Clipping

[Ampster]

I have seen differences of 2 to 10 percent in systems with high DC to AC ratios compared to systems with lower DC to AC ratios. In fact the difference of a generic 1kW system with a DC ratio of 1.2 will produce 1517 kWh annually versus a similar system at DC ratio of 1.5 will produce 1483 kWhs annually, That is 34 kWhs per year and might not justify the additional cost of a larger inverter.

I am clearly not advocating for a system that has a difference of 25 percent. As I mentioned earlier, the important numbers are comparing production. Clipping losses can be compensated by more production so clipping loss is only part of the equation to measure an optimum system. Weather and temperature can affect performance. A system that might be clipping in March when the panels are lower temperature might not clip in the heat of summer. As far as I know inverter output does not degrade over time so inverter life is not any different in a high ratio than it would be in a low ratio.

 

[FilterGuy]

A couple of points.

1) Panels almost never produce at ther STC spec. If you over panel the controller by 15%-20% you are unlikely to get clipping.

2) Yes, having larger capacity SCCs and more SCCs will give more *potential* production but there's other reasons to over panel. The best example is winter vs summer, particularly in higher latitudes. If you're system is sized for winter it might clip in the summer but if you don't need the extra power, who cares?

 

[Ampster]

If you're system is sized for winter it might clip in the summer but if you don't need the extra power, who cares?

Yes, it all depends on where you are standing.

 

[OPTIMALSOLAR]

Panels almost never produce at ther STC spec. If you over panel the controller by 15%-20% you are unlikely to get clipping......................That is not true in Texas. Be surprised if so in Cali. I'm at or near 100% every sunny day with Canadian solar panels for 2 hours and over 85% for 4.5.. Not exactly the high end of panels. While my clipped Qcell 400's are 295 watts the same timeframe.

 

[Ampster]

That is not true in Texas.

Here is an example for zip 75002:
The difference of a generic 1kW system with a DC ratio of 1.2 will produce 1517 kWh annually versus a similar system at DC ratio of 1.5 will produce 1483 kWhs annually, according to PV Watts. That is 34 kWhs per year per kW of system size and might not justify the additional cost of a larger inverter to get that DC ratio much lower than 1.5 to 1. Sure there will be clipping at Noon, but there will also be production increases at other times of the day.

Interestlngly a system with 1 to 1 ratio will actually have slightly less production than a 1.2 to 1 ratio. I cannot say that a 1.2 to 1 ratio will not have clipping but it will have a better annual production that a system with a 1 to 1 ratio.

 

[OPTIMALSOLAR]

So according to the above a 1kw system, identical panels/quantity with no clipping underperforms a 1 kw system with dc to ac ratios of 1.2. That is what the above says?

 

[Ampster]

So according to the above a 1kw system, identical panels/quantity with no clipping underperforms a 1 kw system with dc to ac ratios of 1.2 to 1.5. That is what the above says?

What I have been saying is that just looking at clipping is an incomplete analysis. The PV Watts models does not provide a quantity for clipping, it is an annual production model. What my example is saying is that a system with a 1.2 to 1 ratio will outperform a system with a 1 to 1 ratio. The annual production falls off as you go beyond 1.2 to 1 but the question for an owner or designer is what is the tradeoff of the cost of a larger inverter versus the value of that annual production increase.
As @FilterGuy pointed out, a system may need a higher AC to DC ratio if one needs more panel capacity in winter to compensate for lower insolation values. Yes, in that case lots of power would just be thrown away in the form of clipping.

 

[Sparky7]

I do see several people refer to an AIO as an inverter in discussions.

Yes, size is the factor here.

Clipping is only because the charger is too small, power wise, for the input source's capacity.

Of course the power will be reduced when the battery is full or the system cannot be allowed to sell more power than it is capable of, but that is a different story than clipping or power limiting.

boB

Ok
I am not grid tied so selling isn’t an issue.
Thank you

 

[SpongeboB Sinewave]

Ok
I am not grid tied so selling isn’t an issue.
Thank you

All the better reason not to over-panel. Better to use the energy yourself

boB

 

[Ampster]

All the better reason not to over-panel. Better to use the energy yourself

On the other hand as you said at the beginning of this thread, if you are off grid and size your DC system for winter you will be over-panelled in the summer.

 

[SpongeboB Sinewave]

On the other hand as you said at the beginning of this thread, if you are off grid and size your DC system for winter you will be over-panelled in the summer.

Yes and that is to be expected.

