Solar

[RACarvalho]

Hello, I have a question on what is the real output of solar panels :
I Just installed yesterday a 100W solar panel in our rig.
I chose 100W because the charge controller that came in the rig (GoPower GP-PWM-10) can only handle 10 amps which in my mind would be able to handle a maximum 120W (10A x 12V)...
So I was expecting to see an output from the panel around 8 amps when under direct full sun but the maximum I was able to see so far was 2.8amps (36W - 2.8 x 13V)...
Now I don't know if the controller is showing what the batteries are absorbing or what the panel is producing...
I checked the controller owners manual and it is not clear what that number is...
Any clue on this?

R

[Bob Denman]

Assuming you were making 100 watts of juice with the panel.
That should be about 8.3 amps...
But these things are never 100% efficient.
With 80% efficiency: that would be about 6.7 amps.

You're at half of that
Could it be that you're not at an optimal angle for soaking up the Sun
(Could the controller be working at 24 volts?)

[DavidEM]

The answer is a bit complicated. Let's assume the panel is in full sun and oriented perpendicular to the sun. These parameters are the basis of the 100 W rating.

Panels put out volts and amps that depend on the load. At zero load the amps are zero but the voltage of the panel is high, about 21 volts for a nominal 12V panel. This is called the open circuit (OC) values. When the load is just right enough to load down the voltage to about 17V the amps are 100/17 = 5.9A. This is the maximum power point (MPPT).

But your batteries need approximately 13 volts when charging. The amperage goes up a little, maybe to 6.5 amps but 13 * 6.5 = 85 watts not 100. Such is the nature of PWM controllers like the Go Power which just pulse the load to keep the voltage where it needs to be. So you inherently get about 15% less real output with a PWM controller.

You could install a MPPT controller. These have circuitry that keeps the load at its MPPT value of about 17 volts but converts it to 13 volts for the battery. They do lose several percent in efficiency doing this conversion but all in all are about 10-15% more efficient than PWM controllers.

But your panel will never be perpendicular to the sun at least not in North America. So you lose power due to the sun angle.

All in all the best you can hope for is about 5.5 amps at 13 volts or about 70 watts. That is the most I have seen with my single 100 W panel and a Go Power PWM controller.

David

[RACarvalho]

Originally Posted by DavidEM

All in all the best you can hope for is about 5.5 amps at 13 volts or about 70 watts. That is the most I have seen with my single 100 W panel and a Go Power PWM controller.

David

Thank you All!
the controller is keeping the voltage at 13V and now during lunch I had time to stay there and see the amps fluctuate.
It seems the sun angle is the culprit for I saw peak of 4 amps when the clouds gave us a break but the sun is not really high yet...
The only reason I added solar is because I'm tired of connecting and disconnecting the power cord to trickle charge the battery and I want to run the refrigerator for a couple of days before we leave and it runs on battery...
The refrigerator when running consumes 80W so under the current reality I will probably need 300W of solar in order to do what I want...
So that means a new charge controller too....
This laziness is becoming expensive!!!