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Inverter size when maximum system is 11.56 kW (34 panels)

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nwdiver

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Feb 17, 2013
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you may be loosing a bit of efficiency as the inverter spends less time at it most efficient operating range (maximum output).

Very small. Unless your inverter is ridiculously oversized it's gonna be in the tenths of a percent.

Screen Shot 2021-03-12 at 12.45.53 PM.png
 
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Two-rocks

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Jan 18, 2021
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Help me with the graph - inserting time into the loss (or flattening the system output curve) would compound any losses?
Or put another way, the fractions of a percent * time in years, becomes more than a bit?
 

nwdiver

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Feb 17, 2013
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Help me with the graph - inserting time into the loss (or flattening the system output curve) would compound any losses?
Or put another way, the fractions of a percent * time in years, becomes more than a bit?

Having a slightly oversized inverter is going to have a <0.5% effect on output and efficiency actually drops slightly from 50% to 100% so you could argue an oversized inverter in this range is better... Solar inverters are designed to operate under a wide range of outputs. You're not going to want to lose a lot of power on a cloudy day when solar resources are already scarce.

IMHO the effect is too small to really worry about. There's reasons to not install a 11.4kW inverter when a 7.68 is fine but IMO efficiency isn't one of them.
 

Southpasfan

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Jun 2, 2019
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If one knew they were never going to upgrade their solar, either more panels or larger, I agree. But in my case, I am looking at adding a lot more. If I had just put in inverter to the size I have now, I would have been forced to buy new ones.

Thats a great point. An early adopter friend of mine just swapped out some 240 panels for 340s, and he actually could have waited a bit and gotten 400s.

If the inverter can handle it the panel swap was very cheap compared to the entire installation, and the installer gave credit for the 10 year old panels.

Its not like you could request an oversized inverter in the initial install with Tesla without some discussion at least, if they would even do it. I would imagine it would be an up charge from them, but that's speculation.
 

hurjio

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Feb 3, 2018
130
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Southern California
Sorry for hijacking the thread, but if I go with a Tesla 5.5-6kW system, I'll likely get stuck w/the 3.8kW inverter, right? Is that a good and ok option? This means the system generally won't produce more than 3.8kW power at a given time? Location is southern California, with zero to minimal shading on panels. Panels would likely be split evenly between SW and SE facing.

Would I get better output if I went 3rd party installer 360W panels w/Enphase IQ7Plus microinverters (max power 290W)?
 

nwdiver

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Feb 17, 2013
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Sorry for hijacking the thread, but if I go with a Tesla 5.5-6kW system, I'll likely get stuck w/the 3.8kW inverter, right? Is that a good and ok option? This means the system generally won't produce more than 3.8kW power at a given time? Location is southern California, with zero to minimal shading on panels. Panels would likely be split evenly between SW and SE facing.

Would I get better output if I went 3rd party installer 360W panels w/Enphase IQ7Plus microinverters (max power 290W)?

I'm curious to find out... not sure where Tesla will bump people from 3.8 to 7.6. You're not gonna get a better deal with a 3rd party installer. Would you rather pay ~$12k for 7300kWh/yr or ~$18k to get 8000kWh/yr? If it makes sense for Tesla to install a 7.6kW inverter... they'll install a 7.6kW inverter.
 

charlesj

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Oct 22, 2019
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Monterey, CA
Sorry for hijacking the thread, but if I go with a Tesla 5.5-6kW system, I'll likely get stuck w/the 3.8kW inverter, right? Is that a good and ok option? This means the system generally won't produce more than 3.8kW power at a given time? Location is southern California, with zero to minimal shading on panels. Panels would likely be split evenly between SW and SE facing.

Would I get better output if I went 3rd party installer 360W panels w/Enphase IQ7Plus microinverters (max power 290W)?
Well, unfortunately people forget that if an inverter doesn't clip, the duration of that 3.8kW will be shorter duration time wise than if the system had a 1.5 ratio, and clipped. Clipping increases time duration for that 3.8kW, hence many hours of 3.8 kWh of energy generation.

Here is the image of that clipped inverter from one of the link above with a long duration of max energy production. Yes, counter intuitive perhaps but it is the case:
 

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hurjio

Member
Feb 3, 2018
130
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Southern California
I'm curious to find out... not sure where Tesla will bump people from 3.8 to 7.6. You're not gonna get a better deal with a 3rd party installer. Would you rather pay ~$12k for 7300kWh/yr or ~$18k to get 8000kWh/yr? If it makes sense for Tesla to install a 7.6kW inverter... they'll install a 7.6kW inverter.
Oh, I definitely know I'm not going to get a better deal w/3rd party installer. Quotes I'm getting from 3rd party are 21-23% higher than Tesla's price, which for a "smaller" system like I'm looking at amounts to $2500-3000 difference (pre-ITC rebate. Post ITC rebate the difference is more like $2000). Whether that is worth it for microinverters/better panels/better service/better warranty is something I'm trying to decide.
 

Southpasfan

Member
Jun 2, 2019
460
586
Pasadena
Very small. Unless your inverter is ridiculously oversized it's gonna be in the tenths of a percent.

