Inverter sizing

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Eros

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I have read multiple articles on inverter/PV clipping by having a DC-AC ratio up to 125%. If the price/watt of the array is not determined by the inverter cost, what is the benefit of "down sizing" the inverter (ex: 7.6kw vs 10kw inverter on a 8.5kw system). It seems to me that you would want to harvest all the solar possible. What am i missing ?
 
PV arrays don't usually don't produce at their nameplate capacity. This can be due to shading, azimuth, tilt, etc. It doesn't make much sense to pay more for an inverter and have it be overkill instead of just downsizing (depending on the system) and possibly have some days where you're clipping 5-10% during the peak hours. This is especially true on E-W facing arrays where all of the panels won't produce at full capacity at the same time.

If cost isn't an issue than that may be an entirely different issue. For us, cost is an issue so we take it into consideration for each job we install.
 
Eros said:
I have read multiple articles on inverter/PV clipping by having a DC-AC ratio up to 125%. If the price/watt of the array is not determined by the inverter cost, what is the benefit of "down sizing" the inverter (ex: 7.6kw vs 10kw inverter on a 8.5kw system). It seems to me that you would want to harvest all the solar possible. What am i missing ?
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Another reason you might want to do this, is if you are AC constrained for one reason or another. Maybe by amp capacity due to the electrical infrastructure, maybe by an existing ISA with the utility, maybe to avoid costly extra equipment requirements by the utility that suddenly apply once you exceed a big round number, whatever the reason may be.

Packing more kWdc onto an inverter might be in your interest, if you are OK with the inverter curtailing some of the possible power. It could mean more kW-hrs for the same kWac that you cannot increase.

If you are not OK with the inverter clipping power, run a simulation and look for the peak hourly production on a yearly basis. Make sure the inverter can handle that.
 
Ctay005 said:
PV arrays don't usually don't produce at their nameplate capacity. This can be due to shading, azimuth, tilt, etc. It doesn't make much sense to pay more for an inverter and have it be overkill instead of just downsizing (depending on the system) and possibly have some days where you're clipping 5-10% during the peak hours. This is especially true on E-W facing arrays where all of the panels won't produce at full capacity at the same time.

If cost isn't an issue than that may be an entirely different issue. For us, cost is an issue so we take it into consideration for each job we install.
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You often have a maximum AC system size that's dictated by the utility or the existing equipment on site. In this case the only way to increase the D.C. system size is to also increase the ratio. I've often designed up to 140% in these scenarios.

Even when you have the ability to increase the AC size, there is a relatively small yield benefit to going lower than ~120%-125%. And increasing the inverter size does cost more, which will increase the array cost per watt. You also need to increase all AC equipment from the inverter to POI.

Finally, inverters are typically designed to operate most efficiently at ~70%-90% capacity. Too low of a loading ratio may put you at a reduced average efficiency.


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Other considerations...

A smaller inverter may avoid electrical upgrade costs that will never pay themselves back. For example, suppose a residential 100A service could only take a 3800W inverter, but the sold system is 5kW based on the customer energy needs. Upgrading the service will cost $3000 but will likely only result in $1000 more savings from increased energy production over 25 years. The extra kW of modules may only deliver 75% of what the rest do, but still is cost effective at delivering energy at lower than utility cost.

On utility scale systems, the contract arrangements may greatly benefit having the inverter deliver a more or less constant full output for as much of the day as possible. And since adding more modules has gotten cheaper and cheaper, it often makes sense to just add modules to make that happen.
 
Just today I looked at some plans for a ~3 meg system. They are using 100 string inverters (it doesnt really matter that they are string inveters for the the point I am making, but seeing quantity 100 makes it sound more dramatic :)). The DC:AC ratio was 1.2. Do the math on how much more the inverter bill would be for a 1:1 ratio.
 
electrofelon said:
Just today I looked at some plans for a ~3 meg system. They are using 100 string inverters (it doesnt really matter that they are string inveters for the the point I am making, but seeing quantity 100 makes it sound more dramatic :)). The DC:AC ratio was 1.2. Do the math on how much more the inverter bill would be for a 1:1 ratio.
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120% DC to AC is very reasonable for most systems. I almost never design at 1:1.
 
Bringing this back up.
What do you do when you design with a bifacial module like Trina Duomax?
STC is 390W(0~+5) but backside power max gain is 25% putting it at 488 power output. How do you base your inverter selection? Do I even bother with NOTC?
 
Just winging it here, I do residential, not large systems.

Unless I had more concrete data, I'd probably choose a midrange like 15% for the backside power boost.

If you use NOCT in your normal approaches I don't know why you wouldn't still use it. Backside power gain should be proportional to STC.
 
jaggedben said:
Just winging it here, I do residential, not large systems.

Unless I had more concrete data, I'd probably choose a midrange like 15% for the backside power boost.

If you use NOCT in your normal approaches I don't know why you wouldn't still use it. Backside power gain should be proportional to STC.
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Thanks, that is how I approached it.
 
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