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System Size vs Estimated Annual Production

SoCal Dave

Member
Jul 30, 2020
421
317
California
Funny question. I have heard of PG&E limiting installs to 110% of consumption (but don't actually enforce it). Is it possible Tesla is low-balling the estimate to please the utility?

I would think they would get in trouble if they were systematically trying to do that.

I have SCE and my production needs is better aligned to the 8kW system, but I decided to oversize it to give me some extra room for growth (more AC usage, electric car, etc.). Tesla made me sign a form for SCE acknowledging my system production is higher than my current usage but I expect my usage will increase. I assume they do something similar for PG&E. Here is the wording from the document:
"I have compared the estimated annual production of my Generating Facility with my most recent 12-month usage history. I am aware that my Generating Facility is expected to produce more energy than I used in the last 12 months. However, I affirm that I expect to increase my usage accordingly in the upcoming year. My Generating Facility was sized to meet this expected increase;"​
 
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jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
11,514
13,424
Riverside Co. CA
Directionally what you are saying makes ton of sense. That said fog/cloud completely burns out after 10am. I thought the energy production is fairly small at those times even when there is full sun.

Either way, even if I get what Tesla is predicting I should be fine because of net metering (thanks California). The cost of electricity 2-9pm is 40 cents vs midnight to 7am Is 12 cents. So I’ll be selling at a much higher price than the price I’ll be buying at night. This of course assuming I understand how net metering works ;)

You will be selling at 12 cents at all times outside of 2pm to 9pm, and they may make you choose a new plan as i dont think they offer plans anymore with peak times starting at 2pm. Peak times start at 4pm (or 5pm) depending on plan, because of exactly what you say. They want to buy your solar for 12 cents and then sell you electricity for 40 cents when you are actually using it.

Batteries offset the "buy at 40 cents" part, but you wont be generating a ton of electricity at 4pm on no matter what direction your solar is facing.
 
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getakey

Active Member
Jan 28, 2020
1,497
483
95762
Directionally what you are saying makes ton of sense. That said fog/cloud completely burns out after 10am. I thought the energy production is fairly small at those times even when there is full sun.

Either way, even if I get what Tesla is predicting I should be fine because of net metering (thanks California). The cost of electricity 2-9pm is 40 cents vs midnight to 7am Is 12 cents. So I’ll be selling at a much higher price than the price I’ll be buying at night. This of course assuming I understand how net metering works ;)

Are you on EV1-A? Those peak rates are above 50 cents now. Also, at some point you will be changed to EV2-A
 

Dcd7041

Member
Jun 9, 2020
73
63
Bay Area
I am on E-TOU-B but just purchased my EV and planning to change to EV2-A. But if they buy the electricity at the lowest TOU rates, maybe I should stay at E-TOU-B as in this plan, the lowest price is ~20 cents.

Also, going back to original question, Tesla just updated my documents, two days after surveyor visited my house. The annual production increased materially from 12,723 kWh to 15,254 kWh. Much closer to my calculations now.

upload_2020-7-30_17-13-16.png
 

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willow_hiller

Active Member
Apr 3, 2019
3,288
17,111
Maryland
Also, going back to original question, Tesla just updated my documents, two days after surveyor visited my house. The annual production increased materially from 12,723 kWh to 15,254 kWh. Much closer to my calculations now.

Good to hear they sorted that out. It could have just been a typo. My first layout said 14.2 kW system size and 12,185 kWh per year. It ended up 12.24 kW system size and 14,000 kWh per year.
 
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Dcd7041

Member
Jun 9, 2020
73
63
Bay Area
They did change the layout but to a less efficient layout. That was per my request as I did not want the panels to be visible from front, which looks south. They put those originally south looking panels to now west looking roof.

Original Submission: Tesla Prediction 12,723kWh vs PCWatts Calculator Prediction 15,033 kWh
Revised Submission (post surveyor visit): Tesla Prediction 15,254kWh vs PCWatts Calculator Prediction 14,936 kWh

As you see in revised Tesla and PCWatts predictions are almost the same. Maybe Tesla put the right roof pitches, maybe something else is going on.
 

getakey

Active Member
Jan 28, 2020
1,497
483
95762
I am on E-TOU-B but just purchased my EV and planning to change to EV2-A. But if they buy the electricity at the lowest TOU rates, maybe I should stay at E-TOU-B as in this plan, the lowest price is ~20 cents.

Also, going back to original question, Tesla just updated my documents, two days after surveyor visited my house. The annual production increased materially from 12,723 kWh to 15,254 kWh. Much closer to my calculations now.

