Is this clipping?

[Matt-FL]

That does not look like clipping to me. When mine does, it is 100% flat lined at the top

It is clipping, but the second inverter that is underloaded makes it difficult to see. I have 29 T400w panels on one inverter, and 22 on the other. The 29 panels are definitely clipping, as they are all south facing. The 22 panels inverter will never reach 100%, as 11 are east facing and 11 are west facing.

Why were the panels installed like this? Well, not according to the designer's specs. The installers altered the design once they reviewed my house. They altered the design (in my opinion) to make installation easier rather than what was best long term. At the time, I simply didn't know any better and was just happy they were performing the install after 14 months of post-order waiting.

If I go outside at peak, inverter #1 is fan blowing 100% to cool down, and many times, inverter #2 is just barely keeping the fans blowing.

So my curve, while not flat, is flatter than it should be during the peak hours.

 

[h2ofun]

It is clipping, but the second inverter that is underloaded makes it difficult to see. I have 29 T400w panels on one inverter, and 22 on the other. The 29 panels are definitely clipping, as they are all south facing. The 22 panels inverter will never reach 100%, as 11 are east facing and 11 are west facing.

Why were the panels installed like this? Well, not according to the designer's specs. The installers altered the design once they reviewed my house. They altered the design (in my opinion) to make installation easier rather than what was best long term. At the time, I simply didn't know any better and was just happy they were performing the install after 14 months of post-order waiting.

If I go outside at peak, inverter #1 is fan blowing 100% to cool down, and many time, inverter #2 is just barely keeping the fans blowing.

So my curve, while not flat, is flatter than it should be during the peak hours.

After mine we installed on 2 inverters, I had them come back out and move some sets around between them to better
balance them for what they were used for, and the angle they were facing.

 

[mongo]

I have yet to see any documentation mentioning "overdrive". I think it is a myth. How can an inverter with a rate max output of X do more than X?

It can't. The term means the peak PV capacity exceeds the inverter capacity. However, just like plugging a 2.5W USB charger into a wall outlet doesn't result in the charger pulling all the power in the grid, the inverter is in full control of the amount of power draw.
Key parameter is to not overvoltage to inverter, that is damaging.

Given that, why is there a limit on DC vs AC (besides decreasing gains)? Only thing I can think of is that the maximum power point tracking doesn't work well when the panel response is too flat.

 

[BGbreeder]

It can't. The term means the peak PV capacity exceeds the inverter capacity. However, just like plugging a 2.5W USB charger into a wall outlet doesn't result in the charger pulling all the power in the grid, the inverter is in full control of the amount of power draw.
Key parameter is to not overvoltage to inverter, that is damaging.

Given that, why is there a limit on DC vs AC (besides decreasing gains)? Only thing I can think of is that the maximum power point tracking doesn't work well when the panel response is too flat.

The MPPT is like a micro bucket brigade. As long as there is some water (electricity) to be scooped up, it works, but it will never pass more than a bucket at a time. From an MPPT perspective, you would want DC output of the panels to be less than or equal to, the AC capacity of the inverter.

I think that what throws many people off is that a "6kW solar system" will only produce 6kW of DC if all,of the panels are perpendicular to the sun, and it is cool enough for the panels not to be losing capacity. On a fixed array, like a roof, that never, or almost never happens as the roof doesn't point exactly south and the roof is too flat most of the time. So having DC kW > AC kW makes reasonable engineering sense most of time as the DC power is less than the listed capacity. Many people have done the calculations for their particular installation against PVWatts and found that clipping represents a small percentage power loss over the year, even with south facing arrays.

So, some amount of DC solar capacity beyond the AC rating is not unreasonable as an engineering choice, and as a financial choice, while from the perspective of maximum output from a given set of solar panels, it represents a small loss. (Insignificant in my view).

Does that help?

All the best,

BG

 

[dhrivnak]

I have yet to see any documentation mentioning "overdrive". I think it is a myth. How can an inverter with a rate max output of X do more than X?

Over drive may be the wrong word. I mean 10.5kW DC solar on a 7.7kW inverter. No more than 7.7kW AC but due to earlier start up, later shut down and higher efficiency the losses at minimal.

 

[mongo]

The MPPT is like a micro bucket brigade. As long as there is some water (electricity) to be scooped up, it works, but it will never pass more than a bucket at a time. From an MPPT perspective, you would want DC output of the panels to be less than or equal to, the AC capacity of the inverter.

I think that what throws many people off is that a "6kW solar system" will only produce 6kW of DC if all,of the panels are perpendicular to the sun, and it is cool enough for the panels not to be losing capacity. On a fixed array, like a roof, that never, or almost never happens as the roof doesn't point exactly south and the roof is too flat most of the time. So having DV kW > AC kW makes reasonable engineering sense most of time as the DC power is less than the listed capacity. Many people have done the calculations for their particular installation against PVWatts and found that clipping represents a small percentage loss over the year, even with south facing arrays.

