Tesla string shenanigans or sorcery?

[OCJ]

Before tax credit and before sales tax, $24k for 9.6 kW and $10.5k for the Powerwall+. I checked a couple of local installers as well as https://www.californiadgstats.ca.gov/find_installer/ and decided the price was too good to pass up. Order placed 1/11/22, installed 5/11/22, PTO 6/23/22. The biggest ball drop was 6 weeks between the site visit and them telling me I needed to replace some roof tiles.

All in all the experience was fine for me, even though I hear others have major issues. I managed to generate $703 in credits through the beginning of August before getting an EV so now it's just my "hobby" to tweak whatever settings I can.

[J.P.M.]

Thanks for your comments J.P.M. At the time I ordered solar we had only been living in this house a month. Since I was on SDG&E at the previous house, though, I pulled up the previous 1 year of data and made some guesstimations. The old house was 2,100 sqft, the new one was 2,800 sqft, a +33.33% difference, but the new house was also "newer" so I settled on +20%. The previous one year usage at the old house was 7,800 kWh with August being the worst at 1,259 kWh. So my guesstimate for the new home was around 9,400 kWh per year.

We also planned on an EV and a pool. Our mileage was pretty much exactly 30 miles per day on average, so around 10,950 miles/yr. I went with a 2.4 mi/kWh so that gave me an annual usage of 4,560 kWh. So now at around 13,960 kWh per year. And for the pool I believe I estimated around 1,500 kWh per year. That gives me 15,460 kWh per year expected.

Now, we have an EV. And our house turned out to be even more efficient than I thought, my annual house projection actually went down to 7,400 kWh.

Current: 11,960 kWh (higher efficiency and EV, 7,400 kWh house usage + 4,560 kWh EV usage)
Expected: 13,460 kWh (with pool, +1,500 kWh)

So for now I'm overproducing quite a bit, which is also why I'm looking at some electric space heating for winter for our bedroom and nursery.

As far as the roof goes, I don't think there is room for more panels SW and SE. I've linked to the diagram and pic. I think Tesla did a decent design given the limitations of my roof.



Regarding the NW and NE panels. I calculated them with a marginal cost of $2.70 per watt ($2.50 plus tax) since the SE and SW panels were going to be purchased no matter what. So the worst producing one, the NE panels, is $5,400, which is $2.70 * 5 panels * 400 watts with an annual production of 2,599 kWhs. Assuming every kWh is used, that's $1,170 per year from those panels (at $.45 per kWh). I estimate a 6-year ROI on that lowest productions set of panels.

I'm going to dig into the NW and SW data a bit more, maybe spot check a day in June and July. See if they exhibit that same variance.

I was looking forward to that SW set of panels bringing me good returns late in the evening, as you pointed out, especially going into fall/winter. I was disappointed they put it in parallel with the NW panels, but I really am curious how that NW set of panels is going to hold up towards the end of the year. I'm thinking since I'm overproducing anyway maybe see how it goes out of curiousity. Also, no shading at all. I did also get storage to give me options, even though the ROI is further out, but that's another post.

Thank you for the information.
What did you pay Tesla for the array whole array and, separately, how much was the storage, both before fed. tax credit ?
At this time you've got what you've got.
Since SDG & E and their NEM tariffs and rate plans don't care which panels are where, and only about their total impact on what you draw and pay them for, I'd stop compartmentalizing which panels produce what and for what purpose at what times.
If minimizing how much you send to SDG & E and/or minimizing your bill at the gas pump are two of your priorities, your best chance to do those things now lies in use reduction and conservation efforts.

A bit off topic, but my condolences on choosing Tesla for your roof.

[OCJ]

Thanks for your comments J.P.M. At the time I ordered solar we had only been living in this house a month. Since I was on SDG&E at the previous house, though, I pulled up the previous 1 year of data and made some guesstimations. The old house was 2,100 sqft, the new one was 2,800 sqft, a +33.33% difference, but the new house was also "newer" so I settled on +20%. The previous one year usage at the old house was 7,800 kWh with August being the worst at 1,259 kWh. So my guesstimate for the new home was around 9,400 kWh per year.

We also planned on an EV and a pool. Our mileage was pretty much exactly 30 miles per day on average, so around 10,950 miles/yr. I went with a 2.4 mi/kWh so that gave me an annual usage of 4,560 kWh. So now at around 13,960 kWh per year. And for the pool I believe I estimated around 1,500 kWh per year. That gives me 15,460 kWh per year expected.

