Actual Solar Panel Watt Capability

[Mike90250]

......Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage....

I think you need some edumucation about solar power. And the difference between sinus and sine.

http://en.wikipedia.org/wiki/Current_source

And you will have to wear a heavy leather Farrier's apron to protect from Sunking's sharp answers. This is not the solar molly-coddle form. You will (I guarantee it) get brutally honest, true answers. If you buy first and ask questions later, well, that's your style. I think there are better ones, but we'll try to deal with everyone.

[Sunking]

Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage.

Like I said you do not know as much as you think you do because you can certainly have current without voltage. That is exactly what you do with the Isc test. Riddle me this.

What is the voltage developed with 2.9 amps flowing through 0 Ohms of resistance. A solar panel is a current source, not a voltage source like a battery. When you use a PWM controller you are not operating the panel at MPP, you are operating below MPP. A 50 watt Battery panel specs are:

Voc = 22 volts or open circuit no current.
Vmp = roughly 17 volts
Imp = roughly 2.9 amps
Isc = 3.1 amps @ ZERO volts short circuit.
Pmax = 50 watts or 17 volts x 2.9 amps.

Panel manufactures specify panel watt age in two measurement known as STC (Standard Test Configuration which is a lab measurement in a controlled enviroment) and is what they published for your panels. To be used in Grid Tied Application they are also required to specify PTC and CE ratings which are real world ratings. PTC and CE are roughly 90% of STC.

But that does not have much to do with what you are seeing because you do not understand current sources and PWM controllers. A PWM controller is going to pull down the panel voltage to the battery voltage. So you drop from 17 to 18 volts down to 12 volts. The input current = output current of 2.9 amps. 12 volts x 2.9 amps = 35 watts all day long.

A MPPT controller is a different animal as it is a POWER CONVERTER where Input Power = Output Power - Conversion Losses of a bout 2 or 3%. So now with that 50 watt panel you input 17 volts @ 2.9 amps (MPP aka maximum power point of the Current Resistance Curve) ) and on the output you have 12 volts @ 4.08 amps or 49 watts.

[Experimentor]

Wow, quick with an answer and quick to criticize. I wonder if you fully read my post? What you said is exactly what I did, what I knew I was doing, and what I said I was doing. Your quote from my post even reinforces this (I said you can't have amperage without a closed circuit which is what a short-circuit is). Is that how you always answer questions on this forum?

Also, saying something "is a current source, not a voltage source" is ridiculous. You can't have current without voltage.

But my question was, is this industry standard, and according to your incredibly rude answer, apparently it is.

[Sunking]

So I wrote them and told them this. They said measure the voltage and amperage without a battery or charger. I replied that you can't have amperage without a load (meaning a closed circuit) and assured them that I knew a bit about electricity.

Then you clearly do not know near as much as you think you do. A solar panel is a current source, not a voltage source. What the manufacture was telling you to do was perform a Voc test (voltage open circuit), and Isc test (current short circuit). Measure th evoltage with the leads open only connected to the meter. Then switch th emeter to read current and your meter will short out th epanel and reas Short circuit current assuming you have noon bright sun with the panel facing direcly into the sun.

At very best a 50 watt panel with a PWM controller can only deliver is 35 watts. What you seen is exactly what you should have seen. A 50 watt panel has a Vmp of roughly 17 volts, and an Imp of roughly 2.9 amps. With any PWM controller Input Current = Output Current. So at full power on the output you will see battery voltage x 2.9 amps assuming the battery demads a charge. 12 volts x 2.9 amps = 35 watts.

Read this thread on performing a Voc and Isc test.

[Experimentor]

So, I am new to solar power (and this forum) and wanted to get started with solar in a small way with experiments just for the fun of it and to learn as much as I could (and to have a minor backup if the power went out). I've recently become fascinated with generating my own power. I'm also living overseas temporarily and don't necessarily want to travel with tons of solar panels in my baggage! So I mail-ordered a couple of 50 watt mono-crystalline panels, a good charger/regulator, a sinus wave inverter and I bought a few deep-cycle batteries. I bought two 12v panels so I could make either a 12 or 24 volt system. I also bought a 130 amp DC in-line wattmeter for measurements, besides my regular multi-meters.

I wired it all up and noticed I was only getting about 35 watts per panel. I wrote the supplier and complained. Of course I got the mantra about clear sky, facing sun, humidity, temperature, wire size, etc. I assured them that I understood all that and it was all optimized. Then they mentioned an MPPT charger, which I was not familiar with, and upon reading up on that, I ordered one (which hasn't arrived yet) but it made sense about the voltage limitation of the battery and the charger. So I did an experiment with a ni-cad battery pack. It was a 12-cell pack which can easily go up to over 20 volts while charging and will accept a pretty hefty amperage. I wired it directly to the panel (for a short time). Guess what? Still 35 watts.

So I wrote them and told them this. They said measure the voltage and amperage without a battery or charger. I replied that you can't have amperage without a load (meaning a closed circuit) and assured them that I knew a bit about electricity. They assured me that they did too and sent me a video showing them measuring the no-load voltage of the panel, and then changing the meter to amperage and checking the short-circuit amperage. Of course, when I did this, I got about 20 volts and then 2.5 amps. Voila' - 50 watts! But I explained to them that this is an invalid measurement, since they are two separate measurements under two different conditions. I then made my own video using two meters, to show that when you short-circuit the panel, the actual voltage drops to just over one volt.

They then contradicted themselves by saying the panel wattage is figured with the maximum voltage (18 volts) and the maximum amperage (2.78) but that you will never get the maximums (what?!). Of course, both those are under load, since it also lists the open circuit voltage (22.18) and the short-circuit amps (3.34). So even though I clearly made my argument correctly, their ultimate answer was "you'll never get maximum." I could understand if it was 48 watts, or even 45, but 35? That's fraud in my book.

Here are the specs on the panel.
IMG_0288.JPG

My ultimate question is: is this industry standard? Are all panels grossly over-rated so that "you never get maximum"?