Solar + Lux

[jasonck08]

So I'm testing out some solar panels, they seem to be performing very poorly overall.

I started reading about how solar panels are rated, and I found that the standard is to rate them at 1000W/m2 irradiance.

Doing some research, I found that this translates to 668,449 lux (or 668K lux for the sake of rounding).

I went outside with my extech lux meter today on quite a sunny day with not many clouds (none appear to be blocking the sun) and I get a reading of around 115K lux, which is 17% of the lux needed for max output, not even close to the Lux needed to get full output from the panel.

Does relative light output = relative solar output? For example in my case with a 10W panel, would I only be getting ~1.7W with 115K lux?

Are the above figures I found accurate? 668K lux / 1000W/m2 for full solar panel output?

 

[lwknight]

It is estimated that solar energy is about 120 watts per square meter/hr.
I wonder if this is of any value to calculate solar panels output by knowing their efficiency rating.
Its probably inflated like everything else anyway.
Oh, well... fodder for thought.

 

[ChrisGarrett]

What kind of current and voltage output are you measuring?

My 30w mono-crystaline panels are rated at about 1.47A. I think that this would be a more significant metric, don't you?

I have a small inline volt/ammeter to install, so I'll be able know how close I'm getting to that number, but I've yet to do it.

Chris

 

[hiuintahs]

I agree with Chris Garrett here, that the best way for you to check out your 10w panel is to place it so that it is pointed at the sun the best you can and then short the output through a DMM measuring current and see what you get. I'll make a guess here. If your 10w panel is 12v then open circuit voltage would be around 17 to 20volts or so. That means you should see around 0.5 amps max. 0.5 x 20 = 10w. When determining the wattage of the panel they will take the point of maximum power on the I-V curve. And that is usually where the open circuit voltage is at a maximum before rolling off.

 

[lukesan]

I agree with Chris Garrett here, that the best way for you to check out your 10w panel is to place it so that it is pointed at the sun the best you can and then short the output through a DMM measuring current and see what you get. I'll make a guess here. If your 10w panel is 12v then open circuit voltage would be around 17 to 20volts or so. That means you should see around 0.5 amps max. 0.5 x 20 = 10w. When determining the wattage of the panel they will take the point of maximum power on the I-V curve. And that is usually where the open circuit voltage is at a maximum before rolling off.

Interesting stuff. I've also been testing a 40 Watt panel here in the Benelux.
Since solar panels have been heavily subsidised by the government here I wanted to check the hype about these panels since everyone always talks about how much money they have been saving or even making .... on a (very very) sunny day. Unfortunately on cloudy, rainy, etc days you don't hear these people.

What I have noticed, and due to the lack of a Lumen meter, is that a cloud or even a greyish day can easily half the output of the rated power.
Even if it is a very bright day changing the angle of the panel will make a hugh difference.
I have my panel connected to a 12V solar charger and feeding a 7AH batt. I have a voltmeter over the input and a Amp meter in series of the solar charger so I can fairly well check on how to get the max out of it.

Since you mention several panels I wonder if you see differences between them in efficiency towards the angle that it is being aimed at?

 

[jasonck08]

I have a watts up meter style meter connected to the panel, and the 6W 1.2A 5v panel I have isn't even enough to initiate a charge on a USB device that requires just 250mA current. My main question was if 668K lux is needed for full solar panel output, if so that sounds insane as I measured just 115K lux on a decently sunny day.

 

[lukesan]

I have a watts up meter style meter connected to the panel, and the 6W 1.2A 5v panel I have isn't even enough to initiate a charge on a USB device that requires just 250mA current. My main question was if 668K lux is needed for full solar panel output, if so that sounds insane as I measured just 115K lux on a decently sunny day.

mmm, it might be better to check the actual Voltage when connecting stuff to it. These panels are not Volt stable. The panel that I have has 24 Volts with no load on it and should produce about the max power when it measures 18.3 Volts (full Lumens, moon behind the earth and winning lottery ticket in the pocket).
So that 5 Volt panel might actually be 7 or more Volts when nothing is connected.

