[RE-wrenches] inverter sizing and microinverters

[Solar Plexus]

Wrenches.

I need some help on inverter sizing.   In Montana we have a lot of  
cold weather with bright sun.  What about  the following

With an array of 50 panels at  235-watts, I would select an  11.4 KW  
string inverter.   The string sizing chart says the 11.4 KW inverter  
would be fine, but it also says a 10 KW inverter would be fine.

Best guess is that my nominal  11,750 watt DC array would make about  
80% conversion to AC with irradiance of 1000 watts at  77°F,  so  
9,400 watts AC.  Either inverter doing fine.

On a clear cool day insolation would still be 1000 watts, temp would  
go to 0°F or -18°C, so my 11,750 watt DC array would be 14,217 watts  
DC  times 80% is  11,374 watts AC.   11.4 KW inverter working fine,  
10 KW inverter ignoring over 10% of array.

We have pretty clear air and some high elevations, and code figures  
possible plus 25% over nominal on  irradiance.  So possible 1200  
watts irradiance and still moderately cold, 0°F, so 11,750 x 1.2 x  
1.21 is 17,061watts DC times 80% is 13, 648 watts AC.  My 11.4 KW  
inverter is ignoring 16% of the array and my 10 KW inverter is  
ignoring 27% of my array.

Now I decide to go to a micro-inverter instead.   50 microinverters  
in this case.  So my maximum AC output is now 50 x 190 or 9,500  
watts.  I could be producing 13,648 watts AC but my micro-inverters  
are limiting  me to 9,500 watts.   Not all the time, and not that  
much that much of the time, but in cold clear climates a fair amount  
of the time.

Am I missing something here?


Thanks


Lee Tavenner
Solar Plexus