Undersized inverter with my new Tesla Solar panels installation

[yolostreet]

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:

Given that my roof is unshaded, the power output curve should be sinusoidal and not capped. The overlaid hatched graphic shows power that my panels are producing but that the invertor limits to a threshold value of ~5.3kW. Below is an image showing the DC side of the inverter (what is coming in from the panels) and you can see that it sums to 5.73kW. This is power that I have paid for (in terms of panels and installation) but that I am not receiving any benefit from.


Am I misinterpreting this entirely? What do other people's power output curves look like? As a consumer, I expect my solar provider to provision an inverter capable of handling the output of the panels that they installed. I have complained about this to Tesla technical support and they have been downright dismissive. They claim the system is working perfectly and as designed and refuse to address the case further. I am angry and disappointed and not sure what to do next.

Thanks,
Will

 

[yolostreet]

First thumbnail links to the wrong master image for some reason, here is the correct image

 

[cwied]

It is common to design solar systems that clip in the summer because the extra inverter capacity required to avoid it has a low ROI because it is only used part of the year (and then only for part of the day). Clipping does not indicate a badly-designed system, although they should have told you about it up front. Usually the actual amount lost to clipping is a tiny percentage of annual production.

Also remember you're at the highest production of the year right now. It'll look different in a few months.

 

[power.saver]

I think your inverter is too small for your panel size. I've had three solar arrays since 2006 and I have never experienced clipping. My inverters are sized for maximum panel output at lowest temperature. I can't view either of your attachments, but from the thumbnail, it looks like your inverter clipped for a few hours. That's lost power from your solar panel purchase, and while only a small part of the annual production, it is not great for the inverter either. The inverter is throttling the DC power for the entire time it's clipping, and that means the inverter is working at its maximum limit. I would not want my inverter doing that because over time those stresses could degrade the electronics.

 

[cwied]

One other note: there is always a loss when converting from DC to AC. The DC power you're seeing on the display is already clipped. The pvwatts calculator would probably be a better way of estimating your actual maximum production.

As to whether the clipping adds stress, I don't know. I would have thought that the inverters would be designed to do that without too much trouble. As I understand it, they just the voltage away from the optimal so the panels produce less power, which does not sound like something that should add a lot of stress.

What is your inverter rating, and how many and what type of panel do you have?

 

[liuping]

I would not worry about the clipping. It is minor.

The panels will loose about 1% production per year, and will also loose a few percent when dirty. They loose up to 10% on very hot days. On top of all that, most of the year the sun will be lower at noon, reducing production further. Most days you will probably not get any clipping.

 

[NuShrike]

I think you should get your inverter replaced with one that you're happy with as Tesla undersized the inverter without having a solid agreement with you of why so.

For example, this is a 6kW system with what looks like an equally matched 6 kW SolarEdge inverter:
https://www.pvoutput.org/intraday.jsp?id=51527&sid=47806&dt=20180526

On one of the on/off cloudy days, it hit 6389W @ 13:55. I'm sure that's what you expect where the inverter has some overage tolerance instead of just clipping.

On this other system, which is micro-inverter based: https://www.pvoutput.org/intraday.jsp?id=64598&sid=57421&dt=20180526 , on the same day you can see it slightly clipping, and there's nothing that can be done vs normally just upsizing the main inverter.

April/May, July/August appears to be the peak insolation months, so scale expectations accordingly.

 

[nwdiver]

I think your inverter is too small for your panel size. I've had three solar arrays since 2006 and I have never experienced clipping. My inverters are sized for maximum panel output at lowest temperature. I can't view either of your attachments, but from the thumbnail, it looks like your inverter clipped for a few hours. That's lost power from your solar panel purchase, and while only a small part of the annual production, it is not great for the inverter either. The inverter is throttling the DC power for the entire time it's clipping, and that means the inverter is working at its maximum limit. I would not want my inverter doing that because over time those stresses could degrade the electronics.

Another way to look at it could be that there aren't enough panels to take advantage of the inverter capacity.... As the price of panels have decreased the ideal AC : DC ratio has shifted A LOT since 2006. When panels were >$3/w it made sense to collect every kWh from the panels possible. Now that they're <$0.50/w it makes more sense to make better use of the inverter and more importantly the infrastructure supporting the inverter. It's not just the inverter that would need to be upgraded but also the wiring, breaker and possibly the service panel and service drop.

An 8kW inverter from a reputable manufacturer is designed to operate at 8kW. If the inverter overheats they are designed to curtail power. This doesn't place any additional strain or stress on the inverter the surplus power is simply not harvested.

Most Arrays are now oversized by at least ~20%. The array on my house is oversized by ~30% and I'm planning a ground mount that will be oversized by ~50%. (33% facing SE and 66% facing SW)

As solar penetration increases oversizing will become more and more common since the clipped portion is the least valuable energy and the extra energy produced by a larger array... especially in the late evening is far more important.

Array Oversizing

7 reasons why you should oversize your array

You can play around with PV Watts a bit... the energy lost from oversizing up to ~30% is <5% for most locations. A larger inverter to capture those losses is generally to worth the cost.

