1% clipping too high?

Would you think 1% to 1.25% clipping is too high or is it reasonable?

Anyone familiar with what would be an ideal amount?

thank you
 

jaggedben

Senior Member
I think it's totally reasonable. In fact for residential below 1% is kind of my rule of thumb for not worrying about it anymore. But the acceptable percentage is really not fixed, it's an economic or contractual question. For example:

-Is it cost effective to pay for a higher power inverter, let alone electrical upgrades that might be required for it? Does the extra energy pay for that?
- Are you already exceeding the customers promised energy production by a healthy enough margin? 1% is within the annual variation of weather in most places anyway.
-Is it more cost effective to just add a module or a few to make up the extra desired production instead of upgrading the inverter?

Utility scale projects often have massive clipping because module costs are less than interconnection costs and I believe there are advantages for grid operators in promising a flat-lined output in the middle of the day. In residential the biggest risk is failing to explain your choices up front to a nitpicky or confused customer and then having to make a change after the fact or getting a bad review. It's all about meeting the expectations.
 

Hv&Lv

Senior Member
I think it's totally reasonable. In fact for residential below 1% is kind of my rule of thumb for not worrying about it anymore. But the acceptable percentage is really not fixed, it's an economic or contractual question. For example:

-Is it cost effective to pay for a higher power inverter, let alone electrical upgrades that might be required for it? Does the extra energy pay for that?
- Are you already exceeding the customers promised energy production by a healthy enough margin? 1% is within the annual variation of weather in most places anyway.
-Is it more cost effective to just add a module or a few to make up the extra desired production instead of upgrading the inverter?

Utility scale projects often have massive clipping because module costs are less than interconnection costs and I believe there are advantages for grid operators in promising a flat-lined output in the middle of the day. In residential the biggest risk is failing to explain your choices up front to a nitpicky or confused customer and then having to make a change after the fact or getting a bad review. It's all about meeting the expectations.
Not an expert by no means at this

question. Aren’t invertors too large less efficient? Wouldn’t it be more productive even if your clipping for two or three hours a day on an ideal day?
Wouldn’t you want to be even higher than 1% to gain on the other days, and make up for dirty panels later that never get washed, or panel degradation than to oversize an invertor so it doesn’t clip at all or less than 1%?
ive seen some followers on our system on a graph that clip from about 10 AM to 3PM.
They are limited to 2MW though.
 
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jaggedben

Senior Member
I don't think inverters that are too large are necessarily less efficient, if you mean converting DC to AC. They tend to be less cost effective if you are paying for inverter output capacity that is unnecessary or underutilized.

Aside from soiling and degradation, the other reason that nameplate DC to AC ratios can be above 1 is simply that real world conditions usually underperform the test conditions (STC) that module nameplates are based on.

When the OP says 1% clipping he is presumably talking about estimated annual energy lost due to the inverter limiting output. It is not a direct reference to the DC-to-AC ratio. I suspect that our OP is getting a 1% clipping figure from software I suggested in another thread. Regardless, to see 1% clipping it probably means his DC to AC ratio is already meaningfully above 1, probably at least 1.05 if not 1.1. So as you add more PV to the inverter, you increase the energy production but also the clipping percentage, and you get diminishing returns from each additional module added. At some point it may be more effective to increase the size of the inverter to boost energy production than to continue adding more modules, but it all depends on the prices for everything involved and is not a simple calculation. For residential systems it isn't worth the time to do such calculations with precision, just make sure the customer understands your choices, or give them a larger inverter if you cannot reason with them. (Usually if they need to pay for a service panel upgrade for the larger inverter then they understand your reasoning.) For MW scale systems it's worth having someone spend weeks estimating the right ratio given whatever prices and constraints there are.
 

ggunn

PE (Electrical), NABCEP certified
Would you think 1% to 1.25% clipping is too high or is it reasonable?

Anyone familiar with what would be an ideal amount?

thank you
It really depends on your starting point; are you sizing your array to fit the inverter or sizing the inverter to fit the array? If you are sizing the array to the inverter, a little clipping in the middle of the day will likely be made up for by increased production on the shoulders of the power curve. If you are sizing the inverter to the array, any clipping will reduce the overall production.

Either way it boils down to what the system costs vs what the system will produce over its expected lifetime.
 

GeorgeB

ElectroHydraulics engineer (retired)
One significant thought here ... as the panels age, their output goes down. Give it a couple of years and some combination of dirt and aging will take care of 1 or 2% (or more!). As mentioned, it isn't uncommon to over-panel installations ... if for no other reason than the fed tax rebate is based on panel specifications. WRONG!!!! MY MISTAKE ... Duke Energy had a rebate based on DC Power when I had mine installed. BUT ... I'd suggest, if practical, tilting maybe half of the panels a few degrees to extend the power energy production and drop the peak.

