Panels + Sonnen Battery = is this working ok?

[J.P.M.]

From a practical perspective, adding more panels to an inverter is less expensive than a variable tilt system on a roof. Since most systems are roof mounted, higher DC to AC ratios are an economical way to get the optimal benefits of variable tilt without having to go on the roof to change the tilt.
Solar Farms just over panel because that is less expensive that tracking systems for them to get optimal return on their investment and from the footprint of land.

If one of the goals of PV system design is, in the end, to maximize the financial benefit of the system, I don't understand why or how (assuming the array is sized to meet the design goals) under sizing an inverter system makes any sense. Part of my ignorance I guess.

Nor do I understand how an AC/DC ratio has anything to do with a system with adjustable orientation.

Under sizing or oversizing of any system component makes no sense and wastes resources and design efforts. That's not to say however, that as part of system optimization design tradeoffs and compromises aren't necessary. But I've not read that as part of this thread so far.

Seems to me the rub here is what the optimum inverter size might be relative to the application and the design goals, with one of those goals being optimum long term cost effectiveness.

[Ampster]

It is pretty clear there is a diversity of opinion about optimum DC to AC ratios. In the meantime the OP has not raised any further questions.
Other readers are free to form their own opinions based on their own use case or circumstances. Some may say the glass is half full and some may say the glass is half empty. It all depends on where you are standing. There will also be an opinion that the glass is the wong size.

[J.P.M.]

It is pretty clear there is a diversity of opinion about optimum DC to AC ratios. In the meantime the OP has not raised any further questions.

Yea, and some of it even makes sense, but maybe the OP got tired of all our blather, yours mine and everyone else's.

[J.P.M.]

An analogy to oversizing the inverter might be: Why pay for a sports car designed to run 100 mph if you will never drive it over 80-90 mph?

I don't agree. I'd say that analogy is closer to: Why design a system to generate 10,000 kWh/yr. to meet an annual load of 8,000 kWh/yr. ?
But as Ampster writes, there seems to be some diversity of opinion on inverter oversizing.

[Ampster]

Why design a system to generate 10,000 kWh/yr. to meet an annual load of 8,000 kWh/yr. ?
......

One reason would be the impact of Time of Use rates on the financial equation. If it takes 10 megaWhs to break even financially then that is the optimal solution financially. Of course that would be after exploring other strategies to shift loads to times when rates were lower. It is a three dimensional model where timing has become another variable to the two dimensional supply and consumption model..

[foggysail]

I analyzed my contracted September installation using Canadian Solar HiDM5 400 watt panels along with IQ8 Plus microinverters. This is what I came up with.

The IQ8's are power limited to a maximum of 300 watts with continuous ratings of 290 watts. At first is seemed to say....WRONG INVERTERS but not so fast. The solar panel has a normal maximum power Pmax of 400W Standard Test Data. In real life though, it appears that the STD ratings are not as important as the NMOT ratings that represent real life expected output. In my case, that expectation is only 295 watts which almost exactly matches the continuous power ratings for the IQ8 Plus. I expect I will see some clipping but it is not worth paying an extra 40-50 bucks each for 26 higher power devices.

It will cost an extra $K or so to move up to a 366 watt maximum rated inverter. I need to do more analysis to learn if there is any life advantages to using a higher wattage inverter.

I don't believe in rules of thumb such as the earlier mentioned 1.5 ratio. I do believe actual expected performance numbers though.

[Ampster]

I analyzed my contracted September installation using Canadian Solar HiDM5 400 watt panels along with IQ8 Plus microinverters. This is what I came up with.
........
The IQ8's are power limited to a maximum of 300 watts with continuous ratings of 290 watts. ....
I don't believe in rules of thumb such as the earlier mentioned 1.5 ratio. I do believe actual expected performance numbers though.

The DC to AC ratio is not a rule of thumb it is a ratio or mathematical relationship. Myself and others earlier quoted a rule of thumb or common practice of designing to a ratio of 1.25 to 1 and your configuration works out to be1.37to 1. You could go to PV Watts and enter data for the two configurations and see what that model predicts. A lot will depend on tilt, orientation, and location. I believe there is an input for the DC to AC ratio so you can compare your two options. .

