Calculate solar panel output

[alfie1111]

OK, I'm out of here. Good luck.

can I ask one more question before you leave? If my installer would have installed 1800w micro inverters instead of 1500w inverters would I have the capability of generating more than 9000w with the 24 455w panels and 6 miceo invertesr?

[SolTex]

can I ask one more question before you leave? If my installer would have installed 1800w micro inverters instead of 1500w inverters would I have the capability of generating more than 9000w with the 24 455w panels and 6 miceo invertesr?

Yes, increasing the "size" of your inverters would allow your system to generate more than 9000W. HOW MUCH more depends on a number of factors.

With the bigger inverters, you would be close to a 1.00 DC/AC ratio. The only way you could utilize the max capability of the bigger inverters would be for your panels to actually output their STC rating. This almost never happens in the real world. Most system owners are lucky if they ever see 80% of the STC rating out of their panels. Panel DC output is almost always reduced below STC levels by geographical location, mounting azimuth and tilt angle, snow, clouds, dirt, shade, bird droppings, leaves, ambient temperature, age of panels, etc.

The reason most installers shoot for a 1.25 DC/AC ratio is because over sizing the panel output in relation to the max inverter output provides the most advantageous return on dollar spent. Plus, inverters like to work hard. They are most efficient when running close to max output.

If you want more power output from your system, the best way to do that would be (as you have already mentioned) to add 4 more panels and another 1500W inverter. This would increase system output and keep the DC/AC ratio at a favorable number.

All of the above is only my opinion. TIFWIW

[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.

[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.

[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.

[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.

[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.

[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.

[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?

[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?

[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.

[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

[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

[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]

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.