Undersized inverter with my new Tesla Solar panels installation

[nwdiver]

My system has been flat lining (clipping) since February and I am not concerned. Occasionally I look at that flat top and wonder but then I remember the simulations that my vendor ran for me.

+1; This is how solar production curves SHOULD look... especially with modules <$0.50/w

Add to that the fact that as solar penetration increases that lost energy due to clipping will become less and less valuable while the increase in production from 5-8pm will be come increasingly MORE valuable...

I really hope I get the chance to build a 24kW system I'm pitching. It's gonna use 2 7.7kW inverters. Each inverter will have 8kW facing West and 4kW facing East so production should max out at ~15.4kW at ~11am and stay there until ~6pm

 

[PWBedesem]

My system has been flat lining (clipping) since February and I am not concerned. Occasionally I look at that flat top and wonder but then I remember the simulations that my vendor ran for me.

The simulations your vendor shared with you are the key. Tesla Energy seems to be so busy that they can't find anyone who is willing or able to do that. The sales people here don't seem to be technically oriented and the technically adept people are sequestered such that us mortals may not bother them.

I'm still working on them to let me talk with someone. I really appreciate the information here. It's clarified my thinking quite a bit.

 

[hacer]

In many cases the inverter efficiency is reduced when operating below max power. Since that happens more often than clipping does (for a small clip design) it turns out you can have less energy overall if you size the inverter for absolute max power. In addition as time goes on the panel output will decrease so that the inverter becomes even more oversized.
Because inverts are designed in discrete max power capability you may have to jump to say 10kW from 7.5kW even though peak is only 8 kW. The oversized inverter could be 3% less efficient at 7kW and even more so lower. The only way to know the best solution is to run a good simulation. Unless your actual peak output happens to be very close to an available inverter capacity it's often more efficient to clip.

 

[NuShrike]

Add to that the fact that as solar penetration increases that lost energy due to clipping will become less and less valuable while the increase in production from 5-8pm will be come increasingly MORE valuable...

This is where tilt-mount panels will pay for themselves. Not only does tilt-mount milk TOU times better, but it also can prevent clipping at peak insolation (as panels would not be at optimal tilt).

 

[nwdiver]

This is where tilt-mount panels will pay for themselves. Not only does tilt-mount milk TOU times better, but it also can prevent clipping at peak insolation (as panels would not be at optimal tilt).

'Tilt-Mount'? Like a tracking array?

A lot of this simply comes down to $/kWh. That's really what Solar PV should be for... how do I get the most kWh for the least $$$. The AC/DC ratio isn't even one of the top 3 design decisions that reduce the cost-effectiveness of a PV system. IMO the top 3 areas are Needless optimization, Improper tilt of ground mounts and remotely locating inverters. I recently visited a 30kW array that suffered from all 3 issues. They spent an additional $6k on an optimized solar edge system that will have ~0 effect on performance since it's a ground mount with no shade. The array is tilted at 20 degrees when 30 degrees could have increased annual generation by ~10% at no additional cost and they installed the inverters at the array and ran AC to the meter instead of installing the inverters near the meter causing an additional ~5% in line loses AND the extra expense of heavier wire....

In contrast I helped with a 27kW system that cost $30k in materials. 2 string inverters cost ~70% less than the solar edge system with near identical performance and we ran DC ~250' to the home with smaller, cheaper wires AND less line loss. The two arrays are ~20 miles apart so it will be interesting to see the difference in performance... I expect the smaller cheaper system will actually outperform the larger more expensive one.

Point being... don't fret over the smaller issues. Module type and AC / DC ratios are rather negligible. Anything between ~1.2 and 1.4 is fine.

 

[NuShrike]

'Tilt-Mount'? Like a tracking array?

As in literally tilt-mount:
https://stellarsolar.net/wp-content/uploads/2018/05/west-irvine-solar.jpg?quality=100.3018052418030

The 18 panels in the middle are tilted 24° cumulatively -- the roof's tilt is 18°.

If the city rules were more flexible, it would've been tilted 32° cumulatively.

 

[Ampster]

Not only does tilt-mount milk TOU times better, but it also can prevent clipping at peak insolation (as panels would not be at optimal tilt).