As a consumer, I expect my solar provider to provision an inverter capable of handling the output of the panels that they installed. I have complained about this to Tesla technical support and they have been downright dismissive. They claim the system is working perfectly and as designed and refuse to address the case further. I am angry and disappointed and not sure what to do next.

Thanks,
Will

According to PV Watts you're losing <10kWh per year.... from the 'undersized' inverter. See for yourself... change the AC-DC ratio from 1 to 1.17 (Under Adv Parameters) to see the effect... If you had another $200 to spend on a larger inverter or more panels it would actually be more beneficial to spend it on more panels...

 

[yolostreet]

@cwied @power.saver @liuping @NuShrike @nwdiver - I appreciate the fast and objective responses, as well as the links and references. I have done a good bit of analysis at PWWatts this evening. Giving that I am clipping ~9% of my power currently, and we are at solar radiation peak of 7.5kWh/m2/day for my location, I estimate that the output will only be clipped for ~3.5 months of the year, which reduces the extent of the loss. The over-rating may in fact help in the winter months by keeping the inverter operating longer at its maximum efficiency.

In retrospect, having been educated a fair bit today, what troubles me more is that none of this is discussed as part of the sales process. The Tesla contract has on its opening page , in 84pt font, the power rating of the panels. As the links on this thread show, this is only one of many factors that contribute to the efficiency of the system. Buried on page 17 of the 31 page contract is the table that actually states their performance liability for the system, which is that it produce 16,452kWh over the first two years. PVWatts calculates for my panels and location an annual output of 9,479kWh/yr, meaning that Tesla are only holding themselves to ~86% of the realistic output. Given an annual reduction in efficiency of 0.5% for panel degradation, they have lots of wiggle room left. Since I purchased the system, they are incentivized not to design the optimum system to extract all 9,479kWh, but rather to have the minimum capex spend that will ensure that they do not fall below the 86%, as that is all that they are legally bound to provide. I do wonder if the same design would have been put in place if I had chosen one of the zero-down options, in which case it would be in their financial interests to maximize the annual power recovered.

Just like truth-in-lending laws for used cars, which were established to protect naive consumers from complex loan offerings, it would be nice if there were similar requirements for solar install quotes, which requires the installer to state what fraction of the estimated system output they are committing to provide.

For those just entering in to contracts and comparing providers to purchase a solution - don't be distracted by the "system size" in kW. What you should compare is the minimum committed annual production for years 2-20. Divide that by the amortized price and you have a good basis, not only for comparison between vendors, but for your expectations.

Cheers
Will

 

[nwdiver]

Giving that I am clipping ~9% of my power currently, and we are at solar radiation peak of 7.5kWh/m2/day for my location, I estimate that the output will only be clipped for ~3.5 months of the year, which reduces the extent of the loss. The over-rating may in fact help in the winter months by keeping the inverter operating longer at its maximum efficiency.

In retrospect, having been educated a fair bit today, what troubles me more is that none of this is discussed as part of the sales process.

Looks like you had good solar weather today... ~70F, partly cloudy with a little wind. You actually see higher peak generation on partly cloudy days than sunny days due to reflected light from scattered clouds yielding slightly more power. When it's a bit warmer you won't have the clipping. The power output of a solar panel decreases ~0.5% / degree C.

This is a 9.6kW array on a nice cool day left vs a hot summer day right. (The scale is off but you can see that the cool day starts clipping at 8.29kW)

I founded a PV installation company recently and I'm struggling with how to sell systems. 'Oversizing' and other elements of system design might be things I'll touch on but I think it just makes things more complicated for the typical consumer and my job is to make things easier. I agree Tesla should not have been dismissive and if someone asks why I installed more kW of panels than kW of inverter I'll be happy to explain it to them but it's not information I would volunteer. Not for any nefarious reason... it would just cause unnecessary confusion for most people. The vast majority of potential clients have a hard enough time grasping kW vs kWh; Import, Export, Usage and Production. They don't need to mull over the benefits of various over-size ratios... that's what they pay me for. I generally discuss 2 numbers. Cost and annual production... And yes the annual production estimate will be lower than what they should see... under promise and over deliver

 

[Ampster]

I have a 5.8kW system with a 3.8 Solaredge HD inverter. I saw clipping as early as February and had an extensive dialogue with my installer over a 30 day period. I had him run several simulations with the same system and different sized inverters. The differences were minimal. My ratio is 1.53 and the max that the Solaredge design software will go is 1.55 so I am right on the cusp.

Ironically I have another 4.3 kW system on an investment property. The 8 year old Xantrec inverter broke last November and I converted to a Solaredge HD inverter. The guy doing the work (not the same installer) specified a 5kW inverter and the when I asked the supplier about using the next size smaller inverter (3.8kW) he said it was not recommended. I have since read a lot about the issue plus had the later discussion with s much more knowledgeable designer. My takeaway is you will get different answers depending on who you talk to and HOW you ask the question.

