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11.6kw Solar Roof, but only one Inverter?

ericl

Member
Aug 19, 2021
68
25
florida
Getting the Tesla Solar Roof installed right now. They upsized my system from 7.5kw to 11.6kw (limit for Tier 1 in Florida). I currently only have one Powerwall+ that will be installed. I thought I was going to get two because they promised a free one, but apparently the free one is the one I am already getting.

My question is, will a single inverter inside my sole Powerwall+ handle the a full load? I live in Central Florida so there are definitely days where its sunny and the majority of panels are all south facing with no shading at any time. Is it possible to get another inverter but not another Powerwall+? Or just get another Powerwall+? I just thought of this today, my project took only 1 month from contract to install so this didn't even cross my mind in the last month.

Edit - Apparently the Powerwall+ can kick into Overdrive to 12kw as just responded from my Tesla Advisor. This is done through software. Adding another inverter is not ideal as it will cause energy loss because the inverters themselves consume energy. Anyone have real world examples?
 
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Bitslizer

Member
Jun 16, 2021
237
82
IL
I have never seen my pw+ get up to 12kw

Are you sure it's overdrive and not oversize?

Pw+ have a DC/AC Max ratio of 1.7 which mean they can install up to 12.96kw of PV but you will still clip at 7.8 (7.6 but can go a little over spec)

May not be a problem if your PV tile are all spread out in different direction on the roof so they peak at different time.

For reference on my pw+ I have 2.7k east facing and 7.48k west facing. And they will clip around solar noon.

I also have a standalone 3.8kw Tesla inverter. But certain location limit your inverter total to be max of 10kw so 7.6+3.8 would be over that max, have to check your own local rules
 
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ericl

Member
Aug 19, 2021
68
25
florida
I have never seen my pw+ get up to 12kw

Are you sure it's overdrive and not oversize?

Pw+ have a DC/AC Max ratio of 1.7 which mean they can install up to 12.96kw of PV but you will still clip at 7.8 (7.6 but can go a little over spec)

May not be a problem if your PV tile are all spread out in different direction on the roof so they peak at different time.

For reference on my pw+ I have 2.7k east facing and 7.48k west facing. And they will clip around solar noon.

I also have a standalone 3.8kw Tesla inverter. But certain location limit your inverter total to be max of 10kw so 7.6+3.8 would be over that max, have to check your own local rules
Pretty much all of mine will be South facing, I think 70% or so are south. Also my tilt is around only 20 degrees. Yeah they said the Powerwall+ can go over spec with a software update (quote unquote "overdrive") and invert the max amount the 11.6kw roof can produce and perform up to 12kw. This is what an advisor told me after contacting the engineering team; that I shouldn't need another inverter. I am not too familiar with the tech here and conversion so I feel like they could be right? I'll ask the electricians when they come tomorrow.
 
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BGbreeder

Member
Jun 19, 2020
430
255
Bay Area
Pretty much all of mine will be South facing, I think 70% or so are south. Also my tilt is around only 20 degrees. Yeah they said the Powerwall+ can go over spec with a software update (quote unquote "overdrive") and invert the max amount the 11.6kw roof can produce and perform up to 12kw. This is what an advisor told me after contacting the engineering team; that I shouldn't need another inverter. I am not too familiar with the tech here and conversion so I feel like they could be right? I'll ask the electricians when they come tomorrow.
That fails my personal sniff test. When are you most likely to have full power? Bright sunny day in June/July, when it will be warm outside, and your inverter will have trouble staying cool.

Now if more of your tiles are east/west, that would shave the peak power down a lot, but 70% of 11.6 is still 8.12kW. I would double check your orientations and plans. You can try PVWatts to model your your roof output.

All the best,

BG
 
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Vines

Active Member
Jul 20, 2018
2,078
2,495
Silicon Valley, CA
That fails my personal sniff test. When are you most likely to have full power? Bright sunny day in June/July, when it will be warm outside, and your inverter will have trouble staying cool.

Now if more of your tiles are east/west, that would shave the peak power down a lot, but 70% of 11.6 is still 8.12kW. I would double check your orientations and plans. You can try PVWatts to model your your roof output.

