Adding additional panels to existing system, how to figure?

FWIW, some folks around here may have more history and experience with the subject matter but IMO, but no one here is inherently smarter than anyone else, including you.

To the best of my opinion, knowledge and sought out opinion from SDG & E and the CPUC, the written rules on bumping system sizes by up to 1 kW are based on the spec sheet (STC) rating of the panels with no derating for age, orientation or inverter system. I got that from the horses' mouths' several years ago.
Things may have changed but since the wording is pretty clear, I kind of doubt it.

Reading your prior posts you've called SDG & E in the past with other questions. Why not do it again ?
Even if the written rules were to change, I'd seriously doubt they would change in ways that would benefit PV system owners to the POCOs' determent.

I'd give them a call and say you're enquiring for an aged and homebound friend and see if you'd draw a balk for 1.040 STC kW of added capacity. But before I did, I'd find out what paperwork and documentation is required and see what the forms look like.
The last time I checked (~ 4 years ago I think), bumping a system size within the limits didn't require and inspection.

BTW, how much electricity do you use in a year ? How about 3 X 260 STC W of PV + some conservation measures (maybe ~ 350 - 400 Wh/yr) to make up the difference ?

Typically have to kick in about 1k a year on average.
I only sized my system for about 85% or so when I did my install whey back when. Why I can't remember heck I dont even think electric cars were a thing then.

All my lighting is LED where applicable.
Pool pump is variable speed.
Hot tub runs about $40 a month. Wife's gotta have it even though she goes in it 2x a month, but I digress.

Yeah I will probably call SDGE next week to get the skinny.
I can run the permits through my buddy who still runs his Electrical contracting business and do the work myself \.
Hell the kid used to be my apprentice way back when.
Could probably do the whole enchilada for under 1k plus permit and plan fees.
Payback would probably be under 2 yrs or so.

Thank you.

I'd be interested to read about any updates in SDG & E or city/county policies with respect to system sizing, both for paperwork and inspections you may discover in talking to them.

FWIW, I had a hot tub with a good cover that came with the house.
I fired the tub with a 4 X 8 solar thermal panel for a few years after house acquisition which worked well but I got rid of the tub altogether from lack of use when I redid the deck that surrounded it several years ago as it (the tub) was little more than a non-used PITA.

Wife never used the tub and the tub at LA Fitness that I soak my bones in after my daily swim is a lot easier for me to maintain as well as having better conversation and more variety with respect to eye candy. Win/win.

As mentioned it is the STC rating. If you want to be agressive you could take a chance on adding five which would put your production over by 60 Watts. To me it is a risk management decision. I have taken the risk by adding more than 1kW in panels but I have managed that risk by orienting them such that they will peak at a different time than my main system such that my total peak at anyone time will not exceed my existing system plus 1kW. The downside risk is i could be kicked off NEM 2.0. My philosophy is that the power companies only have meter readings to remotely monitor production. I view it unlikely that they will come out to phyically inspect your installation. I have also timed some of my loads like water heating to further reduce my Net generation so that my production is masked by consumption.

Ampster when you added to your existing system and exceeded by 60w didn't the inspector balk at the overage?
Or do they care?
If it's a "gray" area as far as fudging a bit I wonder if I could get by with (5) of the 260w panels since the micros clip anything over 215w. So 5 x 215 = 1075w

I know the utility company doesn't actually come on site except to possibly change out service or replace meter but Im sure all inspection/permit paperwork rolls across someones desk at SDG&E.
Dont want to get greedy.

The building department does not have any knowledge about my NEM agreement. The only time they communicate with the utility is when there is a service upgrade. That is done so the utility knows to reconnect the service. My only experience is with SCE and PG&E. SDG&E may have a different relationship with building officials in the cities it serves. That is definitely a risk that you would want to understand. To be clear, I am not recomending it to everyone. It was a risk I was willing to take but it obviously is not for the faint of heart. There are many caveats to that approach.

Ampster did you need to file an application with the utility dept to make your changes?

I am sure if I asked them they would have said no. However I did not ask, under the theory that it was easier to ask for forgiveness. The first one was a maintenance repair in 2014 in SCE territory. The next two were in PG&E in 2019 and 2021. Again, I caution that this is a contractual violation of NEM agreement and could result in losing the benefits of an existing NEM agreement and being forced into a less favorable arrangement.

Got it.

A few more questions.
How do I figure out if I am producing 100% of my system capacity?
If my production is less than my official rating could that amount also be figured into the equation when adding my 1k?

