New to Forum & Solar - I have a choice to make...

[J.P.M.]

JRqwertyui, don't be concerned about my question. I was merely using the numbers from your system as a hypothetical - not that your system would or could be in a clipping scenario. I noticed your 18 degree pitch and orientation and such. I'm not any self proclaimed expert. I'm just trying to learn, in general, from where these system estimates originate.

Case in point, I was wanting to go to a larger, higher wattage inverter because the panels (or modules for Butch) I was having installed had a combined output (STC) higher than the inverter. I also wanted to have some future expansion options. The installer explained, perhaps poorly or I was asleep that day, that based on the clipping, or implied loss due to the panels (er modules) orientation and tilt and shtuff that I could expand a bit more later with the same inverter due to clipping. With further research on the web (FWTW) I found that 10% was the general rule but some studies stated 15 or 20% was acceptable. This should have been a red flag and he probably meant that I could add more panels (dang, modules) until I start clipping.

Again, FWIW, all these simulations and models and ... are just best case guesses and estimates. You will find that part of the solar sales pitch is (or has been) utter snake oil. You may get close to their "estimates" for modeling and simulations vs real world conditions but they are going to likely give you the best case to make the sale. They would scare people off if they quoted the panel total watts (combined STC) @ $3.50 per watt but tell you that you will never reach that kWp (combined STC). Maybe I was a bit too naive.

As Butch said, "They can easily add 1kW to the east (would need a combiner is all), and should result in less than 0.2% clipping, though likely none depending on how much degradation of the PV modules has happened." Note the words "should", "less than", "likely", "depending" and "has happened" (future past tense?). This is much like installers painting a rosy, sunny picture but leaving many holes to disappear in.

FWIW, after hanging around solar for ~ 40+ years, and having done a bit of modeling of both solar thermal and later PV systems, I've found that most PV models available today can produce reasonably good estimates of system performance given the inherent variability of the required inputs.. The biggest source of error or variability is, IMO only, user ignorance of what is that inherent variability of inputs - mostly but not entirely weather related, and the unrealistic expectations that result from a lack of understanding of those limitations.

[sensij]

As Butch said, "They can easily add 1kW to the east (would need a combiner is all), and should result in less than 0.2% clipping, though likely none depending on how much degradation of the PV modules has happened." Note the words "should", "less than", "likely", "depending" and "has happened" (future past tense?). This is much like installers painting a rosy, sunny picture but leaving many holes to disappear in.

Future weather isn't known, so any estimate of system performance is necessarily probabilistic. Resources are freely available to self-educate about what is an excessively rosy or sunny estimate, vs estimates that are more firmly grounded in science.

On that note, the latest release of SAM is finally including source code. Anyone who wants to look under the hood of PVWatts (which is one of the models embedded in SAM) will be able to do so.

[DrLumen]

JRqwertyui, don't be concerned about my question. I was merely using the numbers from your system as a hypothetical - not that your system would or could be in a clipping scenario. I noticed your 18 degree pitch and orientation and such. I'm not any self proclaimed expert. I'm just trying to learn, in general, from where these system estimates originate.

Case in point, I was wanting to go to a larger, higher wattage inverter because the panels (or modules for Butch) I was having installed had a combined output (STC) higher than the inverter. I also wanted to have some future expansion options. The installer explained, perhaps poorly or I was asleep that day, that based on the clipping, or implied loss due to the panels (er modules) orientation and tilt and shtuff that I could expand a bit more later with the same inverter due to clipping. With further research on the web (FWTW) I found that 10% was the general rule but some studies stated 15 or 20% was acceptable. This should have been a red flag and he probably meant that I could add more panels (dang, modules) until I start clipping.

Again, FWIW, all these simulations and models and ... are just best case guesses and estimates. You will find that part of the solar sales pitch is (or has been) utter snake oil. You may get close to their "estimates" for modeling and simulations vs real world conditions but they are going to likely give you the best case to make the sale. They would scare people off if they quoted the panel total watts (combined STC) @ $3.50 per watt but tell you that you will never reach that kWp (combined STC). Maybe I was a bit too naive.

As Butch said, "They can easily add 1kW to the east (would need a combiner is all), and should result in less than 0.2% clipping, though likely none depending on how much degradation of the PV modules has happened." Note the words "should", "less than", "likely", "depending" and "has happened" (future past tense?). This is much like installers painting a rosy, sunny picture but leaving many holes to disappear in.

[ButchDeal]

Looks like ultimately 11 panels landed on east side, 17 on west side... their measurements were off a bit + I didn't want the panels close to my skylights...

ButchDeal may I ask you to send me the report in PDF you modeled, I'm curious how it compares with PWatts and the SE design tool...

Sure do you have your monthly consumption numbers?
You can send me the site address butch @ geostellar.com and I can do shadows as well.

btw: still no clipping and a little more production, though shifted earlier in the days.

