SolarEdge Inverter(s) and SolarEdge PowerBox(s) Configuration

[Zeigh]

Hello,

I am having a little trouble understanding how many solar panels that my inverters can realistically handle. Here are the details...

- two (2) SolarEdge model SE7600A-US-U inverters (rated at up to 7680 watts each)
- fifty (50) SolarEdge model P400-2NM4ARM PowerBoxes (rated at up to 400 watts each)
- fifty (50) SolarWorld 315 watt solar panels

This is a matched system from a national vendor, yet the math is not adding up. The panels themselves will produce up to 15750 watts, yet the two SolarEdge inverters can only handle up to 15360 watts. What am I missing here? Because of the proprietary SolarEdge PowerBox "micro inverter" technology, none of the research elsewhere online is giving me a straight answer.


Peace,
Dr. Z.

[DanKegel]

- two (2) SolarEdge model SE7600A-US-U inverters (rated at up to 7680 watts each)
- fifty (50) SolarEdge model P400-2NM4ARM PowerBoxes (rated at up to 400 watts each)
- fifty (50) SolarWorld 315 watt solar panels

25 * 315 = 7875

So you're clipping by at most 195 watts, and only when the panels are directly facing the sun.
That's probably less than a 0.5% loss in power over the day.
Using a beefier inverter might lower your efficiency a tiny bit, so you're probably fine.

(At least, I assume that clipping at the inverter is fine. I'm no expert.)

[bcroe]

Hello,

I am having a little trouble understanding how many solar panels that my inverters can realistically handle. Here are the details...

- two (2) SolarEdge model SE7600A-US-U inverters (rated at up to 7680 watts each)
- fifty (50) SolarEdge model P400-2NM4ARM PowerBoxes (rated at up to 400 watts each)
- fifty (50) SolarWorld 315 watt solar panels

This is a matched system from a national vendor, yet the math is not adding up. The panels themselves will produce up to 15750 watts, yet the two SolarEdge inverters can only handle up to 15360 watts. What am I missing here? Because of the proprietary SolarEdge PowerBox "micro inverter" technology, none of the research elsewhere online is giving me a straight answer.
Peace, Dr. Z.

Don't forget, 15360 W is the OUTPUT rating of the inverters. If they are 97 % efficient, it
would take 15,835 W DC input to reach that output. And the DC wiring loss... Bruce Roe

[sensij]

What Bruce said. Also, this isn't quite right:

The panels themselves will produce up to 15750 watts

The panel rating is what they produce under a specific test condition (controlled irradiance and environment). The actual production in the real world will almost always be less than that value, although if weather, atmospheric conditions, irradiance, and panel orientation all aligned in the right way, nothing would prevent the panel from producing *more* than its rating.

For a better idea of what the real world performance might be, look at the PTC rating, or sometimes the CEC rating... not the nameplate STC rating. Another set of numbers worth looking at might be the performance at NOCT (nominal operating cell temp).

[Amy@altE]

It is very common to have a higher DC:AC ratio for inverters, up to 125%. The inverter has a minimum voltage input needed from the panels (or in this case the Optimizers) to turn on. If you oversize the array, it will reach that voltage earlier in the morning, and stay on later in the afternoon. The chances of you actually seeing the rated output of the panels is so slim, maybe mid-day on a bright cold winter day, that the number of times the output is clipped is more than made up by the additional power generated in morning and afternoons.

The max string input is 5250W, with 315W panels in a string of 13 and one of 12, you have up to 4095W per string. Even if you use the 400W rating from the optimizers, that's 5200W. That configuration is good.

[solar pete]

It is very common to have a higher DC:AC ratio for inverters, up to 125%. The inverter has a minimum voltage input needed from the panels (or n this case the Optimizers) to turn on. If you oversize the array, it will reach that voltage earlier in the morning, and stay on later in the afternoon. The chances of you actually seeing the rated output of the panels is so slim, maybe mid-day on a bright cold winter day, that the number of times the output is clipped is more than made up by the additional power generated in morning and afternoons.

The max string input is 5250W, with 315W panels in a string of 13 and one of 12, you have up to 4095W per string. Even if you use the 400W rating from the optimizers, that's 5200W That configuration is good.

+1, really good explanation for the newbies, cheers.

[sensij]

It is very common to have a higher DC:AC ratio for inverters, up to 125%. The inverter has a minimum voltage input needed from the panels (or in this case the Optimizers) to turn on. If you oversize the array, it will reach that voltage earlier in the morning, and stay on later in the afternoon. The chances of you actually seeing the rated output of the panels is so slim, maybe mid-day on a bright cold winter day, that the number of times the output is clipped is more than made up by the additional power generated in morning and afternoons.

This is something I still don't quite understand. When the circuit is open, Voc can be achieved very easily early in the morning, no matter how many panels are in the array. I bet someone out there has done it with a bright flashlight, just because. However, very little current will flow if the inverter sees that voltage and tries to start inverting. Although "turn-on voltage" is thrown around sometimes, is it really "turn-on power" that matters? The effect you are describing is sort of the same... under a given low light irradiance, if you have 5 panels in series it will produce half the power of a string of 10 panels (because the voltage is half), so the "turn-on power" is achieved earlier in the morning with a longer string.

