Clipping more than expected

[nwdiver]

Yes, I meant the 15000TL. The graph they posted had a peak output of 15 kW, so I figured that was their system.

Apologies for the confusion. The graph I posted was of a recent system that just came online last week. (2) 7.7SB on 22.77kW. So oversized by ~148% with E/W facing arrays. The SB8000 I have at my home has been online for ~6 years and I've never observed any derating despite being oversized by ~134% with panels facing 180S. My inverter is usually saturated for ~5 hours/day in the spring and ~2-3 hours/day in the summer.

[discodanman45]

There's no such inverter. Did you mean the 15000TL? That would make more sense.

But in any case the inverter he has - the SB8000 - is a run of the mill 240 volt inverter, often specified while it was available due to its low cost.

Yes, I meant the 15000TL. The graph they posted had a peak output of 15 kW, so I figured that was their system.

[jflorey2]

We are not comparing apples to apples here. It seems like you have a SMA Sunny Tripower 1500TL Inverter. This is a commercial inverter that is designed for oversized DC/AC ratios.

There's no such inverter. Did you mean the 15000TL? That would make more sense.

But in any case the inverter he has - the SB8000 - is a run of the mill 240 volt inverter, often specified while it was available due to its low cost.

[jflorey2]

The other critical consideration here is that not all kWh are created equal. Here in NM SPS pays $0.024/kWh for net excess... currently they won't allow NEM credits to roll monthly. The vast majority of clipping will occur in the Spring and Fall and the systems really need to be sized for high summer consumption so clipped energy is largely only worth ~$0.024/kWh. I modeled <5% loss or ~2000kWh/yr. @ $0.024/kWh that's $48/yr. It would take ~100yrs to justify the $5k service upgrade.

Yep. The math always wins in the end.

[bcroe]

I am not convinced that some clipping will much shorten the life of an inverter. My
Fronius have been doing clipping 8 hours or more every day the sun shines, for
over 5 years without issue. Bruce Roe

[discodanman45]

I have a SB8000 on a 10.7kW array. I've never observed any derating in 6 years.

I thought the graph you posted was your array... You have a 1.34 DC/AC ratio on a E/W array set-up. That is fine and a system that is well designed. Mine was a 1.48 ratio on a mainly E/S with a few W panel arrays. I clipped everyday for 4 hours with no clouds, even during 110 degree days. My inverter was taking a beating.

If you have graphs with your inverter, you would only see an hour or two of clipping max in the spring and fall. That is absolutely fine. The problem is my installer saw that you could put 155% on the SE7600 and they did that without any simulations of data. I later found out that they quoted my friend with over an 11 kW system on a SE7600 with 90% of the panels on a S roof without any shading. That is the issue I have.

[nwdiver]

We are not comparing apples to apples here. It seems like you have a SMA Sunny Tripower 1500TL Inverter. This is a commercial inverter that is designed for oversized DC/AC ratios. Your unit probably has a high powered fan and is an expensive high quality unit. This is a commercial inverter and wouldn't be used for most residential installations. Typical residential inverters don't have high quality fans and do this to keep noise levels down. Your inverter was designed to be tortured. A SE7600 wasn't designed for 5 hours of clipping. It will even derate itself to protect it from high tempertures. Commerical inverters have cooling abilities so it won't derate.

I have a SB8000 on a 10.7kW array. I've never observed any derating in 6 years.

[discodanman45]

Observations of my inverter. The internal temperatures increase more with ambient temperature than with a ~20% increase in output. The inverter can compensate for higher power output with higher fan speed plus heat dissipation is more effective with a higher delta T. Higher ambient temperature means less effective cooling.

We are not comparing apples to apples here. It seems like you have a SMA Sunny Tripower 1500TL Inverter. This is a commercial inverter that is designed for oversized DC/AC ratios. Your unit probably has a high powered fan and is an expensive high quality unit. This is a commercial inverter and wouldn't be used for most residential installations. Typical residential inverters don't have high quality fans and do this to keep noise levels down. Your inverter was designed to be tortured. A SE7600 wasn't designed for 5 hours of clipping. It will even derate itself to protect it from high tempertures. Commerical inverters have cooling abilities so it won't derate.

[nwdiver]

It's the area under the curve, not the length of time - and it's over the course of a year. A well designed system, with west and east facing panels, might well clip all day on a cool, windy day with good angle sun, and not clip at all on a hotter day. If the area under the curve represents, say, 50 kilowatt-hours over the course of a year (about $6 a year) then it's not really worth adding another $1000 inverter - especially since tare losses will go up.

The other critical consideration here is that not all kWh are created equal. Here in NM SPS pays $0.024/kWh for net excess... currently they won't allow NEM credits to roll monthly. The vast majority of clipping will occur in the Spring and Fall and the systems really need to be sized for high summer consumption so clipped energy is largely only worth ~$0.024/kWh. I modeled <5% loss or ~2000kWh/yr. @ $0.024/kWh that's $48/yr. It would take ~100yrs to justify the $5k service upgrade... Even just more inverter capacity barely pencils.

This is going to get more common. Sort of crazy to think that if we achieve ~20% of annual generation from Solar that's ~100% of demand at noon. So once we hit ~20% penetration the value of energy at noon is going to plummet while the value of energy after 4pm may actually rise.