All the best,

BG
I do not sell a Powerwall + so have no idea really, but from a rough look this seems edgy but doable.
Often the penalty for oversizing is pretty small, and from what I understand the Solar Roof tends to underperform a traditional panel by 10-20% due to lack of ventilation, especially in warm climates. 11.6 worth of solar roof panels on a 7.6 kW inverter is a 1.56 DC/AC ratio. If you derate the solar roof output by 10% that's a 1.37 DC/AC Ratio, at 20% it's a 1.22. I understand Florida is quite hot, so this seems a reasonable deration.

So while some peak power will certainly be lost, the system will probably perform well enough for its cost though will need to get rid of a good bit of heat. Probably best the inverter isn't in the sun.

One post here by an installer Optimizers... what are they good for? points out that oversizing a system by 50% only nets a production loss of less than 5%. So as long as the equipment can handle the heat this might be ok.
 

ericl

Member
Aug 19, 2021
68
25
florida
I would ask for an additional 3.8kw inverter at least if they don't charge you more.
Just talked to the engineers that came over, they said Tesla told them the Powewall+ can handle up to 13kw roofs (they said 12kw on the phone lol), but they agreed with me its BS. Their thinking is aligned with mine and they think an additional 3.8kw inverter is required as well.

Anyone doing Tesla Solar in the future, take your Tesla Advisor's comments with a grain of salt. Also try to get contact with the engineering team.

Edit - Engineers called the "hotline" said Powerwall+ is designed to handle it.... lol
 
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BGbreeder

Member
Jun 19, 2020
430
255
Bay Area
If it were me, I would focus on the big picture; Tesla is guaranteeing you a certain annual performance. At the end of the year, if they don't make, ask for them to make good. How they remedy any shortfall is really their issue. You are paying for performance. Tesla's job here is to deliver it.

All the best,

BG
 
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ericl

Member
Aug 19, 2021
68
25
florida
If it were me, I would focus on the big picture; Tesla is guaranteeing you a certain annual performance. At the end of the year, if they don't make, ask for them to make good. How they remedy any shortfall is really their issue. You are paying for performance. Tesla's job here is to deliver it.

All the best,

BG
The annual performance has varied from 102% offset to 44% offset to 7x% offset. My current layout page is blank because of how many changes the project has been going through.... lol. At this point I just want to the max amount of kw if I am getting this 11.6kw roof.
 

Bitslizer

Member
Jun 16, 2021
237
82
IL
Just talked to the engineers that came over, they said Tesla told them the Powewall+ can handle up to 13kw roofs (they said 12kw on the phone lol), but they agreed with me its BS. Their thinking is aligned with mine and they think an additional 3.8kw inverter is required as well.

Anyone doing Tesla Solar in the future, take your Tesla Advisor's comments with a grain of salt. Also try to get contact with the engineering team.

Edit - Engineers called the "hotline" said Powerwall+ is designed to handle it.... lol
It will handle it, just clip like crazy, if my 10.2kw east west on the pw+ clip for up to 2 hours a day. Imagine how much your 100% south facing would clip
 
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Vines

Active Member
Jul 20, 2018
2,078
2,495
Silicon Valley, CA
The annual performance has varied from 102% offset to 44% offset to 7x% offset. My current layout page is blank because of how many changes the project has been going through.... lol. At this point I just want to the max amount of kw if I am getting this 11.6kw roof.
Tesla wants to focus on area under the curve. The variables are cost, and kWh of production.

Though it might personally bother people who are focused on squeezing every watt out of the system, shedding a few kW instead of harvesting it is exactly what happens to every other square foot of ground. Optimize the largest production for the smallest dollar input and don't even worry about clipping.

For instance you can increase production (really reduce losses) by upsizing all string wiring to #8. You can clean your panels every month to reduce soiling losses to almost 0%. However the cost outlay is way larger than the production you will regain for that cost.

Focus on the important things, cost vs kWh generated.
 

Bitslizer

Member
Jun 16, 2021
237
82
IL
Tesla wants to focus on area under the curve. The variables are cost, and kWh of production.

Though it might personally bother people who are focused on squeezing every watt out of the system, shedding a few kW instead of harvesting it is exactly what happens to every other square foot of ground. Optimize the largest production for the smallest dollar input and don't even worry about clipping.

For instance you can increase production (really reduce losses) by upsizing all string wiring to #8. You can clean your panels every month to reduce soiling losses to almost 0%. However the cost outlay is way larger than the production you will regain for that cost.