To your 1st question, 100% of system capacity under what conditions and compared to what ? What kind of a number are you looking for ? % of STC output ? System raw efficiency % ? System efficiency compared to STC efficiency? With/Without inverter efficiency taken into account ? Temperature effects ? Other ways ?

Partial answer is you probably won't be able to figure your actual working system efficiency (or whatever it is you're looking for) in any way similar to how you would, for example, measure ICE output on a dynamometer.

Before you can estimate how much your system is putting out as a percentage of its maximum or it's rated output, you need to define the conditions of instantaneous operation by measurement of the system input and output and adjust the efficiency by accounting for all the major factors that impact system efficiency (and thus output) to be able to compare that output to what a set of data sheet specs will allow you to estimate what the performance ought to be if the panels were performing the same way the data sheet would say the performance ought to be under those conditions.

It's possible to roughly estimate an instantaneous system or panel efficiency (and then a time integrated value over, say a day), but to do that you need to measure system input (P.O.A. irradiance as f(angle of incidence, diffuse irradiance portion, albedo)) while simultaneously measuring system output and inverter efficiency as well as the major environmental factors that will influence system efficiency such as ambient temperature wind vector and also some way to determine cell temperature. Also, you'll need to make sure your array is clean and washed for any such measurements or figure out how fouled your array is in a quantitative way and include that in your efficiency adjustments.
I've done that for 6+ years several hundred times - long, involved measurements, calcs and story but I believe I've got my system dialed in pretty good.
Most folks wouldn't bother or even know where to start.

On your 2d question, I'm guessing that you're under the impression that you'll actually see your system's STC output.
Reality is you won't.
More reality: I'm not sure you know what you're looking for or I don't understand what it is that you are looking for. Or, most likely it doesn't exist in the way you think it does.

On a very sunny, cool spring day with a stiff breeze and with optimum array orientation (that will give you near vertical P.O.A. angle of incidence and which you don't have with your mostly low sloping to near horizontal panels) around San Diego you might see an instantaneous array output of something like maybe 82% to 86% or so of your array's STC output (which uses a fixed 1kW P.O.A. irradiance and a fixed 25C cell temp. with zero wind vector). Your best output will in all likelihood be less than STC mostly due to the low array slope, likely higher cell temps. and a fouled array under in situ conditions.
But, all that doesn't matter squat in terms of the part of the NEM agreement that limits increases in existing NEM systems' sizes to 10% of system size of 1 STC kW, whichever is greater.

Also, and not that it matters much but back in the day (2013) when you got your PTO and when the state rebates were still available, for incentive purposes max outputs were based on something called PTC output (see your CA solar incentive paperwork for that #). It's less than what's actually or likely to happen as a max. instantaneous output value as described above.

Look, for once, I think Ampster and I agree on something and that is that it's probably possible maybe even likely that no one from either the county or SDG & E is going to care much less come and look at your system based on some increased output of maybe 8 - 10% or so that they may have observed (unless of course they happen to be reading SPT and this thread).
Hell, regular weather variation will mask most of that anyway, but it could happen and they (SDG & E) could rescind your whole PTO if they got really porky about it.

My guess is that's a situation with very small risk but big potential consequences.
Sort of like a Dirac function whose probability of happening is zero until it isn't. Then it's one.
So, you're staring at something like a Dirty Harry choice of did he fire six times or only five and then you got to ask yourself if you feel lucky.
All for what amounts to (besides following the agreement you signed - FWIW these days) the life cycle value of the output of a 260 STC W panel ?

Take what you want of the above. Scrap the rest.

I realize that I am never going to see full wattage output form my system as you have described.
Direction orientation, azimuth angle, temperature, wind, and much more variables Im sure.
Throw in the micros and even if I was using 400w rated panels there still going to be clipped at the maximum AC output rating of the M215's so there's that.

As far as system full capacity compared to what I am actually producing it's pretty simple.

Just took a spin over at PV Watts and changed some perimeters on my existing system specs as far as angle and direction.

(17) panels facing south at 22 degrees should produce 7.286Mw annually.

(6) panels facing west at 12 degrees should produce 2.307Mw annually.

(11) panels facing west at 5 degrees should produce 4.226Mw annually.


Now if I change the direction of the (6) and (11) panel groups to the south at 22 degrees also I get an increase of .752Mw annually.


Which is about 5.2% increase in total production not adding ANY additional panels.

So if I factor that into my equation of adding additional panels that will be more than the 1k originally allowed.

Another note since the new panel locations, for asthetic purposes are concerned, will be facing in an east direction I could probably factor in the production values for that set-up against a south direction and 22 degree angle and be able to add those additional watts to my cumulative total for possible a bit more.