In conclusion, I spent $200 more on the replacement inverter than I needed. The supplier made a little more because I bought a larger inverter. On the other hand my 5.8kW turn key system was optimized for cost/performance by that installer.

 

[renim]

For a cheap chinese invertor, i would match both sizes.
For a quality German invertor, i would oversize the panels by the max allowable under state regs.

Panels are cheap, invertors are wear items.
Cheap Panels also lose efficency, they have a 1st year boosted effect, best to ignore that anyway......

Look at it this way, the system will always be limited by either the panels or the invertor. Where is the last $100 best spent? On another panel or invertor capacity.?

 

[Ampster]

In retrospect, having been educated a fair bit today, what troubles me more is that none of this is discussed as part of the sales process.

I agree. In my case the turn key installer missed an opportunity to sell me a slightly larger system. Instead of a 5.8kW system with a 3.8kW inverter, they could have explained that a few more panels ie 6.3kW and the next larger inverter (5kW) would not be as close to the max DC/AC ratio. That was an install after recently purchasing that home. I had no usage history and had guessed at the capacity I needed based on that location. On the other hand as @nwdiver has said keeping it simple for most customers is probably better. This forum probably represents a higher technical knowledge than the average solar purchaser. I do appreciate the knowledge sharing here.

 

[PWBedesem]

I'd like to thank everyone for the great information posted on this thread. I am in the same situation - a 7.8 kW DC system with a 6.6 kW inverter that went online yesterday. I recognized it would be subject to clipping right away. I expected a 7.6 kW invertor.

Tesla Energy and I are still talking.

My question your all of you relates to inverter sizing when feeding batteries. I presume that at some point prior to a hot place freezing over, I will get two PowerWalls.

Don't we want to maximize feeding the batteries when we can? In my case, wouldn't a 7.6 kW inverter make more sense than a 6.6 kW inverter?

 

[ai4px]

Oversizing the PV power by 25% is common. It isn't harmful to your inverter as long as the string of PV panels doesn't exceed the Voc on a cold morning. The oversizing allows you to make more power on cloudy days, more power earlier in the day and later in the afternoon. Also, I've noticed that in spring my panels put out the most right after a rainy day, then as pollen builds up I can see the peak power drop each successive day. I have considered a misting system to run at night after the dew comes out, but haven't bothered yet.

 

[cwied]

Think about size vs cost of a solar system. As you grow the size you add solar panels. The point where you get more power per $ by upgrading the inverter rather than adding a solar panel is not until the inverter is significantly undersized compared to peak production. This is because even if the sytem clips at peak power, the extra panels are contributing at all other times.

 

[nwdiver]

Don't we want to maximize feeding the batteries when we can? In my case, wouldn't a 7.6 kW inverter make more sense than a 6.6 kW inverter?

No. It all comes down to a cost-benefit analysis. Unless you're space constrained and want every. single. kWh. you can get it's going to be more cost effective to invest in more panels than a larger inverter. 7.8kW is the STC (Standard Test Conditions) which are so incredibly ideal that you will likely only see that power level <1% of the total operating time. STC assumes a cell temperature of 25C and 1kW per sq meter irradiance... very very rare conditions. 'Normal' is usually defined as 44C and 800w per sq meter <80% the power of STC. So nominally you have a 6.24kW array.

Having onsite storage is irrelevant to the debate. If the actual output of your panels is rarely >6.6kW then a 6.6kW inverter is more than sufficient. I helped install a 27kW system on 23kW of inverters a few months ago. I don't think the output has exceeded 22kW for more than 30 minutes since April....

 

[PWBedesem]

No. It all comes down to a cost-benefit analysis. Unless you're space constrained and want every. single. kWh. you can get it's going to be more cost effective to invest in more panels than a larger inverter. 7.8kW is the STC (Standard Test Conditions) which are so incredibly ideal that you will likely only see that power level <1% of the total operating time. STC assumes a cell temperature of 25C and 1kW per sq meter irradiance... very very rare conditions. 'Normal' is usually defined as 44C and 800w per sq meter <80% the power of STC. So nominally you have a 6.24kW array.

Having onsite storage is irrelevant to the debate. If the actual output of your panels is rarely >6.6kW then a 6.6kW inverter is more than sufficient. I helped install a 27kW system on 23kW of inverters a few months ago. I don't think the output has exceeded 22kW for more than 30 minutes since April....

Admitting that the system is only a few days old, it's summer, and all the other factors that contribute, the system has been flat-lined for the last four hours. Bad on yesterday, it'll stay there for another hour.

 

[nwdiver]

Admitting that the system is only a few days old, it's summer, and all the other factors that contribute, the system has been flat-lined for the last four hours. Bad on yesterday, it'll stay there for another hour.

You've got perfect solar weather right now...

be interesting to see how much clipping you experience Mon-Wed. The effect of wind can be rather dramatic... it helps to cool the panels. If you look at the example I posted up thread that system was similar clipped for ~4 hours but now it barely gets to 90% of inverter capacity... granted that system is located in West Texas which see MUUUCH warmer days than where you're located.

 

[Ampster]

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.