### Author Topic: Calculation of insolation from solar panel output  (Read 2374 times)

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#### helgew

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##### Calculation of insolation from solar panel output
« on: January 14, 2015, 12:18:38 PM »
I posted this query in another section but have not had any responses... basically, I am wondering if anyone has put together the math to calculate insolation from solar panel output data? Given geographic location, time of day, roof angle, and orientation of the panels as well as their efficiency, it should be possible to do this.

#### SLOweather

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##### Re: Calculation of insolation from solar panel output
« Reply #1 on: January 14, 2015, 01:59:28 PM »
I think it would depend on allowing for extraneous factors like loading to get an accurate number. Certainly, if you had a panel dedicated just to that, it could be done fairly easily. Put a load resistor on it and log panel voltage and current. But panel output is non-linear, so it would take some math to normalize it.

#### helgew

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##### Re: Calculation of insolation from solar panel output
« Reply #2 on: January 14, 2015, 02:09:54 PM »
I think it would depend on allowing for extraneous factors like loading to get an accurate number. Certainly, if you had a panel dedicated just to that, it could be done fairly easily. Put a load resistor on it and log panel voltage and current. But panel output is non-linear, so it would take some math to normalize it.

My system uses micro converters, one for each panel. Each controller reports energy production in kW/h and that figure is not dependent on the load on the system. I believe the panel's efficiency is a fixed value, but even if not, efficiency data are somewhere in the specifications for the panels and are probably the easiest factor in all the math

#### Josiah

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##### Re: Calculation of insolation from solar panel output
« Reply #3 on: January 14, 2015, 08:06:40 PM »
I was just reading up on what "insolation" is. Interesting...

Now you have me thinking about it...

I've been searching around and this is the best I've come up with so far.

EDIT: Are you trying to figure out how much solar radiation is hitting the panel adjusted for panel efficiency, or how much Solar radiation would be hitting its surface if it were laying level?
« Last Edit: January 14, 2015, 09:23:09 PM by Josiah »

#### helgew

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##### Re: Calculation of insolation from solar panel output
« Reply #4 on: January 15, 2015, 12:10:38 PM »
Are you trying to figure out how much solar radiation is hitting the panel adjusted for panel efficiency, or how much Solar radiation would be hitting its surface if it were laying level?

To be honest, the definitions of insolation are not too specific about how it is actually accurately measured by "real" instruments. However, I believe irradiance is usually measured as solar radiation hitting a surface perpendicular to the sun's rays.

There is a package for R named solaR that looks promising and I will report back once I have had the chance to play with it some.

#### wmh

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• Posts: 1
##### Re: Calculation of insolation from solar panel output
« Reply #5 on: April 09, 2016, 07:30:08 PM »
Simply connect an ammeter directly across a small horizontal solar panel to measure the short circuit current. Panel voltage changes little with insolation but panel current and hence panel power varies directly as insolation.

To convert the result in Amperes to something in Watts/square metre would require a calibration factor which would vary a little with panel temperature so this would have to be measured as well. Perhaps the calibration factor could be found by comparison with the (accurate?) insolation numbers from a nearby Government weather station.