View attachment 643893


This. The bottom axis is shows an inverter crosses 95% efficiency at around a tenth (.1) of output as opposed to rated. In my case, that means for a 16.32 system, if one assumes 16.32 of inverters, it reaches efficiency at 1.632 kw. Today, it only took one half hour to reach that level of production.

I sort of don't buy the concept that undersizing the inverters gives you some sort of efficiency gain.

Its also true that one can calculate the loss from clipping and it might not be that much. My point would simply be that the inverters ought to be sized so that they don't clip. If, on a $60,000 job that means it costs Tesla another $1k wholesale on a larger inverter they ought to spec it.
 
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nwdiver

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Feb 17, 2013
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If, on a $60,000 job that means it costs Tesla another $1k wholesale on a larger inverter they ought to spec it.

So if spending $1k increased annual production by 100kWh with less inverter saturation they should do it? ...... why? $1000 for 100kWh/yr? Seems a bit dumb because numbers..... doesn't it?
 

bevo

Member
Mar 10, 2021
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25
Irvine
So if spending $1k increased annual production by 100kWh with less inverter saturation they should do it? ...... why? $1000 for 100kWh/yr? Seems a bit dumb because numbers..... doesn't it?
You're letting facts get in the way :)
Yes, they should do it because they have a fixed pricing strategy and as a customer, I want the maximum output I can get.
 

nwdiver

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Feb 17, 2013
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You're letting facts get in the way :)
Yes, they should do it because they have a fixed pricing strategy and as a customer, I want the maximum output I can get.

They're able to keep their prices lower than everyone else by not wasting $$$ on unnecessary inverter capacity ;)

Do you want a system with an inverter that's never saturated or do you want the most kWh/yr / $? Can't have both.
 

CrazyRabbit

Member
Apr 21, 2020
439
133
Fort Worth TX
maximum output depends on your panel layout and pitch of your roof. but i agree, and i would want a system that results in little clipping. my dc/ac ratio is 340/295=1.15 (iq7+ inverters), so i will see minor clipping; but as panels age that will go away.
with string inverter and a balanced east west panel layout you can run with a dc/ac ratio of 1.1-1.2 without clipping, so it depends on layout.
 

wwhitney

Member
Nov 2, 2017
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1,313
Berkeley, CA
i would want a system that results in little clipping.
Just to be clear, that's the same as saying "I would want a system that is designed to have a somewhat higher $/kWh generated than the optimum." Or another way of saying it is "I prefer to underutilize my inverter capacity and fully utilize my panel capacity, I don't care about finding the economically optimal balance between sizing the two resources."

Cheers, Wayne
 
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jrweiss98020

Tessa's Tesla
Jan 9, 2020
468
335
Edmonds, WA
Would I get better output if I went 3rd party installer 360W panels w/Enphase IQ7Plus microinverters (max power 290W)?
I went 3rd party in Aug 2019, got 27x335W & IQ7+. Output was above guarantee the first year. Just added 8x340W & IQ7+ to accommodate added loads. I'm happy.

In SoCal, you will waste a significant portion of the 360W panels' output in the summer (Apr-Aug) around noon (±2-3 hours) with the IQ7+. OTOH, you will enjoy the full 290W output of the IQ7+ for a greater part of the day. You will have to decide with your installer whether the 360W panels are the best choice, or if a 340W panel might save you $$ without sacrificing output.
 

Southpasfan

Member
Jun 2, 2019
460
586
Pasadena
Just to be clear, that's the same as saying "I would want a system that is designed to have a somewhat higher $/kWh generated than the optimum." Or another way of saying it is "I prefer to underutilize my inverter capacity and fully utilize my panel capacity, I don't care about finding the economically optimal balance between sizing the two resources."

Cheers, Wayne
Thats true but that's not the question. We are relying on Tesla to put together package systems. The quote above means basically this. "OK, with these smaller inverters you will get 10000kwh per year. With a larger inverter, changing no other spec, you will get 11,000 per year, but it would cost $1000 more.

At 25 cents per kwh, 1000 is worth $25 per year, or $500 over the twenty year life of the system. So if you are going to get charged more than you get, well, that's what people are pointing out.

But that's different than saying that Tesla can make $1,000 less profit if they size the inverter to allow maximum production.

That question gets to the heart of the matter. Tesla is not charging by kwh. If they were, the price of a system in SoCal would be different than one in Seattle. If a given system, say mine, clips five days a year that's one thing. But if it clips 50 days a year, all that clipping has to do is result in 8 kwh loss in each day to equal 400kwh per year, or 8,000 over the 20 year life of the system (which is what I'm buying) or $2,000 less to me. So if its between Tesla making $1000 more profit v. me getting $2,000 less I vote me.

If its a different scenario, and I pay all the cost items, I could indeed chose more panels over a larger inverter if the price of panels to the price of the larger inverter warrants it.

But that's not the question, is not like many buyers have the option when ordering through Tesla to select more panels. The question is should the inverters be sized in the spec to have only minimal or no clipping?. I say yes.

If Tesla wants to change the whole analysis and say, "we are selling a system which will produce a minimum of X and a maximum of Y (because the maximum production is limited by the inverter)" they ought to say so. But they are not making that representation.
 
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