Calculate solar panel output

[jflorey2]

I totally agree that having one industry wide accepted standard for rating solar panels is a good thing. Although, even under the current rating system, there are actually TWO ratings available for most quality panels: STC and NMOT.

NMOT is very similar to NOCT (nominal operating cell temperature) That is a rating when the cell is as hot as it would be in the sun on a day that was 25C. However, you can calculate NMOT/NOCT from STC if you have the temperature coefficients (which is part of almost every panel's data sheet.)

There's also PTC, which is a rating at a more reasonable insolation (i.e. less than the ideal of 1000 w/m2, which you rarely see.) Again, it is easy to calculate PTC from STC.

Why did the industry settle on the STC rating

Because it's easy to test. The panels can be at room temperature (no ovens involved) and they can be tested with a flashlamp in under a second.

But it's not the accepted rating system that is the problem. It's the fact that when a customer is sold a solar system of a given "size" based on the STC rating they are, in MANY cases, not advised that they will probably never see the power output level they think they are paying for.

By the same measure, they are not told they will see 10,800 watts AC out of a 10.8kW STC DC system. And this is mainly because no one cares what the peak AC output is.

And it's a pretty useless number anyway. I mean, the installer has to know that number to accurately size breakers and conductors and safely install them in a panel, but the person whose house it's on doesn't care. What he DOES care about is how much power it will generate over the year. And every good installer provides that number.

And no matter how accurate you get on the AC production, again, it won't matter. In fact, if you drop the yearly production number and instead get very accurate about the max AC production, you are doing the customer a disservice. Due to panel orientation, local weather, trees, soiling etc a max power number doesn't tell you much about what max energy production will be. A system where the installer tells you "this is a 10,800 STC system that will give you over 8500 watts max output!" but only does that for an hour a day due to the trees all around is not a well installed system, and they will likely be unhappy with it.

And even when you know yearly production there is still a lot of influence on total cost/savings based on whether you are on a TOU plan, the details of your net metering agreement etc.

Over the past decade there has been a trememdous amount of work done on forecasting software that gives you a very accurate prediction of your total power bill reduction based on all the above. Good installers not only have this software, but they are experienced at using it (which is important.) This gives potential customers an accurate way to predict how much they save, and thus make better decisions on price tradeoffs.

As an engineer my big goal is to understand the VI curve of the panel as well as the maximum power over the conditions it will be used in. The available data (Voc, Vmpp, Isc, Impp, Pmax @ STC, temperature coefficients for Vmpp and Impp, fusing and max voltage ratings) does that for me. I would be very opposed to any change in that, because then you'd see some panels rated one way and some panels rated another way, and you wouldn't be able to trust values on a datasheet any more.

In most solar installation companies I have talked to there is "that one guy" who designs their systems and takes all that into account. Usually it's only one guy even at bigger installers because it's really pretty easy nowadays, and both the forecasting software and the installation manuals for the inverters they use tell you in great detail what you can (and can't) do. And because most installers has a few standard designs they try to use for everyone.

So what we have now works pretty well.

It may be true that a "competent solar installer" will give the customer an estimated annual kWh production number, but I would not agree that's the only number people care about.

Agreed. They also care about total savings, which is more complex (but again comes from forecasting SW.)

A new solar system is a very expensive purchase. Most customers will be extremely interested (at least initially) in closely monitoring the system to "see how it is doing". In most cases, the customer is provided a app to install on their computer or their phone. Most app's will tell them what the system is producing on a daily, weekly, monthly and yearly basis. In many cases, this leads to the exact situation described by the OP of this thread: a customer who was sold a 10.8kW system looks at the app and discovers that their output starts clipping at the 9.0kW level. Who can blame that customer for feeling like they got screwed?

Well, I can, if they never asked. It's like buying a gas car that is rated at 300hp but putting it on a dyno and finding out that it only generates 260hp at his home in Denver. Did that car buyer get screwed? Whose responsibility is it to educate him on the effects of altitude on a normally aspirated gas car?

Of course, the simple solution to this problem would be education. Every potential solar customer should educate themselves about how solar systems work and about the difference between DC panel output and AC inverter output.

Well, I think every customer should have the opportunity to educate themselves. Whether they do or not is up to them.