Now, we have an EV. And our house turned out to be even more efficient than I thought, my annual house projection actually went down to 7,400 kWh.

Current: 11,960 kWh (higher efficiency and EV, 7,400 kWh house usage + 4,560 kWh EV usage)
Expected: 13,460 kWh (with pool, +1,500 kWh)

So for now I'm overproducing quite a bit, which is also why I'm looking at some electric space heating for winter for our bedroom and nursery.

As far as the roof goes, I don't think there is room for more panels SW and SE. I've linked to the diagram and pic. I think Tesla did a decent design given the limitations of my roof.



Regarding the NW and NE panels. I calculated them with a marginal cost of $2.70 per watt ($2.50 plus tax) since the SE and SW panels were going to be purchased no matter what. So the worst producing one, the NE panels, is $5,400, which is $2.70 * 5 panels * 400 watts with an annual production of 2,599 kWhs. Assuming every kWh is used, that's $1,170 per year from those panels (at $.45 per kWh). I estimate a 6-year ROI on that lowest productions set of panels.

I'm going to dig into the NW and SW data a bit more, maybe spot check a day in June and July. See if they exhibit that same variance.

I was looking forward to that SW set of panels bringing me good returns late in the evening, as you pointed out, especially going into fall/winter. I was disappointed they put it in parallel with the NW panels, but I really am curious how that NW set of panels is going to hold up towards the end of the year. I'm thinking since I'm overproducing anyway maybe see how it goes out of curiousity. Also, no shading at all. I did also get storage to give me options, even though the ROI is further out, but that's another post.

[J.P.M.]

I did a fair amount of research before getting solar this year, but not enough. I partially blame that on new house, new job, and new baby.

I have a 9.6 kW Tesla system with a Powerwall+. 24 panels x 400 W, 4 different azimuths and 2 different pitches in SoCal SDG&E territory.

Azimuth	Pitch	Panels	DC
147 (SE)	12 deg	7	2.8 kW
57 (NE)	20 deg	5	2.0 kW
327 (NW)	12 deg	6	2.4 kW
237 (SW)	20 deg	6	2.4 kW

Now when I got this initial design I ran it through PVWatts and I thought cool, the NE and SE will charge up my Powerwall in the morning, the SE will give me power into the afternoon, the NW will give me some good production into the late afternoon, and the SW panels with bring it home into On-Peak TOU especially into fall/winter. It seemed that the marginal cost of those NE and NW panels was worth the squeeze.

Some of you might already see where this is going: Tesla put those two 6-panel strings in parallel, NW and SW. Now, I didn't realize this until after it was done and I wasn't even able to talk to the team much because I was slammed with the new job. Still, no excuses, I dropped the ball.

What I was expecting was when the sun was overhead in May when I got it installed, and June, and July, the production would be close as modeled in PVWatts. As the sun gets lower in the Southern sky, I expected the NW panels to bring down my SW production, and was gearing up to call Tesla and ask if they will address it, even though the annual production will still probably be above what they told me and I don't have a leg to stand on.

BUT, what I'm finding is the SW panel production is coming down from what PV estimates, but the NW panels are coming up in production. The average of the panel production is almost the same. So is this expected? Is this sorcery?

Here are the results of August 17:

	SE	NE	NW	SW	Total
PVWatts	16.884 kWh	10.752 kWh	13.251 kWh	14.468 kWh	55.625 kWh
Actual	16.892 kWh	10.186 kWh	13.972 kWh	14.077 kWh	55.127 kWh
% Diff	0.05%	-5.26%	3.34%	-2.70%	-0.90%

At some point will the decreasing angle of the sun hitting those NW panels bring down the average production of that string? I'm morbidly curious if that will happen and am debating letting it go through the winter, especially since I'm still oversized and generating good credits.

Anybody have any thoughts, opinions, explanations? Thank you in advance and I hope this wasn't too long.

Maybe by way of comment/explanation:

If your inputs to PVWatts are reflective of reality, the model's output(s) will, in all probability reflect what you may expect from your array(s) as an annual long term average +/- about 10% or so as the PVWatts help screens will tell you.

As you seem to have done it, a one day comparison of actual output against a model's means little. While that the 08/17 data from PVWatts seems to do a good job of matching your actual output for that day, that close match is more likely due to the string of sunny weather this time of year coupled with coincidence.

