Wondering about how to verify that all 3 strings are working correctly on my new system. The rated max output is about 5,500W DC, the highest I have seen on the monitor is about 3,800 watt. Maybe this is normal for this time of year and my location (250deg facing angle, 20 deg roof angle), but it would be interesting to know for sure.

Is there a safe way to disconnect the strings from the inverter terminal block without causing an arc (for example, do I need to shut off the panels with the roof disconnect, then disconnect a string, turn it back on, etc)...or is there an easier way? I'd like to be able to check output around the same time of day and conditions. Thanks!

Jza80,

Yours array’s full power output spec is based on so-called Standard Test Conditions, or STC. A key parameter of STC is that the module cell temperature be at 25 C (77 F). Since cell temperature can be as much as 30 C to 35 C (54 F to 63 F) above ambient, the modules tend to run much hotter than 25 C.

PV module power output drops as cell temperature increases. A typical coefficient of power is +/- ~0.4%/C. So, if it’s 20 C (68 F) in your location, and the modules are operating at 35 C above ambient, their cell temperature may be ~55 C, or ~30 C above the 25 C STC reference. This environmental condition alone will cause a power drop of ~12% from your array. Attaching the array to/near your home’s roof exacerbates the problem because there’s little natural convection to move cooling air below the array, and the array may operate at even higher temperatures.

Another problem is the array’s 20 degree tilt angle. Assuming you're well into the northern hemisphere, the error angle of the Sun’s irradiation will be quite large. This alignment error will reduce array output current, which of course means lower power output. Other performance reducing factors include dirty modules and power losses in the array wiring and the charge controller.

Your installation is somewhat complicated due to the azimuth angle of 250 degrees. The low tilt angle will compensate for this in the summer, but it’ll cost you in the winter. Producing 3,800 W from a 5,500 W STC array means that part of the system is operating at ~69%. Considering all of the items I mentioned above, that (unfortunately) sounds about right.

In summary, it’s highly unlikely you’ll ever see a sustained 5,500 W from your system unless you live in an area that’s fairly cold and clear in the winter and your system is better aligned, both in az and tilt.

HTH,
Jim / crewzer

I don't know what your latitude is, but 3,800 watts is probably about right for this time of year & your tilt angle.



You could do that, but it's a lot of work. Another way is to use a clip-on DC ammeter. You would just clip the ammeter around one wire from each string (only one wire at a time). Either wire (plus or minus) ok. This will give you a reading in AMPS, NOT WATTS. Just compare the three readings (one reading from each string). They should be about the same, give or take a little. If you want to convert these readings to watts, you will need to know the voltage of each individual string at the same time that you take the amp reading. Then multiply volts times amps to get watts. This is more labor intensive as you will need to separate the strings to take the readings. And yes, shut off the disconnect before disconnecting the string wiring.

Disconnecting an active string WILL cause an arc, and it will be "lively" 300VDC @ 5 amps is pretty resistant to quenching. Easiest way is to use the GT's internal disconnect to shut down the arrays. With no current flow, it's easy to unhook the LIVE 300V wires, and just measure each one by itself, with 5 minutes to let the inverter restart in between.

Looks like my array is working OK.

We had a cool day here today, around 58deg F, and some clouds. When the sun broke through the output shot up to 4,600 - 4,700 watts. Nice to know that the capability is there, when the conditions are right.

jza80,
after taking your precautions for safety and arc you could measure the voltages of each string without a load and with loads, but this may not give the best indication of the string's abilities excepting to verify they are similar to each other. i'm assuming this to be a grid tie no battery setup so the voltages will be high so do be sure the meter used can handle it and err on the side of caution.
the better indicator is the current produced and if you can find an accurate ammeter that can handle the rated current you believe it will produce it will show how close to spec it will be by measuring the short circuit current. dmms are usually able to handle up to 10amps and you may need to place a fuse inline with some of these meters as some lack this feature when measuring high current.
if all of this is greek to you then don't do it. i believe that you are getting pretty much what's expected seeing as how the pvs aren't aimed well. they should be aimed south and at a tilt at least equal to the latitude. maybe repositioning them to a more ideal orientation won't be possible, but if you can at least bump up the high point ot the pvs even an inch this would give a slightly better angle overall with a slight added cooling ability. basically i'm saying that nearly anything you do may help as they are aimed in a near westerly direction. at least they weren't aimed north.