The 82 year old widow two doors down recently got solar because she also got an EV and it seemed to make sense to her. She doesn't care about maximum production, or STC vs PTC. She just likes that her power bill went down.

[J.P.M.]

I never compared prices, Price for the 7600 with a revenue grade meter option was $1986.

Thank you.

Just wondering what the cost differential savings for the inverter might be vs. the value of the possibly clipped electricity by getting the smaller inverter.

[Mike 134]

As perhaps 2 side questions:
1.) Do you happen to remember the price difference between the 2 inverter sizes ?
2.) What was it ?

I never compared prices, Price for the 7600 with a revenue grade meter option was $1986.

[J.P.M.]

I had 2 choices for my inverter size either 6000W or 7600W for my 7.29KW of panels. Glad I choose the larger size both to maximize my output and to not have the inverter clip the excess input from the panels. Since energy is neither lost or gained it would have to be dissipated as heat. And on those cool days like today I'm outputting more than the STC numbers.
IMG_9420.PNG

As perhaps 2 side questions:
1.) Do you happen to remember the price difference between the 2 inverter sizes ?
2.) What was it ?

[Mike 134]

I had 2 choices for my inverter size either 6000W or 7600W for my 7.29KW of panels. Glad I choose the larger size both to maximize my output and to not have the inverter clip the excess input from the panels. Since energy is neither lost or gained it would have to be dissipated as heat. And on those cool days like today I'm outputting more than the STC numbers.
IMG_9420.PNG

[bcroe]

All the above apply. But the worst things never mentioned to me,
were the effects of clouds in this area, and of snow. I solved both
of those problems, by methods of my own.

The PV operation must be capable under a huge range of conditions,
which include the weather, the hour, and the season. There are more
constraints in available electrical plant and contracts. There is no one
perfect system which fully utilizes all resurces, all the time, something
is always limiting operation while the rest is below capacity. Fortunately
modern electronics can be extremely adaptable to whatever is the
current situation. Bruce Roe

[J.P.M.]

[QUOTE=SolTex;n438531]


Of course, the simple solution to this problem would be education. Every potential solar customer should educate themselves about how solar system work and about the difference between DC panel output and AC inverter output.
And other things as well, but few, if any users take the time or effort involved. So, as you write, they fill up the forums with the same self answerable questions.

What are the odds of that ever happening?

If what shows up around here is any indication, pretty small.

[azdave]

So basally whatever the seller can find cheapest is what the end user gets. They could sell you a 10.8k system with one 1500w inverter and that would meet industry standard.

No. You are skipping the second, most important part, of the post made by jflorey2. "Any competent solar installer will give you estimated kwhr per year, and that's the number that people care about anyway. It takes into account far more than just max inverter watts and is a much better guide than any one generation number."

If the seller agreed to provide a certain kWh output annually and you didn't get that (within a reasonable margin of error) then you have a reason to be upset. It sounds like you paid for an annual output goal, not maximum utilization of every single component. If you enter all your data into PVWatts and can prove to them that the system they installed will not likely come close to the annual output you were promised, then you have good cause to go back to them and insist they make it right. If their system will meet those annual goals then why would they be concerned if they aren't making full use of the panel output?

[SolTex]

This is deliberate, and it is done because that's the industry standard.
Any competent solar installer will give you estimated kwhr per year, and that's the number that people care about anyway.

I totally agree that having one industry wide accepted standard for rating solar panels is a good thing. Although, even under the current rating system, there are actually TWO ratings available for most quality panels: STC and NMOT. As I understand it, the NMOT rating is an attempt to provide a more realistic power output estimate under conditions closer to the real world. The NMOT rating on my LG380N1C-A6 panels, for instance, is only 75% of the STC rating. Why did the industry settle on the STC rating, which is only attainable under strict laboratory conditions, as the "nominal" panel size? I would guess it is because the STC rating is a BIGGER more impressive number, and it's always easier to sell people on MORE of something (anything) for the same price.

