96-cell modules worth higher cost to get more power at low irradiance levels?

[brucet9]

In considering proposals for a 4.3kW system using either 96-cell modules rated 69.7 Voc or 60-cell modules rated 38.9 Voc. Cost of 96-cell module is higher, of course, but one of the benefits of the higher cell count, according to the mfr, is that its higher voltage output will get it within MPPT range of the optimizer/micro inverter earlier and later in the day, thus producing more power overall and, importantly, more power late in the day when net metering is credited at peak rates.

Is there some source where I can find an hourly irradiance curve for Southern California?
I assume that cell output is probably linear with respect to irradiance, but is there a minimum irradiance level below which internal impedance overcomes voltage generated?
Will that 19%+ module efficiency rating be lineal across all irradiance levels?

[inetdog]

The conversion efficiency of incoming light will be nearly constant until you get to light levels low enough that the total power is also negligible in the first place.
As for the higher voltage, it will be too high for some optimizers or microinverters, and I would again not expect any improvement in starting time to result in noticeable additional power.
The operating voltage, Vmp, does not depend strongly on incident light. The current, Imp, on the other hand does.

[ButchDeal]

I would suggest that you look at pvoutput and see the curves for near by systems of similar brand etc.

[J.P.M.]

Suit yourself, but spending any more to get questionable but perhaps slightly better performance 3 or 4 hrs. away from solar noon will probably not be cost effective. Horizontal irradiance is lower and what irradiance does get to the cells will be lower yet with reflection losses being a lot greater from increased solar incidence angle despite ARC coatings.

For more electrical production later in the day, most folks orient all or some of the array toward the west by ~ 15 - 30 deg. or so.

If you are considering Sunpower for the 96 cell option, keep in mind that all equal (electrical) size arrays in the same location, orientation and service will produce about equal annual output. Sunpower simply does it in a smaller space and costs ~ 20 % more. The number of cells in a panel has little to do w/ annual system output.

Depending on what you mean by irradiance curve, if you're referring to how much solar energy hits an array, I'd start with NREL. If you mean system performance estimates, see Butch's post.

[brucet9]

ButchDeal, your suggestion to look at pvoutput curves of nearby systems would be perfect. How would you suggest I find local Panasonic HIT installs and nearby high efficiency 60-cell installs whose same-day output data I could compare?
I wouldn't expect the installer who made the proposal for both types of modules to be highly motivated to arrange such a comparison unless his profit on Panasonic is substantially higher.

I suppose it couldn't hurt to ask.

[ButchDeal]

ButchDeal, your suggestion to look at pvoutput curves of nearby systems would be perfect. How would you suggest I find local Panasonic HIT installs and nearby high efficiency 60-cell installs whose same-day output data I could compare?
I wouldn't expect the installer who made the proposal for both types of modules to be highly motivated to arrange such a comparison unless his profit on Panasonic is substantially higher.

I suppose it couldn't hurt to ask.

Create a free account and search for them.

as JPM stated the effect is going to be minimal compared to orientation, and D.C. Optimizers. The only real effect of the 96 cell Panasonic modules is the higher efficiency which only means that you can fit more watts in a smaller space.

[jflorey2]

In considering proposals for a 4.3kW system using either 96-cell modules rated 69.7 Voc or 60-cell modules rated 38.9 Voc. Cost of 96-cell module is higher, of course, but one of the benefits of the higher cell count, according to the mfr, is that its higher voltage output will get it within MPPT range of the optimizer/micro inverter earlier and later in the day, thus producing more power overall and, importantly, more power late in the day when net metering is credited at peak rates.

Sounds like salesmanship. A system with strings of 6 96-cell panels will have an almost identical voltage profile as a system with strings of 8 72-cell panels (same OCV, same MPPT) since they both have 576 cells in series. The 96-cell panels won't "get to MPPT earlier" or anything like that.

Now compare those two systems to a string of 10 60-cell panels. That will have 600 cells in series; it will start producing sooner due to its slightly higher string voltage.

About the only times a 96 cell panel makes sense is if:

1) It's cheaper than the 72 cell panel in $/watt
2) You don't have enough roof space to get a decent voltage with 60 or 72 cell panels
3) You can figure out a string length that gets you (safely) closer to your max system voltage with the 96 cell panels (unlikely.)