If I understand how a tilt mount can milk the TOU times better you are saying mount them facing more west because, the IOUs are shifting the peak periods to later in the day.
Also, I thought clipping was a result of the panels outputting more that the inverter capacity. That is usually a result of a high DC to AC ratio. but I can understand how the tilt angle could optimize winter angles over summer so that one would not see as much clipping in the summer with a mount optimized toward winter angles of inclination. I suppose one could run some simulations to compare annual generation with something like PVWatts that lets you tweak those inputs fo see which is optimal.

 

[Ampster]

/....and they installed the inverters at the array and ran AC to the meter instead of installing the inverters near the meter causing an additional ~5% in line loses AND the extra expense of heavier wire....

I am assuming the DC voltage was above 300 volts requiring smaller wire and having less loss than 240v AC.

 

[nwdiver]

I am assuming the DC voltage was above 300 volts requiring smaller wire and having less loss than 240v AC.

Yep. Usually well north of 350v. AND if there's clipping losses are free

 

[NuShrike]

If I understand how a tilt mount can milk the TOU times better you are saying mount them facing more west because, the IOUs are shifting the peak periods to later in the day.
Also, I thought clipping was a result of the panels outputting more that the inverter capacity. That is usually a result of a high DC to AC ratio. but I can understand how the tilt angle could optimize winter angles over summer so that one would not see as much clipping in the summer with a mount optimized toward winter angles of inclination. I suppose one could run some simulations to compare annual generation with something like PVWatts that lets you tweak those inputs fo see which is optimal.

One can do simulations with NREL's System Advisor Model, the step up from PVWatts, to find an optimal tilt that covered the entire year and gave maximal TOU "savings" vs maximal "output" for the mounted azimuth. FYI, it's almost impossible to get a more-west azimuth mount from what the roof is as installers won't "rotate" the mount to something that's not parallel to the roof's edges. Power to you if anybody finds one that will do that.

Meanwhile, SAM supports specifying the exact modules to simulate as it has a mostly-complete CEC database of panel and inverter specifications. It uses insolation data from a typical-model year, or from actual half/hourly measured insolation, for the geographical area -- the latest dated 2016. You can also 3D position panels and obstructions to model shading losses into the simulation.

 

[ai4px]

I love the graph in NWDIVERS post #21 in this thread.

Here's what "clipping" looks like with regard to solar PV production. Adding a little extra PV power is cheaper incrementally than moving to the next size up inverter. On a clear day, you'll make more power because production peaks out earlier in the day. In this chart, it peaks >2hours earlier and lasts 2 more hours longer in the afternoon. On cloudy days, you'll still probably reach peak power, whereas with a system sized to 100%, you won't make full production.

And one more thing... panels typically lose 0.6%-1% per year of capacity. At the end of 20 years your system will be making 80% of it's initial capacity. It is at that point you'll be glad you over sized by 25%.

 

[Don TLR]

Thanks for all the great info in this thread!!!! I bought a house with a 6K system and trying to get educated on Solar..

 

[NuShrike]

I love the graph in NWDIVERS post #21 in this thread.

Here's what "clipping" looks like with regard to solar PV production. Adding a little extra PV power is cheaper incrementally than moving to the next size up inverter. On a clear day, you'll make more power because production peaks out earlier in the day. In this chart, it peaks >2hours earlier and lasts 2 more hours longer in the afternoon. On cloudy days, you'll still probably reach peak power, whereas with a system sized to 100%, you won't make full production.

One can counter-argue then if you had 95% clipping/loss, you would be at peak-power all day and get even more "energy harvest".

Some premium-plus panels only lose 0.34% per year, so it would behoove to tune the system as best as possible.

 

[cwied]

One can counter-argue then if you had 95% clipping/loss, you would be at peak-power all day and get even more "energy harvest".

If solar panels were free and infinitely efficient, that's exactly what you would install. You either waste solar panel production or inverter capacity. It's the price relationship of the two that determines where the optimum trade off is. Since neither is free, the optimum is somewhere in-between maximizing solar panel production and maximizing inverter utilization. Constraints such as roof space and available inverter/panel sizes complicate the picture, of course, but the basic idea that all clipping is bad is a fallacy.

The main point of the picture is that clipping doesn't mean that you're wasting a solar panel. The panel contributes quite a bit of energy when production is below the inverter capacity.

