[RE-wrenches] Inverters Maximum Input Ratings

[Exeltech]

Marco,
 
The applicability of your statement ...
 
"... what’s beyond dispute is that when we size the solar array higher than the nameplate inverter output, there will be clipping that takes place….i.e., harvestable solar power that goes nowhere."
 
depends to a great extent on the site climate .. as I'm sure you're aware.  Overall, the blanket statement you make in this respect is not correct.


Using Hawaii, and 65F/85F as low and high respectively, Solar World SW270 photovoltaic modules will nominally generate 243 watts at 65F, and 225 watts at 85F under conditions of 1 kW/m^2 irradiance and perfectly orthogonal to the sun.

Microinverters rated 225 watts AC output at ~95% efficiency will input nominally 237 watts DC.  Subsequently, the maximum amount of limiting that could take place under these conditions is approximately 6 watts.  However, that too will not happen, as the 65F condition does not occur during mid-day.

Since 65F occurs early morning, during periods of decreased irradiance, the sun angle will be fairly low, thus the SW270 photovoltaic modules won't be orthogonal to the sun .. nor will there be 1 kW/m^2 irradiance.  Thus the PV module won't be producing 243 watts, but instead will be much less, depending on irradiance and sun angle (among other things).

In your own Hawaiian climate, the SW270 is in fact an almost perfect energy match for the Enphase 225.

Move the scenario to Phoenix, Arizona, and the cold/hot PV power output (using 5C/40F low, and 50C/120F high), the PV power output would be 249W and 202W respectively, again with 1 kW/m^2 and orthogonal to the sun.  The likelihood of 40F during mid-day in Phoenix is exceedingly rare.  Thus you can see that the SW270 module is actually a bit UNDERSIZED for an Enphase 225 in Phoenix.  The amount of energy not harvested due to limiting in the Phoenix environment is essentially zero, even with a 270 watt PV module.

Conversely, the SW270 / Enphase 225 combination in Minneapolis would not be optimal due to the extremely cold winter weather.  Here, power limiting within the inverter WOULD occur when cold, resulting in less than 100% of the available DC energy converted to AC at an ambient temperatures of ~50F or cooler, and under conditions of 1 kW/m^2 irradiance with the sun perfectly orthogonal to the PV module.  This would not be thermally harmful to the microinverter because it would be taking place during extremely cold weather.  Conversely, during summer months, using 35C (95F) as the typical high temperature in Minneapolis, the PV module output would be reduced to 220 watts, all other conditions being equal .. thus LESS than is needed for maximum output by the microinverter.


In summary, in warm-to-hot climates, the SW270 would in fact be a nearly ideal match for the Enphase 225 from an energy perspective.



Regards to all,



Dan Lepinski



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On Tue, 8/26/14, Marco Mangelsdorf <marco at pvthawaii.com> wrote:

Subject: [RE-wrenches] Inverters Maximum Input Ratings
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Tuesday, August 26, 2014, 1:10 AM
 
Look, guys, what’s beyond dispute is that when we size the solar array higher than the nameplate inverter output, there will be clipping that takes place….i.e., harvestable solar power that goes nowhere.  The higher that oversize percentage, the more clipping will take place. 

If the design choice, as noted below, is to use a 270-watt module (from a manufacturer that learned last week that a recall notice was published by the U.S. Consumer Product Safety Commission https://www.cpsc.gov/en/Recalls/2014/SolarWorld-Recalls-Solar-Systems-with-Copper-Grounding-Lugs/ ), does it make sense to use a micro inverter that has a max output of about 225 watts (M215) or 250 watts (M250) when there are micros (e.g., ABB 300) that can handle the full output of the 270 which guarantees NO clipping?

 marco