If it never rains, cloud variation is removed from a model. So, the model's output doesn't drift as much day/day as also the actual conditions making for a high(er) probability of a close(r) match between the model and actual output. For my somewhat nearby to you situation (zip 92026) except for 08/09 and 08/13, the average measured irradiance for the prior 31 days has only varied by +9.3%/-8.5% over the month. Specifically, my 08/17 output was 2.5 % above what PVWatts puked out for that day. So far for the prior 31 days my array's daily output has been 97.6% of what PVWatts modeled for the same time period with a std. dev. of 6.7 %. That's pretty stable as I'd expect from what's been a very and consistently sunny month.

1.) Is there (or was there) more room on your roof at the 147 deg. or 237 deg. azimuths ?
If there is, I'd have loaded those orientations to the max starting with the 247 deg. orientation (because under DR-SES and other SDG & E T.O.U. rate schedules, that's the best orientation of the 4 for most bill offset per STC W generated). I'd have filled that orientation first, followed by the 57 deg. azimuth orientation. I'm not sure I'd have done any northerly orientation at all as they are usually very difficult to make cost effective and so drag the cost effectiveness of the whole project down thereby reducing system cash flow and so overall cost effectiveness. That would also depend on your annual usage and how much of it you wanted to offset with PV, either in terms of bill reduction or (usage - generation).

2.) Is there any shading on any of the orientations ?

3.) On a year round basis either one of those southerly orientations would have been better for production than the other two, with the 237 azimuth favored as it will produce the most $$/kWh offset against a bill. If more PV on those roof sections was not possible then the other orientations might be used but, again, at the expense of cost6 effectiveness.

But all that's water under the bridge now.

The usual primary goal of getting residential is to generate as much electricity as needed with the idea of offsetting as much of a bill as desired by first, as much use reduction and conservation as deemed possible, and then by a mix of array size and orientation. Under most T.O.U. rate schedules, and SDG & E's in particular, northerly orientations are mostly antithetical to cost effective PV design.
Then, and even though it's probably not cost effective at this time - even with SDG & E peak summer time vs. off peak T.O.U. differential prices - maybe use some storage to time shift some of the load, but storage is usually a long way from cost effective at this time.
The more electricity that gets generated per installed STC kW by utilizing the best (that is, the most bang per installed STC kW in terms of bill offset) orientation, the more $$ saved and the more cost effective the array is.

By the way, what is your current electricity usage in kWh per year (not annual $$ billing) and what do you expect it to be in the future ?

[OCJ]

I did a fair amount of research before getting solar this year, but not enough. I partially blame that on new house, new job, and new baby.

I have a 9.6 kW Tesla system with a Powerwall+. 24 panels x 400 W, 4 different azimuths and 2 different pitches in SoCal SDG&E territory.

Azimuth	Pitch	Panels	DC
147 (SE)	12 deg	7	2.8 kW
57 (NE)	20 deg	5	2.0 kW
327 (NW)	12 deg	6	2.4 kW
237 (SW)	20 deg	6	2.4 kW

Now when I got this initial design I ran it through PVWatts and I thought cool, the NE and SE will charge up my Powerwall in the morning, the SE will give me power into the afternoon, the NW will give me some good production into the late afternoon, and the SW panels with bring it home into On-Peak TOU especially into fall/winter. It seemed that the marginal cost of those NE and NW panels was worth the squeeze.

Some of you might already see where this is going: Tesla put those two 6-panel strings in parallel, NW and SW. Now, I didn't realize this until after it was done and I wasn't even able to talk to the team much because I was slammed with the new job. Still, no excuses, I dropped the ball.

What I was expecting was when the sun was overhead in May when I got it installed, and June, and July, the production would be close as modeled in PVWatts. As the sun gets lower in the Southern sky, I expected the NW panels to bring down my SW production, and was gearing up to call Tesla and ask if they will address it, even though the annual production will still probably be above what they told me and I don't have a leg to stand on.

BUT, what I'm finding is the SW panel production is coming down from what PV estimates, but the NW panels are coming up in production. The average of the panel production is almost the same. So is this expected? Is this sorcery?

Here are the results of August 17:

	SE	NE	NW	SW	Total
PVWatts	16.884 kWh	10.752 kWh	13.251 kWh	14.468 kWh	55.625 kWh
Actual	16.892 kWh	10.186 kWh	13.972 kWh	14.077 kWh	55.127 kWh
% Diff	0.05%	-5.26%	3.34%	-2.70%	-0.90%

At some point will the decreasing angle of the sun hitting those NW panels bring down the average production of that string? I'm morbidly curious if that will happen and am debating letting it go through the winter, especially since I'm still oversized and generating good credits.

Anybody have any thoughts, opinions, explanations? Thank you in advance and I hope this wasn't too long.