[JRqwertyui]

Looks like ultimately 11 panels landed on east side, 17 on west side... their measurements were off a bit + I didn't want the panels close to my skylights...

ButchDeal may I ask you to send me the report in PDF you modeled, I'm curious how it compares with PWatts and the SE design tool...

[ButchDeal]

28x330=9240. Note I never mentioned anything about the installed array. I now know to use STC in the future specifically for you.

glad I inspire you to strive not to state falsehoods, though I would think you would do that with everyone.

NOTE: You stated that the installed (Array) system would have 18% clipping, it doesn't have any clipping.

[DrLumen]

If anyone is interested....

Trucks rollup, panels on board, racking going on roof....
Inverter being place on side of house and wired up...
Sub open and ready for wiring...

It is exciting, huh?

Enjoy!

[DrLumen]

Then your theory is wrong. 9240 watts is STC for the Array.

28x330=9240. Note I never mentioned anything about the installed array. I now know to use STC in the future specifically for you.

[JRqwertyui]

I'm really digging the statement: "Highest production is May 17th of 7.14kW"
Take that PG&E !!!!

[ButchDeal]

Yeah... PVWatts has "advanced options" that allow you to set the DC/AC ratio. I'm just guessing that the difference in annual output between 1.21 and 1.00 is about 1%, but figuring it out is more manual in this case because of the multiple azimuths. You'd need to download the hourly output for each sub-array, add them together, and then subtract out anything over the 7600 W limit to model how much would get clipped. (System Advisor Model is more capable, and will do this all in one step).

or use a 3D simulation system like Aurora that will also take into account the shadows

[ButchDeal]

If the panels can (theoretically) output 9240watts and the inverter can only output 7600 then that is about a 18% decrease. (7600/9240=.822 or 1.21 reciprocal)

Then your theory is wrong. 9240 watts is STC for the Array.
But in this case the Array is made up of two Panels, one facing East and one facing West.
The East panel is made up of 7 panasonic 330W modules 2.31kW with an azimuth of 99 and pitch of 18
The West panel is made up of 21 Panasonic 330W modules 6.93kW with an azimuth of 279 and pitch of 18

So unless you move house to a planet with TWO suns instead of the northern CA area it is now located in.

I know the actual panel output is subject to a lot of variables and was wondering what determines the starting point for those variables.

for most people the starting point is a model and simulation.
I don't have the exact location or other shadows other than the building but I modeled it with Aurora, there is ZERO clipping, generating 9.5MWh/year (remember my no shadows).

Highest production is May 17th at 1:00 pm of 7.14kW

Further Note that the OP wants the StorEdge inverter for future backup capabilities.
Also Any larger of an inverter would require much more money for the interconnect or MSP upgrade.

Further OP expressed interest in future expansion. They can easily add 1kW to the east (would need a combiner is all), and should result in less than 0.2% clipping, though likely none depending on how much degradation of the PV modules has happened.

[JRqwertyui]

If anyone is interested....

Trucks rollup, panels on board, racking going on roof....
Inverter being place on side of house and wired up...
Sub open and ready for wiring...

[sensij]

Is that where your possible 1% loss is coming from? The pvwatts calc in relation to the 7600w inverter?

Yeah... PVWatts has "advanced options" that allow you to set the DC/AC ratio. I'm just guessing that the difference in annual output between 1.21 and 1.00 is about 1%, but figuring it out is more manual in this case because of the multiple azimuths. You'd need to download the hourly output for each sub-array, add them together, and then subtract out anything over the 7600 W limit to model how much would get clipped. (System Advisor Model is more capable, and will do this all in one step).

[DrLumen]

Even if the panels all had great orientation, a DC/AC ratio of 1.21 is only going to have clipping loss around 1% of annual energy production. With this particular system split between east and west, clipping loss will be even less.

Acceptability of clipping should just come down to evaluating the value of the loss relative to the cost to eliminate it... Spending more on a bigger inverter doesn't always make sense, especially if it means the MSP needs an upgrade to handle it.

I guess I either misunderstood or it was explained to me incorrectly.

If the panels can (theoretically) output 9240watts and the inverter can only output 7600 then that is about a 18% decrease. (7600/9240=.822 or 1.21 reciprocal) I know the actual panel output is subject to a lot of variables and was wondering what determines the starting point for those variables. I am now getting the feeling this is fodder for pvwatts...

Is that where your possible 1% loss is coming from? The pvwatts calc in relation to the 7600w inverter?

[ButchDeal]

Did my paperwork state 18% clipping ? I must have missed that... the SE tool stated no clipping which according to sensij is inaccurate.

No it is unlikely that your system will clip as designed. User DrLumen is just confused about over panel vs clipping.