Oversizing inverters

[Ampster]

.......

Any thoughts on that approach?

I would go with the bigger inverter for that cost increment assuming it is reasonable. If you know the inverter brand you can easily find the retail price difference. That could be worth getting another 2 or 3 MWhs of production. . I would check with your installer to make sure that AC increase would not meant a bigger main panel.

[darkskies]

Cut some or all of tree shading will be your biggest bang for the buck......

Unfortunately, I can’t. We border common land that has wet lands, and the trees are all in the “protection zone” and can’t be touched. It would have been my first choice.

[darkskies]

I would go with the bigger inverter for that cost increment assuming it is reasonable. If you know the inverter brand you can easily find the retail price difference. That could be worth getting another 2 or 3 MWhs of production. . I would check with your installer to make sure that AC increase would not meant a bigger main panel.

They are talking $1500 (list price difference) to go from the 7.6 to 11.4 inverter (SolarEdge with energy hub). I’m thinking that although on the high end, it’s probably the best way to gain output and allow for future expansion.

[DanS26]

Unfortunately, I can’t. We border common land that has wet lands, and the trees are all in the “protection zone” and can’t be touched. It would have been my first choice.

A few of the most obnoxious branches removed may not be missed if discretely done.

[Ampster]

They are talking $1500 (list price difference) to go from the 7.6 to 11.4 inverter (SolarEdge with energy hub). I’m thinking that although on the high end, it’s probably the best way to gain output and allow for future expansion.

I did not see that big of a difference in prices but I am not familiar with the energy hub models. at $0.15 per kWh that is still $300 per year that you would save by producing 2 more mWhs.

[davidcheok]

They are talking $1500 (list price difference) to go from the 7.6 to 11.4 inverter (SolarEdge with energy hub). I’m thinking that although on the high end, it’s probably the best way to gain output and allow for future expansion.

Why not get the same inverter, split the strings up between the two and run parallel?

[darkskies]

A few of the most obnoxious branches removed may not be missed if discretely done.

They are huge pines that are obstructing height wise and being evergreens, year round.

[darkskies]

I did not see that big of a difference in prices but I am not familiar with the energy hub models. at $0.15 per kWh that is still $300 per year that you would save by producing 2 more mWhs.

Good point, and here in NH, we're at $0.25-$0.31 per kW.

[darkskies]

Why not get the same inverter, split the strings up between the two and run parallel?

It's possible. I think the backup interface supports like 2-3 inverters, but the cost of another inverter would be more than the upgrade cost, from what I see of discount prices on the 7600 inverter.

[Ampster]

If indeed the trees are affecting production, upgrading the inverter way not gain much. It might be worth finding a tool that can measure the impact of shade.

[J.P.M.]

i think this is key.

Solar/inverter ratio is pretty far down on the list of things to consider when you start designing a system.

agreed, but i'd suggest its use is limited to describing nothing more than dc/ac ratio and of little practical use even then. It's a convenient artifice and meaningless as a tool in system design. It's a ratio that's about as much use as the panel stc size to system stc size. Kind of like a big "so what ?". A safe, well designed and cost effective system can be had without using dc/ac ratio at all. I've done it and shown the basics on this forum.

are you limited to 40a backfeed due to the 20% rule, and are all your inverter options (up to 7600 watts) pretty similar? And will 6kw of solar get you to your goal? Then there might be no reason to ever overpanel.

after we agree on a starting point for the design process like the annual design duty (annual load) maybe we ought to decide if this is discussion about overpaneling or under inverting as well as some definition of just what makes an inverter oversized.

do you want to expand in the future? Then again maybe don't overpanel so expansion is just adding another string.

i'd decide upfront (and very carefully) if future expansion is to be a design goal and if so make any added duty it part of the design duty while keeping close mind that system oversizing can quickly make the lcoe of a system greater than the lcoe of poco bought electricity.
depending on how high system cost effectiveness is as a design priority, i'd suggest trying to ensure a high probability for the future expansion that's more than a maybe. I'd also suggest that not making any future expansion part of the initial design load complicates any future expansion more complicated.