 

[Magistrate703]

Hello all! This thread is exactly what I was looking for. I just had Tesla install a 9.425 PV optimized system with their 325watt panels and they installed the new Solar Edge HDWave 7600h. I'm not a math guy at all but I figured out it is over-sized by about 124% and It'll be clipping about ~1800 watts at peak hour here in Orlando, FL. The system is not turned on yet due to city inspections failing the first time but hopefully I'll be up and running in the next couple weeks... or month.
Anyway, from what I've read here it's calmed my nerves a bit since I thought Tesla was just trying to save money and maximize their profit from my system... I am still leaning towards demanding the new 'SolarEdge HDwave 10,000h' just to get every last drop of power out of my $30,000 system (before tax credit of course). But, I am wondering if you all had any more info on this. I just don't get why you wouldn't want every bit of power from your array... I am not shaded at all and panels are installed on roof due south at less than 30degrees- perfect for production here in sunny Orlando.

 

[Dan123]

I just got a 6.175kWh 19 panel Tesla solar installation mounted on my roof here in California and received PTO from PGE. After turning the system on, I notice that the peak power produced by the panels during the day is being truncated and discarded by the inverter, which appears limited to 5.3kW of output. Here is my my power production curve:
View attachment 306933

It is quite common to undersize the inverter. The thinking is that the solar panels will produce peak power only a few days out of the year, and the energy lost to this "shaving" is negligible (1-2%). Moreover, your panels degrade over time and they will accumulate some dirt and bird poop on them, so very soon you will not see any of this "shaving" loss.

 

[nwdiver]

Hello all! This thread is exactly what I was looking for. I just had Tesla install a 9.425 PV optimized system with their 325watt panels and they installed the new Solar Edge HDWave 7600h...... I am still leaning towards demanding the new 'SolarEdge HDwave 10,000h' just to get every last drop of power out

~7600 is kind of a magic number in the residential PV world. Like most people you probably have a 200A main panel. Most PV systems are connected in this panel. The max back-fed breaker in a 200A panel is 40A to prevent overloading the bus. The max sustained current for 40A is 32A or ~7600w. So to go any larger would require a different and more expensive connection. Definitely not worth the cost. 120% Rule.

According to PV Watts you'll lose ~13kWh/yr due to your undersized inverter... how much are you willing to pay to produce another ~13kWh/yr? My limit would be ~$20.... the 10kW is ~$400 more than the 7.6kW.

 

[Magistrate703]

~7600 is kind of a magic number in the residential PV world. Like most people you probably have a 200A main panel. Most PV systems are connected in this panel. The max back-fed breaker in a 200A panel is 40A to prevent overloading the bus. The max sustained current for 40A is 32A or ~7600w. So to go any larger would require a different and more expensive connection. Definitely not worth the cost. 120% Rule.

According to PV Watts you'll lose ~13kWh/yr due to your undersized inverter... how much are you willing to pay to produce another ~13kWh/yr? My limit would be ~$20.... the 10kW is ~$400 more than the 7.6kW.

Very interesting... I am beginning to understand the solar madness. Cool stuff! Yes I am on a 200A breaker and hopefully our wonderful City inspector wont make us downgrade like they did with our neighbors Tesla 10.4 system to pass inspection. Thanks for the info though! I will let this one go... haha Now to get this thing switched on..

 

[nwdiver]

Very interesting... I am beginning to understand the solar madness. Cool stuff! Yes I am on a 200A breaker and hopefully our wonderful City inspector wont make us downgrade like they did with our neighbors Tesla 10.4 system to pass inspection. Thanks for the info though! I will let this one go... haha Now to get this thing switched on..

Yep... if your neighbors had a 10kW inverter on a 200A panel that's probably why they failed the inspection. Wonder if they just swapped it for a 7.6kW... 10.4kW on 7.6kW would be fine too

 

[Magistrate703]

Yep... if your neighbors had a 10kW inverter on a 200A panel that's probably why they failed the inspection. Wonder if they just swapped it for a 7.6kW... 10.4kW on 7.6kW would be fine too

He has the older SolarEdge SE10000A which can make up too 10.9kWh according to the spec sheet. I think they downgraded him to a 150A breaker if I remember correctly. Apopka inspectors are finicky. I am going through failed inspections due to not having a separate AC disconnect... I think, they aren't very specific - City just wrote "Plans don't match installation". The plans have it on there but install crew decided I didn't need it according to national reqs. so now I'm in limbo...