Confusion re module rating, microinverter rated output power, and PVWatts.

[wexcellent]

I just ran the numbers again using your 8% loss instead of my 18%. In that scenario with all other parameters identical to above the M250 would produce slightly more power than the M215 (23 kwh annually for my system). However, using the M250 over the M215 would cost me an additional $1200 or so. I don't have much of a grasp about how loss is calculated. Perhaps 8% is idealistic and 18% is pessimistic? PVWatts default is 14% but I guess microinverters mitigate some of that? Anyway, even best case scenario of 8% loss, 23 kwh a year isn't worth the $1200, especially if my loss is actually greater and thus the M215 outperforms the M250 in my installation. Again, please correct me where I am misguided.

[sensij]

Again, please correct me where I am misguided.

Based on what you've written, I think your M215 vs M250 decision making is fine. I'm curious about the comparison of M215 costs vs a string inverter or Solaredge system.

Have you budgeted in the trunk cable? With a max of 17 M215's per circuit, will you be running two circuits for the SSW array, and one for the ESE array, or can you wire the arrays together for just two circuits? Does your AHJ expect to see a separate subpanel to combine those circuits?

[AzRoute66]

[...]

Annual output 6,604 kwh @ 1.35 ratio
Annual output 6,603 kwh @ 1.3 ratiio
Annual output 6,600 Kwh @ 1.25 ratio
Annual output 6595 kwh @1.16 ratio (this would be the ratio with the m250 microinverter paired with 290W panels)

Now I am confused.

How is it that the output is getting LOWER as the DC to AC ratio approaches 1? I thought 1 was the epitome of 'no clipping'.

[sensij]

Now I am confused.

How is it that the output is getting LOWER as the DC to AC ratio approaches 1? I thought 1 was the epitome of 'no clipping'.

I think it is because of the way PVWatts handles inverter efficiency. A higher DC to AC ratio implies a smaller inverter, and a smaller inverter running closer to 100% will be modeled as more efficient than a larger inverter that is not so heavily loaded. The output shown is basically saying there is no difference in clipping, so the inverter efficiency is driving the change in output. The differences are so small that I wouldn't assign much weight to them. As usual, if you want a more sophisticated (inverter specific) model, you have to step up to SAM.

[AzRoute66]

I think it is because of the way PVWatts handles inverter efficiency. [...] As usual, if you want a more sophisticated (inverter specific) model, you have to step up to SAM.

Thanks, as always. I say 'bah' to the inverters for the moment at least. That is one of the things I really don't embrace about SAM. In PVWatts, I can just say give me a 1kW array, in Sam I have to go specify 4 Canadian Solar CS6J-250LMNOPs (or find something else to represent 1 kW). In PVWatts, I can just say give me a decent default inverter, in SAM I have to go pick out the specific inverter(s) from a list of hundreds and just pray they represent something that SAM won't balk on.

Don't get me wrong, I understand and admire the power in being able to simulate real world more closely, but if I am only interested in some of the DC aspects of a situation, the lack of a 'unspecified' panel or 'blatantly average' inverter is a pain. At least SAM will let me proceed with 'No Financial Model', if it didn't I would probably chuck it 90% of the time.

[ButchDeal]

Thanks, as always. I say 'bah' to the inverters for the moment at least. That is one of the things I really don't embrace about SAM. In PVWatts, I can just say give me a 1kW array, in Sam I have to go specify 4 Canadian Solar CS6J-250LMNOPs (or find something else to represent 1 kW). In PVWatts, I can just say give me a decent default inverter, in SAM I have to go pick out the specific inverter(s) from a list of hundreds and just pray they represent something that SAM won't balk on.

Don't get me wrong, I understand and admire the power in being able to simulate real world more closely, but if I am only interested in some of the DC aspects of a situation, the lack of a 'unspecified' panel or 'blatantly average' inverter is a pain. At least SAM will let me proceed with 'No Financial Model', if it didn't I would probably chuck it 90% of the time.

It is simulating real world "more closely" it is actually simulating real world, where pvwatts is only doing a half baked calculation based on rule of thumb.
making advanced calculations and charts based on simple rule of thumb, and not understanding the limitations of such a rule gets people to make silly statements.

For a more advanced real world simulation we use Aurora which does all of the electrical as well as 3D shadow rendering calculations for a true real world simulation.

[NEOH]

Now I am confused.
How is it that the output is getting LOWER as the DC to AC ratio approaches 1?

I don't know?
Because I agree with you.
The OP's numbers do not agree with my PV Watts numbers.

From PV Watts ...

Production Ratio
========== =====
5,093 KWHr 1.1 <<< 4KW PV with 3.6 KW Inverter (Per PV Watts help file)
5,092 KWHr 1.2 <<< 4KW PV with 3.3 KW Inverter
5,073 KWHr 1.3 <<< 4KW PV with 3.1 KW Inverter
5,031 KWHr 1.4 <<< 4KW PV with 2.9 KW Inverter = Smallest inverter

It appears that PV Watts assumes a constant PV DC Size, while the Inverter AC size gets smaller as the DC-to-AC Ratio increases from 1.1 to 1.4.
PV Watts shows me lower and lower production with increased DC-to-AC Ratios, ie smaller and smaller Inverter.
Smaller Inverter = Lower Production = Very logical.

[sensij]

I don't know?
Because I agree with you.
The OP's numbers do not agree with my PV Watts numbers.

You didn't run the model with the same inputs as the op, so you didn't get the same outputs. The conditions you ran are showing clipping. The conditions the OP ran didn't show clipping (high loss factor + low tilt for the latitude), so the difference in inverter loading affected the OP's output. RTFM if you want to see the equations involved, but both your output and the OP's are "correct" for the inputs used.

Edit : if you continue to drop the ratio further (0.9, 0.8, etc), you should eventually see the energy start to drop, as the OP did.

[NEOH]

You didn't run the model with the same inputs as the op, so you didn't get the same outputs. The conditions you ran are showing clipping. The conditions the OP ran didn't show clipping (high loss factor + low tilt for the latitude), so the difference in inverter loading affected the OP's output. RTFM if you want to see the equations involved, but both your output and the OP's are "correct" for the inputs used.

OK,
I re-ran the PV Watts for 20% Loss instead of 10% losses
The change from 10% Losses to 20% Losses does change the output from PV Watts.
So, increasing the "Losses" field essentially lowers the "NET" DC-to-AC ratio used in the calculations vs what we actually entered