Also be careful with Watt meters. I did a couple of tests that made my hair change colour with success. Nice example was my tiny little clock radio that has been waking me up for the last 13 years.
Measuring with the meter it drew 12 Watts ... while the transformer inside is only 1.5VA (which approx. equals 1.5 Watts). Connected a 60 Watt bulb on the same outlet and it measured 60 Watts .... added that same radio ... 61 Watts.
I would go with a separate Volt and Amp meter, and even then you really have to know what you are measuring since you have to take into calculation that the Amp meter actually is a resistor which they measure the Voltage over.

 

[SemiMan]

So I'm testing out some solar panels, they seem to be performing very poorly overall.

I started reading about how solar panels are rated, and I found that the standard is to rate them at 1000W/m2 irradiance.

Doing some research, I found that this translates to 668,449 lux (or 668K lux for the sake of rounding).

I went outside with my extech lux meter today on quite a sunny day with not many clouds (none appear to be blocking the sun) and I get a reading of around 115K lux, which is 17% of the lux needed for max output, not even close to the Lux needed to get full output from the panel.

Does relative light output = relative solar output? For example in my case with a 10W panel, would I only be getting ~1.7W with 115K lux?

Are the above figures I found accurate? 668K lux / 1000W/m2 for full solar panel output?

No your figure is way off 668KLux would only be the case if all the suns radiation for concentrated at just one wavelength which obviously it is not.

A more realistic number is in the range of 100-110KLux with a preference towards the lower end of that (100K). That 1000W/M2 is based on a specific sun angle yielding a specific atmospheric density.

Your 115K measurement may simply be meter error, though I wonder if the solar angle is less than the standard used for 1000W/m2. A more likely scenario is meter error and/or you were picking up reflections from something else (even your body/clothes/etc.) that was increasing the value.

Best thing as pointed out is to find out what the short circuit current is for your panel and measure it and see how close you get. Just make sure it is perfectly clear and the panel is pointed directly at the sun.

Semiman

 

[SemiMan]

I have a watts up meter style meter connected to the panel, and the 6W 1.2A 5v panel I have isn't even enough to initiate a charge on a USB device that requires just 250mA current.

There are several issues here. One, the USB device may not like the variable output of the solar panel and just those little bits of clouds often drop the output below 250mA causing the USB device to stop charging. Two, the panel output is likely closer to 10V open circuit and again the USB device does not like this. Three, the USB device has a switch mode power converter on the input and the high inrush current causes the voltage on the solar panel to collapse causing the device to stop charging or doing whatever it is it is trying to do.

In general, you really need to be charging a battery and then have the output of that battery going to the USB device. Direct connections often do not work.

Semiman

 

[hiuintahs]

.....

In general, you really need to be charging a battery and then have the output of that battery going to the USB device. Direct connections often do not work.

Semiman

I agree. Everyone wants to charge their Eneloops or lithium ion directly from a solar panel and that voltage source is not regulated like you have from a power supply or stable as you'd get from a storage battery.

Most chargers are designed to be used from stable power sources because that is where the majority of the market is.

 

[lwknight]

I would think that the most logical way to get a stable power supply would be from a high capacity battery.
The solar panel would feed through a charge controller to the hi-cap battery.
Then you can power your charging device.

Even without the high capacity battery you could still use a panel that is rated higher than you need and feed through a charge controller set to the appropriate voltage. Since sun power is so unstable , a large deep cycle battery seems to be the only real ticket

 

[jasonck08]

No your figure is way off 668KLux would only be the case if all the suns radiation for concentrated at just one wavelength which obviously it is not.

A more realistic number is in the range of 100-110KLux with a preference towards the lower end of that (100K). That 1000W/M2 is based on a specific sun angle yielding a specific atmospheric density.