View attachment 570742

With PWs, you are able to shift your solar to more of peak. You will charge PWs during low rates and power house from PWs during peak. This will maximize the amount of solar going to grid during Peak
 
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BIC1

Member
Feb 19, 2020
90
16
Missouri USA
The other thing that could make a small difference would be the inverter(s) Tesla is using. It is possible the inverter is sized such that there may be some clipping, for example, that could account for a portion of the difference.
This could actually make a big difference. I have 320w panels but my microinverters cap at 250w. Therefore I have a lot of clipping, although they overclock to about 275w. Is this common, for installers to put in microinverters that are "underpowered" compared to the panels? I don't understand why the installer didn't put in 275w or 300w panels and saved me money while still putting out 275w per panel/microinverter. Anybody know about this? Thanks.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
11,514
13,424
Riverside Co. CA
This could actually make a big difference. I have 320w panels but my microinverters cap at 250w. Therefore I have a lot of clipping, although they overclock to about 275w. Is this common, for installers to put in microinverters that are "underpowered" compared to the panels? I don't understand why the installer didn't put in 275w or 300w panels and saved me money while still putting out 275w per panel/microinverter. Anybody know about this? Thanks.


Maybe these threads might interest you?

New Solar Panel Install - Questions.

(some talk about clipping in that one with a link to some more discussion.

Optimizers... what are they good for?

Discussion about cost of optimizers / microinverters from one of our resident PV installers.
 

BrettS

Active Member
Mar 28, 2017
2,150
2,569
Orlando, FL
This could actually make a big difference. I have 320w panels but my microinverters cap at 250w. Therefore I have a lot of clipping, although they overclock to about 275w. Is this common, for installers to put in microinverters that are "underpowered" compared to the panels? I don't understand why the installer didn't put in 275w or 300w panels and saved me money while still putting out 275w per panel/microinverter. Anybody know about this? Thanks.

The short version is that the panels only put out their max output for a short time during the day, so you might get some clipping during the peak periods. However, during the rest of the day you can take advantage of the bigger panels. For example, a 320W panel can produce about 16% more power than a 275W panel. So during the mornings and the late afternoons you’re getting 16% more output than you would with smaller panels. In the afternoon on bight sunny days you might lose a little potential power to clipping, but it’s really not a significant amount.

The cost to upgrade the inverters would be much more than the return in kWh that you would get back by avoiding that small amount of clipping.
 
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DrPhil

Member
Oct 30, 2019
25
16
Indianapolis
Someone spoke of the power company being acutely interested in systems that go over their typical usage. I just had my homeowners insurance agent tell me that they would only insure system sizes up to the size that is less than or equal to 125% of your usage. Thought that was particularly odd and unexpected for sure.
 

SoCal Dave

Member
Jul 30, 2020
421
317
California
Someone spoke of the power company being acutely interested in systems that go over their typical usage. I just had my homeowners insurance agent tell me that they would only insure system sizes up to the size that is less than or equal to 125% of your usage. Thought that was particularly odd and unexpected for sure.

That is interesting. I guess it make sense that they don't want to be in the business of insuring solar farms. Is it 125% usage at the time of install or at the time of loss?
 

BIC1

Member
Feb 19, 2020
90
16
Missouri USA
Good to know. Had it affected your rates much? We've got USAA too.
Depends how you look at it. If you have a $100,000 house and it's insured for $100,000 and you add $10,000 in solar, you now have to pay for $110,000 in insurance. If you have a $90,000 house insured for $100,000 and you add $10,000 in solar, no change to your premium.

They raise your rates to cover the added cost, same as if you built a garage or deck. They're not penalizing solar, per se.
 
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wjgjr

Active Member
May 11, 2020
1,349
1,063
Silver Spring, MD
Depends how you look at it. If you have a $100,000 house and it's insured for $100,000 and you add $10,000 in solar, you now have to pay for $110,000 in insurance. If you have a $90,000 house insured for $100,000 and you add $10,000 in solar, no change to your premium.

They raise your rates to cover the added cost, same as if you built a garage or deck. They're not penalizing solar, per se.
Exactly. Also have USAA, and they said no problem covering solar roof. The only concern was to review the coverage to make sure it was sufficient for the added cost to re-build. We did up the numbers and premium as a result, but there is no higher rate structure for having the solar/PWs insured than the rest of the dwelling.
 
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SoCal Dave

Member
Jul 30, 2020
421
317
California
I have Farmers and my agent wanted to know the size in kW, # of panels, cost of system and if we are purchasing or leasing. I don't know if it will increase my rates.

I also told her about the lift and relay of our roof. I'd hope that would possibly lower costs as they don't have to worry as much about replacing things because of a bad roof.
 

DrPhil

Member
Oct 30, 2019
25
16
Indianapolis
That is interesting. I guess it make sense that they don't want to be in the business of insuring solar farms. Is it 125% usage at the time of install or at the time of loss?
Basically he did say they didn't want to be in the business of insuring power producers. At the time of install, they wanted some documentation regarding our usage as well as expected production going forward. Don't know what would happen if our real world data turned out differently or we became much more efficient with our power consumption. Guess I'd have to buy another tesla...
 

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