I understand the voltage matters with regard to the MPPT range, but that will be just as true in the middle of the day as it is at sunrise and sunset. Per the typical IV curve, as irradiance drops, the drop in power is found in reduced current, not in reduced voltage.

I guess what I don't understand is that if Voc for a 5 panel string and Voc for the 10 panel string are both within the MPPT range, why wouldn't they turn on at the same time? In either case, if voltage is all that matters, all the inverter has to do is to regulate the array voltage to just a tick below the Voc to start generating current. The 5 panel array will produce half the power because its voltage is half (but still above the MPPT minimum), while the current would be the same for both arrays.

If it is really turn-on power that matters, the system makes more sense to me. The 5 panel array needs to wait until its current is double the current of the 10 panel array to get the same power. That means the sun needs to be slightly higher, however much it takes to get the plane of array irradiance for the small array to be twice as much as the big array (with the imperfect assumption of all else being equal).

[bcroe]

This is something I still don't quite understand. When the circuit is open, Voc can be achieved very easily early in the morning, no matter how many panels are in the array. I bet someone out there has done it with a bright flashlight, just because. However, very little current will flow if the inverter sees that voltage and tries to start inverting. Although "turn-on voltage" is thrown around sometimes, is it really "turn-on power" that matters? The effect you are describing is sort of the same... under a given low light irradiance, if you have 5 panels in series it will produce half the power of a string of 10 panels (because the voltage is half), so the "turn-on power" is achieved earlier in the morning with a longer string.

I understand the voltage matters with regard to the MPPT range, but that will be just as true in the middle of the day as it is at sunrise and sunset. Per the typical IV curve, as irradiance drops, the drop in power is found in reduced current, not in reduced voltage.

I guess what I don't understand is that if Voc for a 5 panel string and Voc for the 10 panel string are both within the MPPT range, why wouldn't they turn on at the same time? In either case, if voltage is all that matters, all the inverter has to do is to regulate the array voltage to just a tick below the Voc to start generating current. The 5 panel array will produce half the power because its voltage is half (but still above the MPPT minimum), while the current would be the same for both arrays.

If it is really turn-on power that matters, the system makes more sense to me. The 5 panel array needs to wait until its current is double the current of the 10 panel array to get the same power. That means the sun needs to be slightly higher, however much it takes to get the plane of array irradiance for the small array to be twice as much as the big array (with the imperfect assumption of all else being equal).

I don't think you will be turning anything on with a flashlight. I tried that under full moon,
and voltage was a couple % (and no current).

My 2 inverters will recognize sunlight, but won't start until there is enough DC power available
to produce about 125W AC. . Probably yours are in the same ballpark. That is for strings of
720 cells; I presume it would be similar for other lengths. Sure 10 panels will reach that power
level sooner than 5. As soon as direct sunlight hits a string, power here rises to the KW level,
and 10 KW when they are all out of the shade.

A question I have been wondering about, is if there is ever a situation where some strings
can make power, but some mostly shaded strings will back feed and steal some power.
The moonlight experiment hinted at that. Guess I could insert some big diodes to block
that, wire some LEDs to show any reverse voltage across them. Bruce Roe

[sensij]

I don't think you will be turning anything on with a flashlight. I tried that under full moon,
and voltage was a couple % (and no current).

Thanks for the response, what you've said makes sense. I think, to be picky about it, the act of measuring the voltage will draw current, so Voc might have been achieved by the flashlight, but could not be sustained under the load of the meter.

[bcroe]

Thanks for the response, what you've said makes sense. I think, to be picky about it, the act of measuring the voltage will draw current, so Voc might have been achieved by the flashlight, but could not be sustained under the load of the meter.

Well, the VOM drew less than 1 micro amp here. With more than 1000' of wiring out
there, other measurements indicate a larger loss to insulation leakage. My suspicion
is, leakage in the cells themselves is dominating the situation. Bruce Roe

[Carl_NH]

Hello,

I am having a little trouble understanding how many solar panels that my inverters can realistically handle. Here are the details...

- two (2) SolarEdge model SE7600A-US-U inverters (rated at up to 7680 watts each)
- fifty (50) SolarEdge model P400-2NM4ARM PowerBoxes (rated at up to 400 watts each)
- fifty (50) SolarWorld 315 watt solar panels

This is a matched system from a national vendor, yet the math is not adding up. The panels themselves will produce up to 15750 watts, yet the two SolarEdge inverters can only handle up to 15360 watts. What am I missing here? Because of the proprietary SolarEdge PowerBox "micro inverter" technology, none of the research elsewhere online is giving me a straight answer.


Peace,
Dr. Z.

I recently posted a similar question (I have 7.2KW array and SE 6KW Inverter and Optimizers). I see clipping at 6KW from the inverter in the middle of the day. Now the SE 6KW inverter max DC input is 7500 W DC, BUT max output is 6WK AC. I was misled to believe they would output above the 6KW AC, and this is the only inverter in the SE product that does not output 10% more AC Watts than rated.

Now the SE 7.6 inverters actually can take max input of 9500 DC Watts, but also the key difference is they can output up to 8450 W AC, so with your panels you should never have clipping.

Carl