Im not trying to take undo advantage of my signed contract with my utility provider and staying with older style panels and micros so I can effectively still monitor it with my existing Envoy display which I have in my garage for a minimal amount of dough.

Hell in a year they will probably change the whole rate schedule again making this all moot.

Then I will start looking at wind power, just kidding mumu.

JPM gave a thorough answer to that.
My goal is annual Net generation to create a small credit. Anything above that gets discounted at True Up. Any thing below that costs me money. I track monthly totals from my bill by TOU period in dollars and kWhs
Yes, I am aggressive and have added significantly more than 1kW to one system at my home.. I just look at the daily totals from the website occasionally in the summer. It never gets close to that in the winter since my usage pattern is much heavier in winter. My True Up is September and by March I can have a 2000 kWh Net consumption. In the following six months I crawl back that number. I just got my bill for August and have a small dollar credit so I will have a cushion at September True Up. With two EVs I have a lot of flexibility about charging from excess solar during the day or just using banked credits by charging at night at off peak and paying NBCs. It has become a hobby to optimize my system, but I also don't waste my time turning off lights or telling my wife when to bake or use the dryer.

Aesthetics are above my paygrade but I'm not following your math.

Taking your PVWatts numbers, if you double your south facing STC wattage that ought to about double that array's output to 2 X 7.311MWh/yr. = 14.622 MWh/yr.
At the same time, if you eliminate the other two arrays, you'll be losing their combined annual output that PVWatts models as 5.435 MWh/yr. for a net pickup of 1.876 MWh/yr.

I don't understand what you mean by "an increase of 594w annually".

I'm probably missing something but I'm now wondering, if you have/had all that south facing area available, why use the other orientations at all ?

All this is NOMB, but I'd also check the city and SDG & E requirements and then call them to see if either one of them thinks rearranging a system layout and orientation as you describe will run counter of some rules (and get the response in writing).

Phone calls are cheap these days and a lot less onerous than trying to extract a tit from the wringer. Just sayin'.

Rant mode on.
As for rate schedules and tariffs, residential electric bills will continue to increase, but there's enough blame to go around to all of us - POCOs and users both.

I've been tracking and studying primarily SDG & E but also a lot of other rates and to a lesser extent rate policies of other CA I.O.U.'s since 1995.
Those rates, tariffs and policies can change at any time but usually at least 4X/yr.
For residential PV users in CA, NEM policies of the I.O.U.'s have been more stable than rates. They have always (since I've been watching them anyway) worked in favor of the POCOs, most usually by taking advantage of user ignorance.

Rates, BTW, have been more mercurial to some extent; that is, and in spite of what a lot of folks think (or more accurately repeat what they read or hear without questioning or even thinking about it), have gone both up and down.
Policies have tended to get changed around in favor of the POCOs as they continue to capitalize on users' lack of diligence to take control of their usage.
All that, combined with users' tendencies to increase usage (particularly residential PV system owners who don't have the draconian $$ incentive to conserve now that their electric bills are much lower) combine to increase the size of electric bills through that increased usage.

Rant mode off.

(17) panel group is fine the way it is.

(6) group panel faces west at 12 degrees giving me an annual projected output of 2.307Mw
Changing the same group to face south at 22 degrees would potentially give me a generation of 2.571Mw. = +.264Mw

(11) group panels currently face west also at a 5 degree tilt with a projected annual output of 4.226Mw.
Changing those panels to a south facing direction and adjusting the tilt angle to 22 degrees would potentially give me 4.714Mw = .488Mw.

.264Mw + .488Mw = .752Mw

I feel these changes were I to make them on my own existing system have nothing to do with the Utility companies approval or concern.
I would not be changing my original capacity specs just allowing for a more efficient system generation.

But I digress.

As far as having all that "south facing area availible" thats where the aesthetics reference I made comes into play.
I made the choice on my own when designing my system to try to blend in my solar arrays to my roof lines and not be an eyesore.

All Im stating is I could make these changes which would be due to me with no recourse.
So in using that logic I feel I should be able to add that "projected" value amount when figuring my max 1k add-on.

Theoretically I should also be able to factor in my existing panels degradation amount from when "new" to present output and add that as well.

But realistically going by PV Watts projections for my existing home system they come up with an annual output when new of 13.819Mw.

My lifetime annual amounts for the full years of 2013 to 2022 have avaeraged 12.664Mw giving me a difference of -1.155Mw output per year.

So I should also be able to factor that value in as well.