Do you have a source for this information? I was thinking 668K is way too high, as I can't imagine 5-6x sun intensity compared to how sunny it was... Found this in a google search:

1 Lux is equal to 0.001496 watts/square meter or 668,449 lux = 1000 watts/square meter

Solar panel technologies range from 8-17% efficiency. So that means that say a solar panel that is 15% efficient (in converting direct full sunlight to electricity) and let's say is 1 meter squared in size then it would produce 150 Watts (1000 Watts/square meter x 0.15 efficiency x 1 meter) or 100,267 lux.

Your 115K measurement may simply be meter error, though I wonder if the solar angle is less than the standard used for 1000W/m2. A more likely scenario is meter error and/or you were picking up reflections from something else (even your body/clothes/etc.) that was increasing the value.

Best thing as pointed out is to find out what the short circuit current is for your panel and measure it and see how close you get. Just make sure it is perfectly clear and the panel is pointed directly at the sun.

Semiman

I doubt it, I measured it several times, and this meter was calibrated when I bought it. 115K lux is the highest measurement I got depending on what angle I held the meter sometimes it would go as low as 100 - 110K. Can't find the short circuit current, as this panel has a built in USB charge controller.

There are several issues here. One, the USB device may not like the variable output of the solar panel and just those little bits of clouds often drop the output below 250mA causing the USB device to stop charging. Two, the panel output is likely closer to 10V open circuit and again the USB device does not like this. Three, the USB device has a switch mode power converter on the input and the high inrush current causes the voltage on the solar panel to collapse causing the device to stop charging or doing whatever it is it is trying to do.

In general, you really need to be charging a battery and then have the output of that battery going to the USB device. Direct connections often do not work.

Semiman

The solar panel has an integrated USB charge controller designed output 5v regulated output, and it does provide 5v's. The current however isn't high enough apparently to even initiate a charge on a USB device requiring 250mA.

To clarify the solar panel has a USB voltage regulator built in, and is quite stable at 4.8-5v.

Also be careful with Watt meters. I did a couple of tests that made my hair change colour with success. Nice example was my tiny little clock radio that has been waking me up for the last 13 years.
Measuring with the meter it drew 12 Watts ... while the transformer inside is only 1.5VA (which approx. equals 1.5 Watts). Connected a 60 Watt bulb on the same outlet and it measured 60 Watts .... added that same radio ... 61 Watts.
I would go with a separate Volt and Amp meter, and even then you really have to know what you are measuring since you have to take into calculation that the Amp meter actually is a resistor which they measure the Voltage over.

The watt meter is quite accurate, I tested it against my Fluke and Agilent DMM's. Also, its much much easier to use as it has the ability to measure both current and voltage at the same time. 2 meters and tons of wire would be just a huge mess for testing, not to mention have higher resistance than this meter, which is designed for currents up to 60A continuous. It has a 0.001 Ohm current sense resistor for current measuring.

I would think that the most logical way to get a stable power supply would be from a high capacity battery.
The solar panel would feed through a charge controller to the hi-cap battery.
Then you can power your charging device.

Even without the high capacity battery you could still use a panel that is rated higher than you need and feed through a charge controller set to the appropriate voltage. Since sun power is so unstable , a large deep cycle battery seems to be the only real ticket

That is my idea as well, however so far these panels appear to be garbage or something, as a 5v 1.2A panel can't successfully charge a 250mA device in perfect sunlight.

 

[HKJ]

I would suggest checking http://en.wikipedia.org/wiki/Sunlight, it says 100klux.

Your watt meter is not accurate, comparing it to DMM's that cannot measure watt does not say anything. You problems is probably that watt is not the same as volt*ampere when measuring AC.

 

[SemiMan]

Do you have a source for this information? I was thinking 668K is way too high, as I can't imagine 5-6x sun intensity compared to how sunny it was... Found this in a google search:

- Lots of references ... you will find them easier when searching. As I pointed out, 668K is ONLY if all the power was concentrated at a single wavelength ...which it is not, hence why the actual lux is about 100K.

