"Efficiency" on PVOutput and comparing performance between differently sized arrays?

[jasonvr]

On PVOutput, there is a quantity that gets calculated called Efficiency with units of kWh/kW (kilowatt-hours per kilowatt). If appears that this is a decent way to judge performance of two systems regardless of their size. I'm part of a "team" on PVOutput composed of other people who used the same installer as me. If I look at all team members (all located in the general LA area) and sort based on efficiency on any given day, my system tends to be at the top on many days. More importantly, when I notice a dip in my production that is most likely weather related (either cloud cover, or heat driving down panel efficiency) I can look at the team results and see that everyone's efficiency number has dropped and the rankings more or less stay in the same order.

So, what exactly is that Efficiency number measuring/relating and is it really a good way to compare the relative performance between systems regardless of system size? It really just seems to be a ratio of the energy generated and the DC system size (kinda arbitrary since one is AC and one is DC)

[AzRoute66]

Not knowing anything about PVOutput, I would place my bet on array orientation, shading, DC accumulation method (micro-invert, optimize, string), and perhaps panel thermal response at the top of the factors that differentiate the DC kWh/kW rankings. I wouldn't even speculate on the conversion to AC.

[tyab]

AzRoute66 - you are way way way over thinking this

https://pvoutput.org/help.html

PVoutput calcs efficiency as kWh/kW. Basically your current total output divided by the system size value in your settings. System size in settings is calculated as the max panel output x number panels.

For example if you have 10 panels and each is a 280W panel, then your system size is 2.8kW, and efficiency is your current total output / 2.8.

[AzRoute66]

tyab - I understand what is being calculated. The OP asked (to my understanding) why size equalized systems, installed by the same installer, in the same geographical area, would have certain systems outperforming other systems in a predictable and repeatable manner. He then asks, "So, what exactly is that Efficiency number measuring/relating" and I speculated on the factors that might contribute to that in the DC arena. Of course, it could be that his system is always near the top of the rankings because of his pure motives and clean living - or perhaps now I am overthinking it...

Edit: We'll see if the inverter gurus can add more factors other than the obvious inverter efficiency number. Perhaps the load type (capacitive, inductive) or if the load is more 'surgy' than 'steady'???

[cebury]

The joys of the internet forum. It appears both of you might have misread a point in the previous post which changes your post, then the next person does the same repkying to yours. I apologize if now I have also done it a third time.

I think we all know what the kwh/kw ratio is (fairly self explanatory). The OP defined it in his last sentence.

Like any other time we accept statistical reporting, we have to know what a number means and does and does not represent. Obviously whomever originally developed the formula realized the inherent simplicity of using the Energy generated (yes, post inverter AC) divided by the STC DC KW rating. Generating kwh is the goal after all and STC system size is, well, standard method of sizing. Two consistent, established, measurable units. It excludes all those influential factors that would have any affect on performance (such as those listed by Az above) other than system size in order to be useful as a common unit of measure. However, using kwh makes it prone to the inverter's quality/grade of metering.

So how about we attempt to answer the OPs ouestion, is this ratio really a good way to measure performance between systems of different sizes? Anyone have any better ideas?

[sensij]

So how about we attempt to answer the OPs ouestion, is this ratio really a good way to measure performance between systems of different sizes? Anyone have any better ideas?

I frequently sort the single day output of "Team San Diego" by the efficiency metric being discussed here, and then play the game of trying to figure out why the ranking is what it is on that particular day. (yes, this is my idea of fun!). AzRoute66 's response pretty much nails the top 4 things that matter, in about right order of significance. (shade can be more important than orientation, but towards the top of the list, most of the systems are relatively shade free except for differences in what time the marine layer / cloud cover clears). I think differences in temperature show up as more important than hardware selection, as well.

Through the forum, I've even taken this to the level of reaching out to system owners when I see something I can't explain... one system has two arrays that performed differently, and the owner explained that one was up on a 2nd story roof that gets a great sea breeze that can make it the most efficient (kwh/kw) system during the right times of the year, and a 2nd array on a sheltered first story roof that underperforms by a noticeable amount.

So, is it a "good" metric? I like that PVoutput presents it, because if it wasn't there, I'd probably be trying to calculate it by hand. As a metric that can be easily misinterpreted, it isn't great, but that it clearly illustrates the significant factors that affect output is nice. if you can find two systems located close enough together, with similar orientation, it is a very good way to try to tease out differences in performance over time, moving to the next level of factors like degradation and fouling.

The way PVOutput implemented "insolation" (butchered the temp coefficient contribution) makes me glad they haven't tried to tackle any more complicated corrections that would lead towards an "adjusted" efficiency that could more directly compare arrays with different characteristics.

[ButchDeal]

Many of them are not set up well in pvoutput, there are way too many single array due south facing systems in there. They are easy to spot particularly if the show insulation button shows a graph way off.
Often for the ones that are close it is a matter of them using magnetic north instead of true north.
i always try to check ours and it is an easy way to find if the original assessment was off.

[jasonvr]

So, if I'm reading the comments right, the absolute number really isn't that helpful as it relies on a lot of factors (shading, orientation, etc), but it seems like as a relative number amongst geographically similar systems, it can help spot trends. So for example, if there are three systems near me, all set up correctly in PVOutput, and I am typically number one in the rankings (@sensij, I also sort and see where I am ranked in the team), but suddenly I drop to last place for an extended period of days, I could suspect that something has gone wrong with my system. It it were a weather phenomenon causing a dip, in the same geographic area each system should be more-or-less equally affected and cause the Efficiency number for each of the systems to go down in a similar fashion.

Sound about correct?

[ButchDeal]

Depending on the inverter system, you generally can get better problem reporting from the inverter monitoring system directly. This is definitely he case for solaredge which has excellent trouble reporting and easily configured and personalized. Though generally installers do not give homeowners access to this alerts section.

[AzRoute66]

[...]but suddenly I drop to last place for an extended period of days, I could suspect that something has gone wrong with my system. It it were a weather phenomenon causing a dip, in the same geographic area each system should be more-or-less equally affected and cause the Efficiency number for each of the systems to go down in a similar fashion.Sound about correct?

Absolutely. Unless you have reason to believe that all of the others in your team upgraded at the same time, you can come to no other conclusion.

Edit: Unless you believe that your installer had some magic juju, it might be instructive to join (or create) some different teams to be on. Perhaps a neighborhood team, or a inverter/panel branded team, or a X degree tilt team. As I said, I have no idea how PVOutput works. Speaking of tilt, if you find yourself drifting up or down in the standings on a seasonal basis, that would be the first thing I would suspect.

[jasonvr]

Depending on the inverter system, you generally can get better problem reporting from the inverter monitoring system directly. This is definitely he case for solaredge which has excellent trouble reporting and easily configured and personalized. Though generally installers do not give homeowners access to this alerts section.

True, but I badgered and pestered my installer until I got full access, to the point that I had full control and had to add their email address to allow them access to be able to even monitor my site

[sensij]

Depending on the inverter system, you generally can get better problem reporting from the inverter monitoring system directly. This is definitely he case for solaredge which has excellent trouble reporting and easily configured and personalized. Though generally installers do not give homeowners access to this alerts section.

I guess, but when I had an *actual* problem with my electrical system, I found it on PVoutput first. No alerts from SolarEdge, nothing from my installer (although my installer knows I'm self sufficient and may have stopped watching my system after communication problems that took a couple months to solve). It may be possible to set up better alerts than what is there by default, but the awareness that watching relative performance on PVOutput brings is worth much more to me.