So, some amount of DC solar capacity beyond the AC rating is not unreasonable as an engineering choice, and as a financial choice, while from the perspective of maximum output from a given set of solar panels, it represents a small loss. (Insignificant in my view).

All the best,

BG

Yeah, I've built power converters/ PPT systems. My early morning pre coffee brain was thinking of the lack of voltage fluctuation based on current variation for a severely oversized system would be an issue, but the MPPT would be looking at power, so it would still track.

Which leads back to my original thought that a comically oversized PV system would run the inverter at it's maximum whenever the sun was up, but shouldn't have any other effect. In other words, once the inverter is clipping, additional parallel panels are a no-op (beyond minor voltage increase).
Unless it's a control loop thing where too much voltage shift due to irradiance change is an issue...

 

[Redhill_qik]

I have yet to see any documentation mentioning "overdrive". I think it is a myth. How can an inverter with a rate max output of X do more than X?

You aren't seeing this term, because it is generally known as oversizing the PV array versus the inverter. The inverter will not do more than its rated spec, but you can put more kW into than it can output.

 

[Matt-FL]

I find this response from Tesla quite humorous. It was the response I received shortly before they installed (from August 25, 2022), when I asked about my 20.4kw of panels and only two 7.8kw inverters. In hindsight, I should have known better. If a 7.8kw inverter was capable of 170% capacity for hours on end... well, that would be impressive.

Hello,

This is <your 8th Project advisor> at Tesla, your Design has factored both efficiency and ease of installation into your design, and although we are able to overdrive up to 170% of our inverters we also ensure minimal clipping with this design . doing a 50/50 split between the inverters may not be the best solution in regards to how the conduit will run.

<8th Project Advisor>

 

[Redhill_qik]

I find this response from Tesla quite humorous. It was the response I received shortly before they installed (from August 25, 2022), when I asked about my 20.4kw of panels and only two 7.8kw inverters. In hindsight, I should have known better. If a 7.8kw inverter was capable of 170% capacity for hours on end... well, that would be impressive.

Hello,

This is <your 8th Project advisor> at Tesla, your Design has factored both efficiency and ease of installation into your design, and although we are able to overdrive up to 170% of our inverters we also ensure minimal clipping with this design . doing a 50/50 split between the inverters may not be the best solution in regards to how the conduit will run.

<8th Project Advisor>

You have two 7.8kW inverters, so if PV is split that is 10.2kW of PV per inverter or ratio of 1.31. Depending on the orientation of the panels it might not make sense for these to be 50/50. You might have more panels that are facing North going into one inverter and less panels facing South going into the other which would match from actual kW output from the strings.

 

[Matt-FL]

That would be great, and is what the Tesla designer had planned. However, the installers adjusted (in my now semi-experienced opinion) based on what was easier to install.

What I ended up with is 29 T400w panels - all south facing on two strings, connected parallel to one inverter. There is a second string, where they probably peeled off an odd number of panels to match the parallel strings, but no idea how many.

On the other side, I have 22 T400w panels, 11 east, 11 west facing panels on the second inverter.

So I have one inverter cranking all day long with 11.6kw (potential) coming in, and the second inverter meandering around all day skipping along.

 

[mongo]

You aren't seeing this term, because it is generally known as oversizing the PV array versus the inverter. The inverter will not do more than its rated spec, but you can put more kW into than it can output.

Pedantic mode on:
Solar can't put power into the inverter (unless you overvoltage it), the inverter draws power from the panels. Clipping occurs when the panels can produce more power than the inverter is drawing.

 

[BGbreeder]

Yeah, I've built power converters/ PPT systems. My early morning pre coffee brain was thinking of the lack of voltage fluctuation based on current variation for a severely oversized system would be an issue, but the MPPT would be looking at power, so it would still track.

Which leads back to my original thought that a comically oversized PV system would run the inverter at it's maximum whenever the sun was up, but shouldn't have any other effect. In other words, once the inverter is clipping, additional parallel panels are a no-op (beyond minor voltage increase).
Unless it's a control loop thing where too much voltage shift due to irradiance change is an issue...

Yup. Short answer, yes, a small inverter hooked up to a large array would chug along at 100%, the panel/string voltage rises, limiting current, and the excess solar energy turns up as heat in the array. MPPTs are just filling their buckets and moving them along. They don't really care how fast they fill; if bucket is full quickly, it gets passed along, and if the fill rate is slower, it waits.

No worries about the morning fog; we have all been there.

All the best,

BG