[jflorey2]

1. The first one I agree with. However, clipping 5 hours a day is too much for a home system.

It's the area under the curve, not the length of time - and it's over the course of a year. A well designed system, with west and east facing panels, might well clip all day on a cool, windy day with good angle sun, and not clip at all on a hotter day. If the area under the curve represents, say, 50 kilowatt-hours over the course of a year (about $6 a year) then it's not really worth adding another $1000 inverter - especially since tare losses will go up.

2. Because if you are clipping at 7.6 kW you are losing energy.

Agreed there. Whether it is significant is the question. (See above.)

The other thing is you are going to hurt your inverter. My inverter was running at high temperatures for 5 hours a day. If you have an industrial inverter meant to have high clipping, then it is no issue. However home inverters are not designed or meant to have this much clipping.

Again, why do you think this? There is no difference (from a thermal perspective) on an inverter clipping at 7600 watts and not clipping at 7600 watts. Keep in mind that in a Solaredge system the inverter always sees the same voltage, so it doesn't even see a different voltage.

Oversizing the inverter also causes the inverter to operate at high power for longer periods, thus affecting its lifetime. Operating at higher power also increases inverter heating and may heat its surroundings. Inverters will reduce their peak power generation in case of overheating.

Well, I think there's a typo there; if it says what you claim it does, it should be "undersizing the inverter."

But the rest is correct. It is the power it operates at that determines its operating temperature. And again, if you are operating at 7600 watts, it doesn't matter if it's clipping or not. The issue is the power level.

[nwdiver]

What proof do you have in this? Oversized arrays on SolarEdge inverters raise the temperature much more than ambient temperatures. I know this for a fact since I had an oversized system in Central Valley California where we have 110 degree days. Oversized arrays for hours stress SolarEdge inverters much more than ambient temperatures. I observed this myself personally.

Observations of my inverter. The internal temperatures increase more with ambient temperature than with a ~20% increase in output. The inverter can compensate for higher power output with higher fan speed plus heat dissipation is more effective with a higher delta T. Higher ambient temperature means less effective cooling.

[discodanman45]

A 10F increase in ambient temperature will probably affect service life more than oversizing an additional 50%.

What proof do you have in this? Oversized arrays on SolarEdge inverters raise the temperature much more than ambient temperatures. I know this for a fact since I had an oversized system in Central Valley California where we have 110 degree days. Oversized arrays for hours stress SolarEdge inverters much more than ambient temperatures. I observed this myself personally.

[nwdiver]

So not only do you get clipping, but you can also derate your inverter. I had points during the day where my inverter basically shut off to cool itself down.

True; But I would argue that environmental factors have a much larger affect than oversizing and that SHOULD be taken into account. A 7.6kW inverter may only TRUELY be a 6.7kW inverter if it's installed in death valley. So oversizing 130% of nameplate would really be oversizing 150% but that same inverter installed in Portland OR would have no issue supporting 11.4kW. A 10F increase in ambient temperature will probably affect service life more than oversizing an additional 50%.

[discodanman45]

1. As always, do the math. Overpaneling to get more production late in the day - even if it causes clipping - may make more financial sense than going to a larger inverter.

2. Why do you think producing 7600 watts while clipping is worse than producing 7600 watts with no clipping?

1. The first one I agree with. However, clipping 5 hours a day is too much for a home system.

2. Because if you are clipping at 7.6 kW you are losing energy. The other thing is you are going to hurt your inverter. My inverter was running at high temperatures for 5 hours a day. If you have an industrial inverter meant to have high clipping, then it is no issue. However home inverters are not designed or meant to have this much clipping.

To nwdiver, please read the documentation for a SolarEdge inverter. I meant to say oversizing instead of overclocking, sorry.

"PV inverters are designed so that generated output power will not exceed the maximum AC power. In many cases, oversizing the inverter, i.e. having more DC power than the inverter AC power, may increase power output in lower light conditions, thus allowing the installation of a smaller inverter for a given DC array, or alternately, installation of more DC power for a given inverter. However, too much oversizing of the inverter may have a negative impact on the total energy produced and on the inverter lifetime. This document provides considerations for oversizing inverters and presents the maximum allowed DC/AC oversizing of SolarEdge inverters."

"On the other hand, too much oversizing may negatively affect the inverter power production: Inverters are designed to generate output power up to a maximum AC power that cannot be exceeded, and they limit (clip) the power when the actual produced DC power is higher than what the inverter can output. This results in loss of energy.
Oversizing the inverter also causes the inverter to operate at high power for longer periods, thus affecting its lifetime. Operating at higher power also increases inverter heating and may heat its surroundings. Inverters will reduce their peak power generation in case of overheating"

So not only do you get clipping, but you can also derate your inverter. I had points during the day where my inverter basically shut off to cool itself down.

[jflorey2]

I have no idea what type of argument is being made here. Clipping should be minimal, especially with the small price increases in inverter size.

As always, do the math. Overpaneling to get more production late in the day - even if it causes clipping - may make more financial sense than going to a larger inverter.

If you think it is acceptable that your installer thinks clipping from 10 am until 4 am is acceptable, then you embraced a lie...

Again, do the math. Math doesn't lie.

This type of clipping is going to stress inverters, I hope you have an extended warranty on them!

Why do you think producing 7600 watts while clipping is worse than producing 7600 watts with no clipping?