do you have two different orientations? Then overpaneling becomes much less of a big deal for the obvious reasons.

any system with multiple orientations will have a lower overall specific annual output (in kwh/yr. Per installed stc kw of panels) than any equal stc size single orientation system of a reasonable but more equator facing orientation. Another way to describe it is that in comparing the annual output curves of the two systems, the area under the single orientation system's curve will be larger. Multiple orientations may be a necessity (or a single orientation an impossibility for the application), but if not necessary, and cost effectiveness is a priority, it's not a good choice.
That written, if a multiple orientation system is over paneled for the design load and inverter(s) i can't see how it would become less of a big deal.

are you limited to 40a but need a 12kw system? Then you're going to be significantly overpaneling out of necessity - and it probably won't be that bad.

if by how bad you mean - how much the specific output will drop as a result of a design decision for less than optimal array orientations to stay under 40a:
Assuming there is/are some array orientation choice(s), while it's a design decision, i'd use an initial guess of a ratio of 12/9.6 as a starting point for an overpaneling penalty for the orientation choices, check max. Total system power output and iterate the orientation(s) to stay under 40a.
As a swag, i'd agree with your statement that it wouldn't be that bad but only in a relative sense. It may be unavoidable, but less than optimal orientations always mean cost effectiveness takes a hit, sometimes to the point of making pv power more expensive than poco power.

to put it another way, the overpaneling ratio should be an output of, not an input to, the equation.

i believe that's about the equivalent of what i wrote 05/01/2023 post time stamped 4:24 p.m.

j.p.m.

[Mike 134]

If indeed the trees are affecting production, upgrading the inverter way not gain much. It might be worth finding a tool that can measure the impact of shade.

I rented this tool to estimate the effects of shade.


Solar Pathfinder - Solar site analysis

[darkskies]

I’ve seen that tool, but didn’t know it handled shading and it was expensive. I didn’t know you could rent it. Will have to look into that.

[Ampster]

...........

To put it another way, the overpaneling ratio should be an output of, not an input to, the equation.

I agree, but somewhere in the design process a decision is made about what size inverter and what total kW the panel configuration based on the criteria you mentioned. Also if using PV Watts as a tool to get an estimate of production it should be noted that PV Watts assumes a 1.2 to 1 DC to AC ratio. Therefore if one wants to get a reasonable estimate of production they should calculate the DC to AC ratio and enter it into PV Watts along with other system parameters such as location, azimuth and tilt. When designing a string inverter system the ratio is affected by the number of panels per string but in the case of micro inverter systems the ratio is a result of matching a particular micro to a panel. The point I am making is that the ratio is not the basis for the choice but as you implied, the ratio is a result of design decisions made about inverters and panels. In that sense it influences output similar to choices about tilt and azimuth.

[J.P.M.]

Therefore if one wants to get a reasonable estimate of production they should calculate the DC to AC ratio and enter it into PV Watts along with other system parameters such as location, azimuth and tilt.

Which PVWatts easily allows and seems obvious to most users. Like the system loss parameter which has a default value of 14%, or any of the advanced parameters the 1.2 DC/AC ratio is a start, not a mandate.
While I understand what DC/AC ratio is and what the term means, I'm not sure of the value of calling it a DC to AC ratio. That seems to mask what it is: The inverter size/array STC size.
As far as PVWatts is concerned. Seems to me it would be just as easy and more straightforward to simply input the inverter type and size the way SAM and other design models do.

When designing a string inverter system the ratio is affected by the number of panels per string...

The way I learned it at least for initial design, the max. string length is initially set by the max. inverter input voltage and the max. expected panel voltage.


...but in the case of micro inverter systems the ratio is a result of matching a particular micro to a panel.

And that's another purpose of the PVWatts' model's allowing input of different inverter sizes by varying DC/AC ratios.

The point I am making is that the ratio is not the basis for the choice but as you implied, the ratio is a result of design decisions made about inverters and panels. In that sense it influences output similar to choices about tilt and azimuth.

Which is what I've been saying all along. Seems like you're agreeing with both Jeff's and my opinions.