Clipping more than expected

[discodanman45]

I finally got my system and I am waiting for PG&E to make it official. I can monitor the system and I experiencing much more clipping than what pvwatts estimated. My system consists of 15 East panels, 11 South panels, and 4 West panels. My roof made it hard to install West panels because of the weird front architecture and the South had a lot of vent systems. The panels are LG 375 and the system cost $2.80 a Watt. The inverter is a SolarEdge 7600, so I expected some clipping. I went with that inverter because eventually they will attach an EV car charger to it and it would be at least $2000 more to modify my panel to accommodate a 60 amp breaker, and I have three arrays. I attached my graph of production from yesterday and you can see it clipped from 10 AM until 3 PM. I also have two panels that aren't producing in the early AM because I have a Direct TV dish that is blocking the Sun. I forgot to get the installers to remove it, so I need to get my ladder out.

The question is how much clipping am I really experiencing? I was expecting a little clipping, but even pvwatts showed me I wouldn't be getting any in March. Getting five hours of clipping in a partially cloudy, windy, and below 60 degree day in March is excessive. I know I shouldn't worry about this and I should be glad I produced almost 67 kWh in a single day in March.

[ButchDeal]

I was expecting a little clipping, but even pvwatts showed me I wouldn't be getting any in March. Getting five hours of clipping in a partially cloudy, windy, and below 60 degree day in March is excessive.

Not sure what you are meaning here but the wind and low temperatures HELPED you produce more and clip more.

Also at 11.25kw on a 7.6kW array is right up against the max the manufactur allows and guaranteed even with the multiple azimuths to clip.

it is not easy to tell how much you are clipping wihout doing a full model of your system with all shadows etc, to see what it could produce.

[Kendalf]

The question is how much clipping am I really experiencing? I was expecting a little clipping, but even pvwatts showed me I wouldn't be getting any in March. Getting five hours of clipping in a partially cloudy, windy, and below 60 degree day in March is excessive.

What inputs did you put in PVWatts that indicated that you would not get any clipping?

[discodanman45]

I didn't realize that panels have positive temperature coefficient when they are colder than 77 degrees. I thought the 77 F was optimal temperature for production, it is just the temperature they are tested at. How can wind help? Is it just by clearing dust and debris off of the solar panels?

I know the clipping may be difficult to model, especially with multiple arrays. I know you can't apply a simple bell type curve with multiple azimuths. The cold weather makes perfect sense with getting peak production, I was actually thinking about upgrading my inverter and electric panel because of the results recently.

[J.P.M.]

The question is how much clipping am I really experiencing? I was expecting a little clipping, but even pvwatts showed me I wouldn't be getting any in March. Getting five hours of clipping in a partially cloudy, windy, and below 60 degree day in March is excessive. I know I shouldn't worry about this and I should be glad I produced almost 67 kWh in a single day in March.

PVWatts showed you nothing of the sort. Read the PVWatts help screens.

The output from PVWatts is based on "representative" weather - not "actual" or "average" weather. Most all of the weather, particularly the irradiance data from what PVWatts and many other output models use is synthetic, or modeled. Little of it, particularly the irradiance data is from actual weather records. See something called the TMY manual for details.

Any or a lot of clipping may or may not occur. Provided a system is well designed and fit for purpose, how much or how little clipping occurs and in what sequence or season is mostly a function of the weather. As such, clipping predictions are about as reliable as a weather forecast and for about as far into the future. No model can predict the weather beyond a few days, much less over a year's worth of modeling.

PVWatts is a model used for system design. It is not a predictor.

If you know you shouldn't be worried about it, why are you ?

[discodanman45]

What inputs did you put in PVWatts that indicated that you would not get any clipping?

I put in the three separate arrays with the pitch of roof, geographic location, azimuth, etc... I took the data from the spreadsheets and added the three arrays together for production for each hour. The results showed clipping, but not until April. Even then, the clipping was minimal. Maybe an hour or two a day max. Does PVWatts seem to underestimate production?

[J.P.M.]

I put in the three separate arrays with the pitch of roof, geographic location, azimuth, etc... I took the data from the spreadsheets and added the three arrays together for production for each hour. The results showed clipping, but not until April. Even then, the clipping was minimal. Maybe an hour or two a day max. Does PVWatts seem to underestimate production?

See my post and then read the PVWatts help screens and the TMY manual for a better understanding of why PVWatts is not "predicting" clipping.

PVWatts will produce a model of annual output based on input. No more. Saying an hour's output or a series of hourly outputs is a reliable predictor of future behavior is unrealistic.

[discodanman45]

If you know you shouldn't be worried about it, why are you ?

I was thinking about upgrading my electric panel to accommodate a larger inverter. It would cost me about $3000 to do this and with the large clipping I thought it may be worth it. $3000 is a lot of kWh and it would take about 15,000 kWh to earn back that money.

[J.P.M.]

I was thinking about upgrading my electric panel to accommodate a larger inverter. It would cost me about $3000 to do this and with the large clipping I thought it may be worth it. $3000 is a lot of kWh and it would take about 15,000 kWh to earn back that money.

Get a better handle on how much clipping you may be experiencing. It probably isn't 15,000 kWh/yr. or as much as a few hundred kWh/yr. More than that and I'd suggest the system design might have been better.