The issue is that the installer didn't explain things well enough.
 
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ggunn

PE (Electrical), NABCEP certified
One significant thought here ... as the panels age, their output goes down. Give it a couple of years and some combination of dirt and aging will take care of 1 or 2% (or more!). As mentioned, it isn't uncommon to over-panel installations ... if for no other reason than the fed tax rebate is based on panel specifications. BUT ... I'd suggest, if practical, tilting maybe half of the panels a few degrees to extend the power energy production and drop the peak.

The issue is that the installer didn't explain things well enough.
Unless the modules are already at a very low tilt, changing the tilt a few degrees won't make an appreciable difference in their output.
 

jaggedben

Senior Member
...if for no other reason than the fed tax rebate is based on panel specifications.
This is not correct. The tax credit is based on expenditure. Although systems are often priced directly on the DC watts, there's no reason that the cost associated with a larger inverter can't be included in a contract and receive a corresponding credit.

BUT ... I'd suggest, if practical, tilting maybe half of the panels a few degrees to extend the power energy production and drop the peak.

The issue is that the installer didn't explain things well enough.
Tilt and azimuth can affect clipping in myriad ways but presumably the OP is using software that already accounts for that.
 
The exact optimal value is completely dependent on the specifics of your system and the constraints you’re working with. But in general 1% is low, and after a few years of degradation you’ll likely see minimal if any. We often design systems with 2-5% clipping in year one.


Sent from my iPhone using Tapatalk
 
this is a residential application. Main service panels are oversized (400A) so no issue with upgrades.

Owner is using a tracker system so basically get perfect tilt/azimuth at all times.

The total STC for the arrays is about a little over 16kW. Can you assume that realistically it will produce only 80% of that in the real world? so around 13kW realistically? This is in Arizona.


I estimate that the owner would save around $20/mo if he goes with the double inverters (+2k more) so I guess it's his choice.


10 years minimum to get your money back lol, solar is a dumb investment in general. He should just invest his 25k in the stock market.
 

electrofelon

Senior Member
The total STC for the arrays is about a little over 16kW. Can you assume that realistically it will produce only 80% of that in the real world? so around 13kW realistically?
Its usually not black and white like that. Ill bet that system will produce almost the full 16kw at times - the question is for how long and how often. You 13KW results in a 1.25 AC/DC which is reasonable. The clipping numbers from the OP, what AC values where those based on?




I estimate that the owner would save around $20/mo if he goes with the double inverters (+2k more) so I guess it's his choice.
Those numbers dont makes sense, although scant details and I am making some assumptions off the OP as to what you mean. What are "double inverters" and what sizes is the comparison based on?


10 years minimum to get your money back lol, solar is a dumb investment in general. He should just invest his 25k in the stock market.
But what happens after the 10 years? Seems like an incomplete analysis. Sometimes solar is a very good investment, sometimes its not. There are too many variables to make a blanket statement like that. Also keep in mind some people do solar for other than investment reasons. Lots of people drive around expensive cars that they dont need and that isnt a good investment.
 

GeorgeB

ElectroHydraulics engineer (retired)
This is not correct. The tax credit is based on expenditure.
You are absolutely correct. Duke Energy had a DC power based rebate when mine were installed and I had my head stuck up somewhere when I replied. Thanks for correcting me.
 

pv_n00b

Senior Member
Once you start clipping then it's a complex financial question as to how much clipping you are ready to allow to meet your project LCOE goals. Basically, as you add more modules past the point where clipping starts you increase your annual energy production but your cost per kWhr goes up since each additional module contributes less to the annual energy production than the one added before it. You can mess around with the financial numbers comparing the LCOE of the energy you are producing with the value of that energy based on the cost from other sources, it gets way beyond me and into the bean counter realm. There's really no rule of thumb for this, it's all in the finances. That's why for smaller systems without a team of bean counters to make that decision most people use a DC/AC ratio of around 1.2 and don't mess around trying to dial in clipping.
 

wwhitney

Senior Member
Basically, as you add more modules past the point where clipping starts you increase your annual energy production but your cost per kWhr goes up since each additional module contributes less to the annual energy production than the one added before it.
Your marginal cost per kWhr produced for adding the first clipped module is more that your marginal cost per kWhr from the unclipped modules. But your average cost per kWhr will still probably go down from adding that first clipped module, as you have more modules to spread the inverter costs over.

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
It depends on whether the clipping you are talking about is 1% of the instantaneous output of the system during max insolation on "perfect" days (no big deal) or 1% of the cumulative output of the system over time (a bigger deal). In either case you need to consider the economic loss to the customer due over the lifetime of the system to the clipping vs the cost of putting in a larger inverter in the case of a fixed array size or the savings from installing a smaller array in the case of a fixed inverter size.
 
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