[foggysail]

The DC to AC ratio is not a rule of thumb it is a ratio or mathematical relationship. Myself and others earlier quoted a rule of thumb or common practice of designing to a ratio of 1.25 to 1 and your configuration works out to be1.37to 1. You could go to PV Watts and enter data for the two configurations and see what that expected model predicts. A lot will depend on tilt, orientation, location. I believe there is and input for the DC to AC ratio so you can compare.

Sure, with an inverter at peak power and a panel at peak power in my case I would have 400/ 300= 1.33 factor. And neither of those numbers represent real life. Panel real life expected wattage is only 295 watts (NMOT) and the continuous wattage from an IQ8 Plus is 290. With these numbers the ratio is much lower than previous at 1.01.

Use whatever method one is comfortable with relating to panel and inverter power. I will use the data sheets to make my trade offs between performance and cost.

One other interesting thing worth mentioning. My installation will have 26 panels. Based on my earlier post, I can expect to have a maximum of 290 watts of useable solar power/panel or 7540 watts maximum. The current at 240VAC is 31.4 amperes. I intend to back feed a 40 ampere breaker but that has to be derated to 80% for continuous (more than 3 continuous hours) or 32 amperes.

[Ampster]

Sure, with an inverter at peak power and a panel at peak power in my case I would have 400/ 300= 1.33 factor. And neither of those numbers represent real life. Panel real life expected wattage is only 295 watts (NMOT) and the continuous wattage from an IQ8 Plus is 290. With these numbers the ratio is much lower than previous at 1.01.

Ratios are relationships of numbers to each other and I have no expectation that they represent real life. Your real life expected Wattage is going to be dependent on real life orientation, tilt, temperature and the number of clouds in the sky.I do not disagree with anything you are saying. My over explanation is for the benefit of other readers.

Use whatever method one is comfortable with relating to panel and inverter power. I will use the data sheets to make my trade offs between performance and cost.

One other interesting thing worth mentioning. My installation will have 26 panels. Based on my earlier post, I can expect to have a maximum of 290 watts of useable solar power/panel or 7540 watts maximum. The current at 240VAC is 31.4 amperes. I intend to back feed a 40 ampere breaker but that has to be derated to 80% for continuous (more than 3 continuous hours) or 32 amperes.

I have been a number cruncher most of my life so I run all kinds of numbers all the time. I even slip a digit once in a while. Let us know what your conclusion is about which micros you decided to use and your real world experience. My panels are 345 Watts and ,my IQ7s cap at 290 Watts and I have not seen any clipping.
My IQ7 Trunk cable only allows 20 Amps per string and I am at 15 Amps for the largest string so I chose to use two 20 Amp breakers instead of a combiner so I could do diagnostics on individual strings if needed. It came in handy when one of my trunk cable connections that I had made went bad.

[J.P.M.]

One reason would be the impact of Time of Use rates on the financial equation. If it takes 10 megaWhs to break even financially then that is the optimal solution.

Can you provide an example of how that might work ?

[foggysail]

Ampster-- I read many of your posts here---- you are doing a fine job trying your best to help. This stuff is a piece of cake for me only because of my earlier years doing circuit design of military radar transmitters and other related stuff.

[foggysail]

I don't agree. I'd say that analogy is closer to: Why design a system to generate 10,000 kWh/yr. to meet an annual load of 8,000 kWh/yr. ?
But as Ampster writes, there seems to be some diversity of opinion on inverter oversizing.

There may be good reasons to over design related to reliability. I certainly would use higher powered inverters than required but as I type, I do not want to spend the extra money and that might come back and haunt me later.

[Ampster]

Can you provide an example of how that might work ?

Yes, but it is really a niche discussion probably only of value to some California ratepayors. It has also been discussed in detail on other forums with specific sections about California rate trends. If you want to get into the details, start a new thread with that in the heading and I would be happy to continue the discussion. This thread has been off topic too long. Although it is a topic of interest to me. I think most people who have been on TOU rates understand a number of ways that the concept would work. If I remember correctly you are still on a Non TOU rate and the concept would not work for you anyway in your two dimensional rate environment.

If others who are on TOU rates want to continue the discussion feel free to start a thread. I do not want to presume that there is much interest in that topic here but as I mentioned it is a lively subject on some other forums with a larger user base of California ratepayors.

[J.P.M.]

This thread has been off topic too long.

You're right. Screw it.