[discodanman45]

I agree the system design could have been better. The clipping is probably a few thousand per year looking at the graphs from the last few days. With the 7600 accepting up to 11.78 kW of panels I think this may be commonplace now. I have three separate arrays, so it probably isn't as bad as other systems. I could see a contractor putting the same system I have on one azimuth. It is not enough for me to complain to the contractor since their price was $2.80 a watt compared to the other 5 quotes I received that were all over $3.10. The contractor also said they would include the EV charger for the inverter once it becomes available. Seeing a flat line graph for 9 hours in three days shows that my system is overpowered for the inverter.

[ButchDeal]

I agree the system design could have been better. The clipping is probably a few thousand per year looking at the graphs from the last few days. With the 7600 accepting up to 11.78 kW of panels I think this may be commonplace now. I have three separate arrays, so it probably isn't as bad as other systems. I could see a contractor putting the same system I have on one azimuth. It is not enough for me to complain to the contractor since their price was $2.80 a watt compared to the other 5 quotes I received that were all over $3.10. The contractor also said they would include the EV charger for the inverter once it becomes available. Seeing a flat line graph for 9 hours in three days shows that my system is overpowered for the inverter.

No it is NOT commonplace. SolarEdge allows that for situations of high shade, or straight East/West installs etc. Most installers would NOT have put in such an over paneled system as yours. However I would definitely try to get a better handle on an estimate of the loss. Since you were looking at an EV, one solution would be to add a SE3800H which can also have an EV charger allowing you to have two EV chargers, and no clipping. it might be cheaper to do a lineside tap instead of electric panel upgrade as well.

[discodanman45]

No it is NOT commonplace. SolarEdge allows that for situations of high shade, or straight East/West installs etc. Most installers would NOT have put in such an over paneled system as yours. However I would definitely try to get a better handle on an estimate of the loss. Since you were looking at an EV, one solution would be to add a SE3800H which can also have an EV charger allowing you to have two EV chargers, and no clipping. it might be cheaper to do a lineside tap instead of electric panel upgrade as well.

I already have a Chevy Bolt and I charge about 40 kWh everyday. The EV rates for the electric company are amazing if you can avoid using energy during peak times. Plus the EV rates don't have any tiers.

I will ask the contractor about the possibility of adding a smaller inverter. I think that this would be the way to go. It also would allow me to maybe get battery storage in the future when prices fall and still have an EV charger at the same time. I really don't want to lose the ability to charge my EV directly from my solar panels, even though I have a hard-wired 40 amp EV charger. Net metering does have a 2 cent charge for each kWh pulled from the grid. I think I will definitely look at that option of adding another inverter.

[discodanman45]

Well about year later and I finally got my system "fixed". Because of the clipping almost everyday for 4 hours from my 11.25 kW system from March until August, the owner agreed last summer to add a SE3800H, free EV charger since they installed an older SE7600, do a line-side tap, and have me $360 for clipping. They then forgot about me and I had to call back. They then came to add the inverter in January this year and were just going to put half of the 11.25 kW panels on a 3800 inverter and make my clipping worse. I had to stop the workers from doing this. They then said I was correct and they would call me that night. One month without contact and i was the one who contacted back. The owner personally apologized again and they came out two days later and now my system is finally finished and I can charge my car with a SE7600.

So my 11.25 kW three array system was on a single SE7600 before. Now my system has two SE7600 inverters with 5.625 kW on each inverter, they did a line-side tap, gave me a free EV charger, will be installing two more LG375W panels on my system for free in a few weeks, gave me a $200 check(now up to $560), and $100 gift card to a nice steakhouse in town. Plus the lead guy in charge of the maintenance part of the company told me that if I ever want to add more panels that they would give me a good deal.

The company dropped the ball many times with me and I think they are hoping I don't leave a bad review. Here is a picture of my production today. It is still February and I would have clipped today with the system that was "designed" by someone that had no idea what they were doing.

[nwdiver]

I think a lot of this has to do with better educating customers. Clipping is going to get A LOT more common going forward. Installers and owners need to start embracing it. As solar PV becomes more common energy generated around noon on a sunny day will be worth less and less while energy on cloudy days and in the evenings will be worth more and more. So it's going to make sense to oversize arrays and face them west to increase the production of valuable energy even if it means losing some at noon that would be worth much less. My most recent system was similarly oversized though the panels are in a E/W arrangement which slightly mitigates clipped production.

NEM also can't last forever... clipped energy is much more likely to be exported => less valuable than energy added in the morning and especially in the evenings.
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[discodanman45]

I think a lot of this has to do with better educating customers. Clipping is going to get A LOT more common going forward. Installers and owners need to start embracing it. As solar PV becomes more common energy generated around noon on a sunny day will be worth less and less while energy on cloudy days and in the evenings will be worth more and more. So it's going to make sense to oversize arrays and face them west to increase the production of valuable energy even if it means losing some at noon that would be worth much less. My most recent system was similarly oversized though the panels are in a E/W arrangement which slightly mitigates clipped production.

NEM also can't last forever... clipped energy is much more likely to be exported => less valuable than energy added in the morning and especially in the evenings.
Screen Shot 2019-03-24 at 7.50.05 PM.png

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. If you think it is acceptable that your installer thinks clipping from 10 am until 4 am is acceptable, then you embraced a lie... This type of clipping is going to stress inverters, I hope you have an extended warranty on them!