Personally, I would get a second controller which will also mean higher reliability overall and less wasting of the PV.

boB

 

[Sparky7]

Yes and that is to be expected.

Personally, I would get a second controller which will also mean higher reliability overall and less wasting of the PV.

boB

Sorry, I have a lot to learn about solar. How will it benefit me to have another cc? I have two midnight classics 150.

My panels are 150 feet from the cc. Would I divide the panels up evenly between three cc? 3 separate wire runs?

 

[SpongeboB Sinewave]

Sorry, I have a lot to learn about solar. How will it benefit me to have another cc? I have two midnight classics 150.

My panels are 150 feet from the cc. Would I divide the panels up evenly between three cc? 3 separate wire runs?

Depends on how much one or both of those 2 Classics are being over-powered and limiting their output.

How often do you see current limit come up or that current limit LED on the MNGP display remote ?
If not very much, then you are most likely fine with what you have.

If they are limiting quite often and you could use the extra power/energy to keep your batteries charged, then a third controller might be a good idea.

Each controller will need at least separate positive wires for that 150 feet of course. BUT if necessary, you might be able to share negative PV wires with the two arrays. Depends also on the wire size and how much loss you are getting for those 150 foot runs of wire.

Separate wire for each will reduce total losses a bit because you are adding more copper to that 150 foot run so less resistance.

Always a compromise somewhere !

boB

 

[FilterGuy]

Each controller will need at least separate positive wires for that 150 feet of course. BUT if necessary, you might be able to share negative PV wires with the two arrays. Depends also on the wire size and how much loss you are getting for those 150 foot runs of wire.

That would be an unusual way to wire it. I don't think I would do that. don't know if that would meet code or not. Regardless,

1) the common negative would need to be sized for the combined current of both arrays
2) the solar disconnect would need to break all three lines at once.
3) the classic has both PV Ground fault protection and PVarc fault protection. I am not sure how/if that would work with a common negative
4) All the normal rules for PV wire sizing and fuse sizing would need to be rethought.

 

[Zwy]

A couple of points.

1) Panels almost never produce at ther STC spec. If you over panel the controller by 15%-20% you are unlikely to get clipping.

I have clipping daily. 8S 530W half cell strings. That's 4.24Kw and the EG4 6500EX will limit the input to 4.0Kw. Cloud edge effect you will see over 4.0Kw for a few seconds until the SCC adjusts.

That is 6% overpaneled. The AIO fans really ramp up. Vmp is around 360V, VOC in winter at -30F would still be under 450V at just under 430V. I see clipping at least 3 hours per day with good sun this time of year.

I went with 8S to get voltage up, array is 420 feet one way from SCC's plus about 10 feet inside the house. I'm actually surprised to see any clipping with some VD and power loss due to the distance.

2) Yes, having larger capacity SCCs and more SCCs will give more *potential* production but there's other reasons to over panel. The best example is winter vs summer, particularly in higher latitudes. If you're system is sized for winter it might clip in the summer but if you don't need the extra power, who cares?

 

[Ampster]

I'm actually surprised to see any clipping with some VD and power loss due to the distance.

Probably temperature related. How cold is it there when you see clipping?

 

[Sparky7]

Depends on how much one or both of those 2 Classics are being over-powered and limiting their output.

How often do you see current limit come up or that current limit LED on the MNGP display remote ?
If not very much, then you are most likely fine with what you have.

If they are limiting quite often and you could use the extra power/energy to keep your batteries charged, then a third controller might be a good idea.

Each controller will need at least separate positive wires for that 150 feet of course. BUT if necessary, you might be able to share negative PV wires with the two arrays. Depends also on the wire size and how much loss you are getting for those 150 foot runs of wire.

Separate wire for each will reduce total losses a bit because you are adding more copper to that 150 foot run so less resistance.

Always a compromise somewhere !

boB

Thank you!

I’m guessing MNGP stands for midnight display thing??

I’ve never noticed Current limit. From what I understand is the cc are maxed out but not over the max.

I am transitioning to 6 48v EG4 batteries soon but I currently have 8 6 v AGM 415 ah batteries. On a sunny day they would normally be charged after 4 hrs. Or so. One of my cc goes into resting mode and the other stays in float for a while (they read the v by .3 difference).

When I hook up the EG4s it might take longer to charge (not sure).

I have 8910 w of solar panels all facing south left in the winter position.

 

[Zwy]

Probably temperature related. How cold is it there when you see clipping?

Temps here this time of year range from about 30F to 50F.