But it's not the accepted rating system that is the problem. It's the fact that when a customer is sold a solar system of a given "size" based on the STC rating they are, in MANY cases, not advised that they will probably never see the power output level they think they are paying for. It may be true that a "competent solar installer" will give the customer an estimated annual kWh production number, but I would not agree that's the only number people care about. A new solar system is a very expensive purchase. Most customers will be extremely interested (at least initially) in closely monitoring the system to "see how it is doing". In most cases, the customer is provided a app to install on their computer or their phone. Most app's will tell them what the system is producing on a daily, weekly, monthly and yearly basis. In many cases, this leads to the exact situation described by the OP of this thread: a customer who was sold a 10.8kW system looks at the app and discovers that their output starts clipping at the 9.0kW level. Who can blame that customer for feeling like they got screwed?

Of course, the simple solution to this problem would be education. Every potential solar customer should educate themselves about how solar systems work and about the difference between DC panel output and AC inverter output. Every customer should be educated by their "competent solar installer" as to what exactly they will see when the look at the app. It should be explained to them that they will never actually see the STC power output numbers that they thought they were buying, even though the system will (hopefully) meet the annual kWh estimated production number. Sadly, in many cases, this does not happen. Solar installers don't always take the time to explain these facts. Customers buy systems without understanding what exactly they are paying for. I see the same question come up over and over on solar forums and online blogs: "Why doesn't my system put out what I thought it would?"

I don't expect the industry standard to change. My only hope is that eventually, by some miracle, every solar customer will have these things fully explained to them before they sign the contract. What are the odds of that ever happening?

[alfie1111]

+1.

Few readers of Jeff's post could have said it better or even understand what he wrote.

So basally what ever the seller can find cheapest is what the end user gets they could sell you a 10.8k system with one 1500w inverter and that would meet industry standard. There should be some standards set. Oh so you know I swapped one of my inverters with a 1600w and my 4 panels on that inverter were at 390w -399w each instead of the 370w im was getting with the 1500w inverter.

[J.P.M.]

This is deliberate, and it is done because that's the industry standard. It's how solar power systems are compared to each other. It would be much, much worse if company A advertised STC ratings. company B advertised PTC ratings, company C advertised STC times inverter efficiency and company D divided the expected production by equivalent hours of direct sun and used that. There would be no way to (effectively) compare systems.

Any competent solar installer will give you estimated kwhr per year, and that's the number that people care about anyway. It takes into account far more than just max inverter watts and is a much better guide than any one generation number.

+1.

Few readers of Jeff's post could have said it better or even understand what he wrote.

[jflorey2]

I agree. Wood sizes are not what they use to be.

They are. The term 2x4 refers to the rough-sawn wet wood. Once it's dried it shrinks, and once it's planed (to get rid of the bandsaw marks) it is about 1/4" smaller in all dimensions. Nowadays it's harder to get rough-sawn wet wood at a lumberyard, but it's still available for people who want it. The problems with it (surface roughness, weight, likelihood of warpage) makes it not that useful for construction.

It's a great example though. If someone demanded REAL 2x4's be sold in a store, it would result in instant chaos, because most framing assumes specific dimensional lumber, and if it were all suddenly 1/2" thicker most things built with it would be just a little off. What's important is that a 2x4 measures what people who understand lumber (i.e. the people building homes) expect it to measure. Same with solar power systems.

[jflorey2]

Which is exactly the problem with how people are "sold" on solar systems. Either deliberately, or because of ignorance, the salesman touts the STC rating in DC watts when selling the system.

This is deliberate, and it is done because that's the industry standard. It's how solar power systems are compared to each other. It would be much, much worse if company A advertised STC ratings. company B advertised PTC ratings, company C advertised STC times inverter efficiency and company D divided the expected production by equivalent hours of direct sun and used that. There would be no way to (effectively) compare systems.

Any competent solar installer will give you estimated kwhr per year, and that's the number that people care about anyway. It takes into account far more than just max inverter watts and is a much better guide than any one generation number.

[SunEagle]

I just bought a 2x4x8 but I checked it and it only measures 1.5"x3.5"x 8'. Should I think that the lumber industry is out to screw me or that possibly I don't understand the how that industry sells their products?

It appears to me that you received what you signed for in the contract.

I agree. Wood sizes are not what they use to be.

[azdave]

If I go to the hardware store and buy a 2x4x8...

I just bought a 2x4x8 but I checked it and it only measures 1.5"x3.5"x 8'. Should I think that the lumber industry is out to screw me or that possibly I don't understand the how that industry sells their products?

It appears to me that you received what you signed for in the contract.