[brucet9]

Sounds like salesmanship. A system with strings of 6 96-cell panels will have an almost identical voltage profile as a system with strings of 8 72-cell panels (same OCV, same MPPT) since they both have 576 cells in series. The 96-cell panels won't "get to MPPT earlier" or anything like that.

Now compare those two systems to a string of 10 60-cell panels. That will have 600 cells in series; it will start producing sooner due to its slightly higher string voltage.

About the only times a 96 cell panel makes sense is if:

1) It's cheaper than the 72 cell panel in $/watt
2) You don't have enough roof space to get a decent voltage with 60 or 72 cell panels
3) You can figure out a string length that gets you (safely) closer to your max system voltage with the 96 cell panels (unlikely.)

You are assuming a string inverter. I plan to use optimizers so that shading and soiling of any panel will have no effect on the rest of the string and so individual module monitoring will be possible. Single-module output would then apply for early/late operation.

[brucet9]

Create a free account and search for them.

as JPM stated the effect is going to be minimal compared to orientation, and D.C. Optimizers. The only real effect of the 96 cell Panasonic modules is the higher efficiency which only means that you can fit more watts in a smaller space.

Please excuse my ignorance. Does pvoutput somehow allow me to look at anyone's system data without their authorization?

[ButchDeal]

Please excuse my ignorance. Does pvoutput somehow allow me to look at anyone's system data without their authorization?

By putting your system on pvoutput you are making it public. So yes you can see the data for all supystems on pvoutput. They can hide their consumption if they choose but not generation.

[inetdog]

You are assuming a string inverter. I plan to use optimizers so that shading and soiling of any panel will have no effect on the rest of the string and so individual module monitoring will be possible. Single-module output would then apply for early/late operation.

I think that you are missing the point.
With a single panel connected to a micro or optimizer the only time the panel output will be below the MPPT range will be when that panel is partially shaded and therefore one or more bypass diodes are activated. If that only happens in early morning or late afternoon, then the panel is not producing much in the first place.
An unshaded panel in low light intensity or grazing angle light will still produce at very close to the same Vmp as is does in full perpendicular sun.
In some cases the Vmp will actually be higher in low light because of the lower panel temperature.
There may be some extreme edge cases where a high cell count panel will work better with micro or optimizer than a lower cell count panel, but it is extremely unlikely that you will encounter such.
Any price premium you pay for the high count panels is basically wasted.
If you have to get more power per unit of roof area instead of more power per dollar, then some premium panels may work out for you.

[jflorey2]

You are assuming a string inverter. I plan to use optimizers so that shading and soiling of any panel will have no effect on the rest of the string and so individual module monitoring will be possible. Single-module output would then apply for early/late operation.

Optimizers are used with string inverters. Again, just choose the cheapest ($/watt) panels and design the string to remain within the MPPT range (without going over) as much as possible. You care about the total number of cells - not the number of cells in each panel. The optimizer will ensure that individual panel losses (shading etc) do not significantly reduce the output of other panels.

Another way to do it is with microinverters. These are designed to operate with a specific number of cells in a panel; it is a bad idea to use the wrong # of cells with such inverters.

[ButchDeal]

Optimizers are used with string inverters.

SOME optmizers are used with string inverters.

SolarEdge has an optimized system. The inverter is not a string inverter in the since that it does NOT have MPPT and does not have the ability to string modules and operate without the optimizers.

This thread seems to be focused on the number of cells in the modules. That just one aspect of the panasonic HIT modules, they are high efficiency modules. The main reason to use them is to fit more kW in a smaller space. With SolarEdge the actual cell count matters little (other than using appropriate optimizers for the higher voltages).

[brucet9]

An unshaded panel in low light intensity or grazing angle light will still produce at very close to the same Vmp as is does in full perpendicular sun.

That is the answer I needed. Panasonic's customer support guy made it seem to me that voltage ramps up linearly with intensity of irradiance (as power certainly does), hence the advantage of 96 cells over 60 cells. If, as you say, voltage at low light is nearly the same as at full irradiance, then only amperage ramps up, and his statement is bogus.

[sensij]

That is the answer I needed.

even if one panel "turned on" with 10% less irradiance than another, the difference in energy generated over the year is probably less than the rounding error on your meter. Seriously, look at PVOutput and see the 5 min data that people share. Beginning and end of the day are worthless... You could replace the first and last data points with zeros and the daily total is not meaningfully affected.