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# How to measure solar energy collected? Answered

If this isn't the right section to post this question, please direct me to a more appropriate place.

So I am pretty knowledge-less about solar energy, so I decided to do some research on it a couple hours ago. Is there a simple way to measure the solar energy that is collected? From what I understand, determining if it is cost efficient to implement solar energy is very dependent on how much sunlight is collectible, which seems to be very location-dependent.

The only solar device I really have available are the \$2 outdoor solar lights made by Westinghouse (purchased at Walmart, impossible to miss). Anyways, the device works by collecting solar energy with the panel on top, and converted to be stored in the rechargeable Ni-Cd AA Battery.

Would I be better off trying to measure the amount stored in the battery for a period of time, or is there a way of measuring the solar light collected accurately?

As I said, I am extremely new to this topic,  so I apologize if any of this was too vague or incorrect. Constructive criticism never hurts :).

Any ideas or suggestions?

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## Discussions

You can measure the electrical power put out by your panels directly.  Use a multimeter.  First, measure the voltage V across your panels when illuminated.  Second, measure the current I across your panels under the identical conditions (just switch the meter without disconnecting, moving the red lead from V to A).  The power is just V x I, if you're measuring everything in SI units (watts = volts x amperes).

Of course, if you change the lighting, or the tilt of your panels, or stand over them when making the measurements, you'll get a different result.  So you need to do multiple runs, spread out over time, and take an average.

Except the VI curve is non-linear and Voc x Isc does NOT equal peak panel power.

Make an electric analogue clock, use  a DC motor, and power the motor with a panel. The elapsed time is a direct measurement of the energy received.

Steve

Hi, Steve. You are quite right about the V-I curve, and not being able to extrapolate peak power output from individual readings. But shouldn't you still be able to get an instantaneous data point? I must be missing something that's obvious to an engineer (pretty common for me :-/ ).

Using an electric analogue clock to integrate the varying power output is a really cool solution.

Hi Kelsey
I don't propose tp bore people with the math, but I reckon you could be out by between 35 and 50% of the "actual" maximum power output. Its why MPPT controllers are so impressive - you can get 35% or more energy from a given panel !

Steve

Pffft! Math isn't boring, it's awesome! :-D It's fascinating that the performance of PV panels are so dependent on the load -- I tend to think in terms of ideal circuit elements, but these things seem awfully far from "ideal"!