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Instruments for testing photovoltaic panels

April 28, 2019 — BarryK

I posted a few days ago about disappointing power output of my "250W" folding solar panel:

...I only got about 120W out of it.

What I want to do is get setup to do more precise measurements, including taking into account the intensity of the sun and the ambient and cell temperatures. To achieve this, I needed to purchase some measuring instruments, that I have done.

Solar power meter

I hunted online to find out if any local store (in Perth, Western Australia) has one of these, and at a reasonable price. Found this, at a clearance price of AU$99.95:


First impressions, quality construction, works OK.

Infra-red thermometer

Measuring solar cell temperature is difficult, as they are sandwiched between materials both in front and back. I decided to use an infra-red non-contact thermometer, and bought this one, AU$34.95, from Altronics:


First impression, easy to use.

There is going to be a temperature gradient from the cells to back surface of the panel. If the thermometer is used to measure back temperature, then I read somewhere that very roughly the cell temp will be 7°C higher.

I also read somewhere, that if the thermometer is used at the front of the panel, at a steep angle, not perpendicular, it can get an IR reading direct from the cell. That remains to be seen.

There is another approach to testing. Instead of letting the panel warm up in the sun, keep it in shade, so the cells are at ambient temperature. When everything is connected to take a reading. move the panel into the sun, take a reading very quickly, then move the panel back into the shade. I saw this technique being used in a YouTube video. However, I don't know whether this will work, as I imagine the cell junction will get hot very quickly. I intend to play with this method.

Resistive load

One way to do a super-quick evaluation of a panel is to test short-circuit current with a multimeter. With the multimeter on DC current-reading setting, and being sure that the multimeter can handle the expected current, connect it directly to the terminals in the panel junction-box. The panel could be in shade, then bring it into the sun and observe  the current.

To obtain the peak power point, though, needs more equipment. An MPPT regulator connected to a not-fully-charged battery would do it. If the MPPT regulator is doing it's job, it will hunt for and find the peak power point. Some means of measuring voltage and current will be required.

An alternative is to use a variable resistive load. For the previous test on the "250W" panel, I used nichrome wire, just some lengths that I had. However, some kind of continuously-variable adjustment would be better, to accurately locate the peak power point. Also, it would need to be designed to handle the power dissipation.

Such a load, I am planning to build very soon. 

Tags: nomad