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Posted by Wayne Langille on February 20, 2006, 5:54 pm
How do you measure the power of sun hitting solar panel? I took my small
solar panel and hooked it up to volt meter, is this a good way to measure
it, not enough sun to really test it but when clouds block sun I get about
20.2 v, when sun hit it I get about 22v, maby my panel is getting old,been
on fishing boat for 8 yrs.

Posted by schooner on February 20, 2006, 6:16 pm
Found this one:

I would assume reading the voltage off an solar panel would give the same
thing but not sure.

Posted by daestrom on February 20, 2006, 7:30 pm

Actually, the *voltage* from a solar panel that is not loaded is pretty
constant.  The *current* is what varies with the strength of the sunlight
hitting them.  These little 'light meters' typically have a circuit where
they short the solar cell and measure the short-circuit current.  This turns
out to be pretty linear with sunshine.

Of course, shorting out your solar panel means you get zip for power.  But
depending on how it's connected to your charger/controller/battery/whatever,
you're usually better off measuring the current.


Posted by Gary on February 21, 2006, 12:24 am
 Wayne Langille wrote:

Hi Wayne,

I've been struggling with the same problem.

I've seen the DayStar meter that Schooner mentions and been tempted to buy one,
but it does not really say what part of the spectrum it measures.  Since they
don't talk about any of the issues that are discussed in the sites listed below,
I'm really not sure what they are measuring.

Here are a couple places that sell stuff to measure sun light, and some material
on the features of a good sensor.




These tend to be on the pricey side.  I was about ready to spring for the $00
Onset one (partly because I have some Onset loggers), only to find out that you
have to buy a whole different logger to read it.

For the time being, I use the Onset Pendant light intensity logger, which is
only $0 and logs light intensity in Lum/sqft -- this is very convenient, and I
can just let it keep logging whenever I am doing a test.  But, I have not found
a way to convert Lum/sqft to BTU/hr-sqft or the like (maybe Daestrom or Nick
have an idea on this?).  Also do not know what part of the solar spectrum it

On very clear days, you can look up solar intensity in a reference, or use my
Radiation On Collector program to calculate it, but I suspect that the intensity
level can vary quite a bit even on clear days?  On the little Pendant meter, I
see variations from around 20K up to around 25K for what seem like clear sky

Another thought that I plan to follow up on is to find the nearest weather
station around here that does sun measurements, and see if I can get these
records to calibrate my pendant logger.

Anyone have any other ideas?




"Build It Yourself" Solar Projects

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Posted by Jeff Thies on February 21, 2006, 2:40 am
 Gary wrote:

Google yields 683lumens = 1 watt and 3.413 watt * hours = 1 BTU (but you
knew that.)

   So I'm thinking 3,243 lumens = 1 BTU/hr

   A lumen doesn't seem to be much!

So take your lumens per square foot and multiply by the area. And a
fungible for how much much of those lumens is convertible to heat.

   Of course, I could be completely wrong!



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