Focus on the important things, cost vs kWh generated.
Tesla charge by per watt PV installed (at least the panels), it is to the customer benefit if they can convince Tesla to install an additional inverter but without added cost. Not like other company that charge you more for another inverter
 

wjgjr

Active Member
May 11, 2020
1,331
1,051
Silver Spring, MD
Does Tesla provide production guarantees? From what I remember when I had them quote panels, they provided estimated production values, but don’t think they stood behind them. Maybe the solar roofs are different?
For our solar roof, they did not guarantee production - just estimated it. So, some care needs to be taken to confirm that the estimates are based on the actual configuration as best one can determine with how difficult it is to get answers from Tesla. That said, they generally seem pretty good (and conservative) with the estimates they do provide. This is likely because, while not necessarily a warranty issue, Tesla does not want to be fielding calls from customers asking why production was low where it might be due to weather.

The only performance guarantee we had was that each module would perform at 95% of its rated peak power after 5 years and then drop by no more than 0.5% a year after that. While it is good to have that coverage, it is difficult as a customer to determine whether there is an issue if it is isolated to a small number of shingles.
 

wwhitney

Member
Nov 2, 2017
979
1,340
Berkeley, CA
If you go for up to a 13.5kw...14kw PV, they will be force to give you a second inverter 🤣
With a corresponding price bump for the second inverter, presumably, causing your $ per kWh generated to go up. If you're not clipping, your system cost too much.

Inverter capability and PV panel capability can never be perfectly matched. Either you have a lot of unused inverter capability during the morning and evening, or you have some unused PV panel capability during midday.

Cheers, Wayne
 

h2ofun

Active Member
Aug 11, 2020
3,028
658
auburn, ca
IMO, some just take this over the top! NOTHING with solar can be guaranteed. with all the smoke last year, and now this year, my solar output is way down. My panels are now real dirty. I get shading. I just spent the money I wanted to. What I get is what I get, no matter what anyone has told me or PVwatts says. Life is just too short to waste all this energy on this :)
 

Bitslizer

Member
Jun 16, 2021
237
82
IL
With a corresponding price bump for the second inverter, presumably, causing your $ per kWh generated to go up. If you're not clipping, your system cost too much.

Inverter capability and PV panel capability can never be perfectly matched. Either you have a lot of unused inverter capability during the morning and evening, or you have some unused PV panel capability during midday.

Cheers, Wayne
Tesla don't charge for inverters, they just charge a flat rate per watt installed (at least for panels) one inverter or two they would charge the same $ per kw.... More PV and more inverters you get a smaller DC/AC ratio if anything the $ power kWh would go DOWN because you aren't losing as much kwh from clipping 14kw/11.4kw = 1.23 vs 11.6/7.6 = 1.53
 

wwhitney

Member
Nov 2, 2017
979
1,340
Berkeley, CA
Tesla don't charge for inverters, they just charge a flat rate per watt installed (at least for panels)
To my knowledge Tesla is marketing panels in units of 4.25 kW, so their inverter sizes are also going to increase accordingly. I.e. the standard DC/AC ratio is fixed and their price per watt covers both panels and inverters. I seriously doubt that if you have a 8.5 kW system being designed with them, and you say "oh, I can fit in two more panels, what would that cost?" that they will give you same price adder for 2 panels only vs 2 panels plus an inverter increment.

If you like, I'll amend my statement "if your not clipping, your system cost too much" with the footnote that maybe you got lucky and the system was mispriced and some of that cost was eaten by another party.

Also, depending on what you care about, certainly at DC/AC ratios just about 1.0, the net environmental impact of increasing the inverter size will be negative. As it will only slightly increase energy harvest, and that increase will not offset the environmental impact of building that extra inverter capacity.

Cheers, Wayne
 

ericl

Member
Aug 19, 2021
68
25
florida
I am comfortable at 11.6 for my setup because over this limit, I am considered Tier 2 and will require that 1 million personal liability insurance at least here in Florida.

Also today I just realized its possible Tesla just improperly removed/disposed of my roof and may have spread asbestos fibers everywhere as the original roof could have been encapsulated by another layer of underlayment. My house was built before 1980 and there is asbestos everywhere, for example I am waterfront so my seawall is made from asbestos.

And that now after all of the roof is gone, they are about to tell me that my panel needs to be upgraded. This is a fun project /sarcasm, seems to be run super rushed and like in the tech mindset "figure things out while they are being done".

-Edit Environmental health and safety team on their way.... hopefully negative everything.
 
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