115K is high, but perhaps only 10%. The lux should change quite a bit as the angle changes away from the sun. Extech is a mid end brand. Good, but not great results. Notice on Extech that results would be off before battery replace came on as well so make sure you have a new battery.

Unless that panel also has a battery in it, or at least a large value capacitor (like the ultracaps and other dual layer very high capacitance types), then it is next to useless as you have no idea how the USB device you are trying to charge works.

There is chance that the panel is defective as well.


Semiman

 

[Barrel mount]

Some other stuffs may influence your max reading:

- airborne pollution, includes humidity. I presume the highest reading would be in dry desert air with no wind kicking up dust.
- your geographical location vs sun position to earth.

European locations are already 50degree latitude or so, and if you're near the coast, you won't get much sun rays to start with, subsidized or not.
California stands a much better opportunity to catch the rays, but should be less than Acapulco.

 

[jasonck08]

I would suggest checking http://en.wikipedia.org/wiki/Sunlight, it says 100klux.

Your watt meter is not accurate, comparing it to DMM's that cannot measure watt does not say anything. You problems is probably that watt is not the same as volt*ampere when measuring AC.

Thanks for the wiki link... So it looks like in ~100K lux, there is no reason a solar panel should not be putting out full output (unless its output is overstated, or its broken etc.).

The watt meter is not a precision tool, I know that, but it is accurate enough to get a general idea of whats going on.

For example, testing it with a USB device on my B&K Precision digital bench power supply with the voltage set at 4.80v, watt meter reads 4.77v. The USB device is pulling 0.52A, and the watt meter reads 0.4A. Measurements verified with my Fluke 179.

As I pointed out, 668K is ONLY if all the power was concentrated at a single wavelength ...which it is not, hence why the actual lux is about 100K.

The ~100K figure makes a lot more sense, however the 668K Lux references I see don't make any sense...considering Lux by definition is precivied brightness according to the human eye (visable light only).

"It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human visual brightness perception."

 

[HKJ]

The watt meter is not a precision tool, I know that, but it is accurate enough to get a general idea of whats going on.

Sorry I mixed up two answers, it was lukesan's watt measurement I was thinking about.




I am no expert on solar panels, but I believe that some ratings are for unloaded voltage and current when shorted, this means that the panel will never be able to deliver volt*current as watt. You do also have to adjust the angle of the panel to get maximum output.

 

[lwknight]

It seems that there is not an enforceable standard in which solar panels are rated.
If a panel is rated at 400 watts and 48 volts it should by all rights hold 48 volts at a 400 watt draw. Really , you might pull it down to 30 volts when trying to get 400 watts out of it.

Some ratings are screwy right on the label.
Example: 12 volts and 10 amps and the wattage rating is 150 watts. That does not add up unless you can say that 15 amps at 10 volts counts as legitimate on a 12 volt rated panel.

 

[SemiMan]

It seems that there is not an enforceable standard in which solar panels are rated.
If a panel is rated at 400 watts and 48 volts it should by all rights hold 48 volts at a 400 watt draw. Really , you might pull it down to 30 volts when trying to get 400 watts out of it.

Some ratings are screwy right on the label.
Example: 12 volts and 10 amps and the wattage rating is 150 watts. That does not add up unless you can say that 15 amps at 10 volts counts as legitimate on a 12 volt rated panel.

Actually there are very very well defined standards by which solar panels are measured and virtually all panel manufacturers follow them.

What is missing is understanding by the uneducated consumer as to what they mean. People who work with the technology know exactly what the figure mean and what the conditions are.


Semiman

 

[jasonck08]

Actually there are very very well defined standards by which solar panels are measured and virtually all panel manufacturers follow them.

What is missing is understanding by the uneducated consumer as to what they mean. People who work with the technology know exactly what the figure mean and what the conditions are.


Semiman

I think you are referring to large commercial panels like those in the 200-300W range or so (Sharp, Suntech, First Solar etc.). The ones from all the big brands should be rated properly.

The smaller panels assembled by smaller companies, however who knows how they rate them. In my case the wattage appears to be BS.