladin]

The sun is going to explode!!!

 

[rommel]

Ninemilshooter,

My bad, my response was to the op not you and I did not clarify that. I was trying to keep it simple by saying the controller will always have 30 volt input from the panels. That is where his calculation was wrong. He was calculating that the controller was always getting 250 watts. What will be variable is the watts and thus the amps. That variability comes from the concentration of sunlight. Less sunlight = less wattage = less amps. In order to size the controller correctly you simply need to add up the maximum output amperage for each solar panel then add 25%. In this case he has about 8 amps per panel or 10 amps with the 25% margin. In this case again, the charge controller rated at 20 amps can effectively handle 2 of those panels. If he wants to go to 4 panels, he will need at minimum a 40 amp controller.

 

[Kaos116]

Ninemilshooter,

My bad, my response was to the op not you and I did not clarify that. I was trying to keep it simple by saying the controller will always have 30 volt input from the panels. That is where his calculation was wrong. He was calculating that the controller was always getting 250 watts. What will be variable is the watts and thus the amps. That variability comes from the concentration of sunlight. Less sunlight = less wattage = less amps. In order to size the controller correctly you simply need to add up the maximum output amperage for each solar panel then add 25%. In this case he has about 8 amps per panel or 10 amps with the 25% margin. In this case again, the charge controller rated at 20 amps can effectively handle 2 of those panels. If he wants to go to 4 panels, he will need at minimum a 40 amp controller.

If I connect one panel and have full sunlight on it, that would be 30V @ 8A going into the controller. This being a MPPT controller , what could I expect for an output amperage. Isn't the purpose of a MPPT controller to convert that excess voltage (30v when ~14 is needed) into usable amperage (like a dc-dc converter)?

That is how I am getting confused.

 

[rommel]

Yes theoretically the mppt controller will be pushing out around 14 volts and a corresponding doubling of amps, around 16. However the limit now will be what the batteries can take. Batteries have 3 stages of charging, bulk, absorption and float. The mppt regulates each stage and adjusts voltage and amperage accordingly. I do not know the specifics of each stage. So yes if you only had one battery and a large solar array then there would be unusable energy. This is why you try to size your system as a whole. You could also try to plan to use the batteries during peak times with excess energy generation, for example run an ac at peak times and the charge controler could transmit otherwise unusable energy.

 

[ninemilshooter]

Ninemilshooter,

My bad, my response was to the op not you and I did not clarify that. I was trying to keep it simple by saying the controller will always have 30 volt input from the panels. That is where his calculation was wrong. He was calculating that the controller was always getting 250 watts. What will be variable is the watts and thus the amps. That variability comes from the concentration of sunlight. Less sunlight = less wattage = less amps. In order to size the controller correctly you simply need to add up the maximum output amperage for each solar panel then add 25%. In this case he has about 8 amps per panel or 10 amps with the 25% margin. In this case again, the charge controller rated at 20 amps can effectively handle 2 of those panels. If he wants to go to 4 panels, he will need at minimum a 40 amp controller.

What you keep missing is that the input current to the controller does not equal the output current. Power is what matters. If each panel is capable of 8A @30V and you could wire the panels in parallel (you can't wire them in series because it would exceed the input voltage spec of the controller) then you would have:

8A x 30V x 2panels = 480W input power

Since input power is being converted to use 12-14V by the controller you would need:

480W = 12V x A where A is the output current your controller would need to be able to source to use all of the power possible in the panels.

A = 480/12 = 40A

So in my opinion, more than one panel per the controller you spec'd here is a waste when there is direct sunlight. Multiple panels will give you better low light performance up to 240W, when the controller starts current limiting the output. The rest of the power (one whole panel of the 2 panel set) is wasted or not used.

Just my opinion, and it's worth what you paid for it...

 

[ninemilshooter]

If I connect one panel and have full sunlight on it, that would be 30V @ 8A going into the controller. This being a MPPT controller , what could I expect for an output amperage. Isn't the purpose of a MPPT controller to convert that excess voltage (30v when ~14 is needed) into usable amperage (like a dc-dc converter)?

That is how I am getting confused.

It is exactly like a DC to DC converter because that is what it is. Use the power in = power out equations. 30V*8A = 12V*A. Solve for A = 30*8/12 = 20A.