solar panel question

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Artic Warrior

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Okay here goes.
I understand 100 watt 12 volt solar panel if decided by 14 gives roughly 7 amps.
But many people are fitting the house roof solar panels that give off 36 volts and some time 60 volts.
Now a 250 watt panel at 60 volts only gives out 4 amps.
So my question is the 250 watt panel regulated through a 12-volt controller is its amperage still 4 amps or does it increease to around 17 amps because now it's regulated for a 12 volt regulator.

Thanks in advance
Colin
 
Measurements for volts and amps are taken after the controller. For instance, my panels are currently yielding 80w at 40v, the controller has stepped the voltage down to 14.4v. So, 80/14.4 = 5.5a going into the batteries.

If you had a panel that was yielding 250w (as in your example about), on a bulk charge the controller would step the voltage down to 14.4v.* Amps into the battery would therefore be 17.36a.

Note * my controller is set to charge at 14.4v bulk, this can vary.
 
Artic Warrior said:
Okay here goes.
I understand 100 watt 12 volt solar panel if decided by 14 gives roughly 7 amps.
But many people are fitting the house roof solar panels that give off 36 volts and some time 60 volts.
Now a 250 watt panel at 60 volts only gives out 4 amps.
So my question is the 250 watt panel regulated through a 12-volt controller is its amperage still 4 amps or does it increease to around 17 amps because now it's regulated for a 12 volt regulator.

Thanks in advance
Colin
Click to expand...

The specification of a solar panel gives its power output in watts at what is called the maximum power point.

So for s so called 12 volt panel rated at 100 watts you will get 100watts of power when the output is controlled to the maximum power point Typically this will be when the terminal voltage is regulated at approx 16 volts. ( or a little more) then the current flowing from the panel is 100 / 16 = 6.25 amps.

The spec for the panel should define the volts and amps at max power. Also the open circuit volts ( typically a little above 20 volts) and the short circuit current which will be a little more than the current at max power.

To get the max power you must of course have a maximum power point tracking (MPPT) controller. This converts the power at 16 or so volts into ( nearly) the same power at 14 ( or so) volts for battery charging by electronic wizardry

To utilise higher voltagepanels, or several 12 volt panels in series, you need a MPPT controller that will accept them and convert their power into the same power at 14 volts. Controllers that handle up to about 75 volts input are readily available.
 
Artic Warrior said:
But what I don't understand is if solar panel is wired and gives let's say 10 amps it will always only be 10 amps ? No matter what voltage it's regulated at
Click to expand...

No a solar panel has a significant internal resistance . As you increase the current drawn the terminal voltage will fall, from the open circuit volts at zero current to zero volts at the short circuit current.

At the extremes either current or volts is zero therefore the power obtained is zero ( 20 volt x zero = zero and zero x ( say) 7 amps = zero.)

Somewhere between the two volts x amps reaches a maximum ........ That is at the MPP.

Thats what MPPT is all about
 
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Ok and cheers for the explanation as it's botherede for a long time now ....as I see many people use the ones from a house..... I have a vicrron 75/50 so I can have a large array but was only going to use 12 volt panels.
So I suppose they make 60 volt panels so to increase the voltage to 240 when putting the power back into the grid if used on a house
 
Artic Warrior said:
Ok and cheers for the explanation as it's botherede for a long time now ....as I see many people use the ones from a house..... I have a vicrron 75/50 so I can have a large array but was only going to use 12 volt panels.
So I suppose they make 60 volt panels so to increase the voltage to 240 when putting the power back into the grid if used on a house
Click to expand...

60v panels use the same cells, just more of them wired in series. You can do the same with "12v" panels (which are closer to 20v open circuit). Just wire three in series.

If you have room, fit an array of quality panels and wire them in series/parallel. You can connect a pair in series, which will give you about 40v max then connect the pairs in parallel. I have two pairs wired like this. My new pair of 100w back contact panels are mounted at the transom on an arch, they suffer no shading and are very, very efficient. Cheap solar is a mistake, IMO.
 
Artic Warrior said:
Ok and cheers for the explanation as it's botherede for a long time now ....as I see many people use the ones from a house..... I have a vicrron 75/50 so I can have a large array but was only going to use 12 volt panels.
So I suppose they make 60 volt panels so to increase the voltage to 240 when putting the power back into the grid if used on a house
Click to expand...

I think Victron say their MPPT controllers are most efficient when the panel output is a nominal 24V, but the 75/50 is OK up to 75 volts max, and a 36V nominal panel will have a max output voltage of about 55 Volts. The MPPT controller acts like an intelligent, continuosly variable transformer, transforming the voltage at which the panel is giving out most power down to the correct charging voltage for the battery.

House installations have intelligent inverters to convert the panel DC to mains voltage AC, and synchronize that AC with the mains AC waveform, so the DC voltage is something of a minor detail for them, but having higher DC voltage means lower DC current for any given power, so reduces voltage loss on the way down from the roof and allows for thinner cable to be used.
 
Artic Warrior said:
So I suppose they make 60 volt panels so to increase the voltage to 240 when putting the power back into the grid if used on a house
Click to expand...

Not only that, but 60V gives far less voltage drop (allowing thinner cabling), whilst still being safe should you accidentally become part of the circuit.
The cabling from our boat's panels runs at 80V (but never as much as 5A), also partly also to minimise voltage-drop. After the controller, that can reach 25A at 14V.
 
billcole said:
I think Victron say their MPPT controllers are most efficient when the panel output is a nominal 24V .....
Click to expand...

Although I think that applies when you have only one panel, in which case one 24V panel is better than the same physically-sized 12V panel. Obviously, when you have more than one panel it's not so important as you can link the panels in series to achieve whatever voltage the controller can handle, subject to shading conditions.

Richard
 
billcole said:
a 36V nominal panel will have a max output voltage of about 55 Volts.
Click to expand...

Slightly pedantic, but my panels, wired in series output around 40v, so i'd expect 36v nominal to be 60v.

Still within the limits of the OPs Victron controller, but more panels in series = more risk/likelihood of shading on a boat. As you say, the Victron MPPT controller is efficient at a nominal 24v.
 
Think also of house wiring: much skinnier than much boat wiring, but at 240V is often carrying thousands of watts. (A typical domestic kettle is about 2400W, or 10A; the same 2400W would run at 200A at 12V, something akin to your starter motor or a rather hefty inverter).
Relatively few 12V circuits on a boat carry more than a mere 100W (but when they do, they need some serious weight of copper).

This is one reason why, on another current thread, there are cautions about domestic sparks rewiring boats.
 
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On really high voltages such as 33kV AC power lines, the amazing thing is that the current tends to only flow in the outer part of the conductor -known as the surface or skin effect
 
Gladys said:
On really high voltages such as 33kV AC power lines, the amazing thing is that the current tends to only flow in the outer part of the conductor -known as the surface or skin effect
Click to expand...

That will confuse 'em. It relates only to AC and the higher the frequency the greater the skin effect. Back to solar panels my domestic 1500w system runs 8 x200w panels in series so voltage is in the order of 320v a max 5 amps. So at higher voltage less current but also less significant is any volt drop. So lose 5 v ina 12v system that is terrible but lose 5v in 320v it is nothing. Anyway well done to Vic for his explanation.
Just one thing to beware of, you will find on Ebay from China many cheap controllers claiming to be MPPT. They are not. An essential part of an MPPT controller is the large coil (transformer) operating at 100s of Kilohertz which transforms the volts down increasing amps out. A cheap PWM controller might cope with high input voltage but will not give more amps into battery than panel gives. olewill
 
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