Amperage

Lauzon · June 20

I have two strings of 3 250W panels installed for my off-grid system. The panels have open circuit voltage of 37.2 and short circuit current of 8.74. I'm getting anywhere from 95 to 100 V at the MPPT which is roughly 200 feet from the panels however only 5-8 amps. My assumption is I should be getting double the amperage. Am I not correct? If so what caused this? I have 4 breakers in the combination box and have tried all 4 to see if it is a breaker issue.

BB. · June 20

Based on my experience with Grid Tied Inverter (GT inverters always draw 100% of available array "energy"--Off Grid systems are always a bit "confusing" interpreting array output because it depends on the MPPT charge controller's interpretation on battery state of charge, DC loading, available sun, etc.)... Assuming a battery bank with less than 80% State of Charge and a reasonably designed, programmed, and installed:

Have a 6 panel array with Vmp-panel~33 volts and Imp (250W/33Vmp= ) 7.58 Amps.

2*7.58a Imp = 15.74 Imp-array at full noontime sun with array within 10 degrees of pointing at sun

Another conversion of Imp = Isc * 0.80 Imp conversion factor = 7.0 amps estimated/guess Imp-panel

Or around 14 Amps Imp-array... Really need Vmp & Imp panel sticker info--But close enough for first pass guess.

Another calculation:

6 panels * 250 Watts = 1,500 Watt array
1,500 Watt array * 0.77 panel+controller deratings = 1,155 Watt Watt array "best case typical output" (cold clear day, near solar noon, a few times a year peak)
1,155 Watts / 100 Volts Vmp-array (measured) = 11.55 Amps "best estimated" array "real life output" (again in "perfect conditions"

Without measuring actual solar irradiation with a reference cell, I would suggest that an arbitrary "normal" day, that output can be upwards of 50% of "best case" output (moisture in air, dust in air, dust on panels, etc.) or approximately:

11.55 amps * 0.50 = 5.78 amps "floor" of good array (before I would "worry" about array performance on an "average" day)

Our human eyes are not great at estimating 1,000 Watts/sq meter sun (100% full sun energy on a very clear day). "We" can barely "see" the difference between a 1,000 W/sq meter day vs a 500 W/sq meter day (dust/moisture in air, etc.)... Using a solar irradiance meter:

https://www.amazon.com/s?k=solar+irradiance+meter&i=tools

is really needed if you are trying to verify array solar performance (again MPPT controller & battery SoC) makes accurate array performance measurements "difficult").

Remember Power=Voltage*Current--Voltage (Vmp or voltage maximum power) falls as panels get hot--On a hot day, no wind, panel/array Vmp can fall by almost to 80% (or 0.80x) "mfg sticker" value (based on lab test of "flash" sun test--I.e., panel/cells are at ~25C/77F vs real life at 65C/149F or so on hot/windless day). The 77% (some folks like 75%) panel "real life summer array derating" is roughly 82% Vmp-array and 5% MPPT controller "best case" losses.

In your case, you have two identical parallel solar array strings... Get an AC+DC Current Clamp DMM and measure the current from each string. Typically, the two strings would be withing a few percent of each other for Imp-operational per string current of each other:

https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (lower cost, good enough, DC capable current clamp DMM)
https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid priced, nicer AC+DC current clamp DMM)

Note there are AC only current clamp DMMs... They are perfectly fine meters, but cannot measure DC current.

If your two array strings are not very close in current flow--Then look for "issues" (shading from dirty panel(s), shading from tree, chimney, plumbing stack, or bad electrical connections, etc.).

Or you throw a blanket/tarp over one string at a time and check the voltage&current on your MPPT controller and make sure that each string is carrying 1/2 the load.

In the end, with the information provided I don't see that you have a "bad/questionable" array--Especially when MPPT controller charging a battery bank + DC loads. Measuring string current is usually a very quick way to isolate array problems.

MC4 connectors issues (corrosion, bad crimps, overheated/burned connections, etc.) are very common wiring issues. And sometimes you can see "browning/blackening" of panels or cells (not good). Having too high of voltage drop in wiring from Controller to Battery bank can also limit "peak array harvest" (suggest 0.05 to 0.10 volt max voltage drop for a 12 VDC battery bank). Keep wiring short and "heavy" AWG from charge controller to battery bank for optimum controller battery charging.

-Bill

Photowhit · June 20

Lauzon said:

I have two strings of 3 250W panels installed for my off-grid system. The panels have open circuit voltage of 37.2 and short circuit current of 8.74. I'm getting anywhere from 95 to 100 V at the MPPT which is roughly 200 feet from the panels however only 5-8 amps. My assumption is I should be getting double the amperage. Am I not correct? If so what caused this? I have 4 breakers in the combination box and have tried all 4 to see if it is a breaker issue.

Yes/No, I don't know...

A few questions, You have 2 strings yet 4 breakers at the combiner box?

The first thing I pick up on is that you appear to be running these together and running 200 feet. So, what gauge wire have you used?

You have given us the highest value of the panels, numbers that have little relation to the panels underload (charging).

Here are examples IOC vs IMP and VOC vs VMP as well as STC numbers and NOCT (Normal Operating Cell Temperature values)

SO first lets look at you VOC (voltage open circuit) values 37.2, SO that would correlate to a VMP (Voltage Maximum Power) of about 82% of that say 30.5 volts.

They have a ISC (current Short Circuit) of 8.74, Which would translate to an IMP (current Maximum Power) of around 94% of that roughly 8.2 amps.

So 30.5 volts x 8.2 amps gets us to roughly 250 watt panels. 30.5Volts x 8.2amps = 250.1 watts.

You have 2 strings of 3 panels these so the voltage should be 3x the individual panel and the current should be 2x for 2 strings in parallel, or roughly 91.5 volts and 16.4 amps before losses.

Losses of about 15% in current and 10% in voltage. Lets call it 14 amps at 83 volts. If you are measuring 100 volts at the controller, I suspect that short circuit.

Now that we have Maximum numbers let's look at further loses. If you compare the STD (Standard Test Conditions to NOCT (Normal Operating Cell Temperature) values, you will see about a 75% difference in total wattage. If the panels are in an Optimal angle to the sun, within 20 degrees of perpendicular, you are looking at them producing about 75% of the rated value. You can expect some further loss if they aren't clean, and more if you have multiple connections. The panels are 200 feet from the controller, but how long is the wire run?

...and how thick is the wire.

If the wire has to go up to the panels and back down on both ends and has a slight curve lets say 240 feet. If it's 12 gauge you may find additional losses of 16% in voltage lost to resistance across the wire.

That said it does appear you should be getting higher current at the controller.

Amperage

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