[RE-wrenches] inverter sizing and microinverters

[frenergy]

Lee,

        It possibly sounds like two inverters to me.

You also need to factor in whether or not the PVs will benefit from the 
reflection of that bright sun you're enjoying there from snow.  So,  is it 
ground mount or roof mount, angle of PVs (the shallower the angle, the less 
benfit from snow reflection)?  Though you may have hecka bright sun, you're 
also at 45-48 degree latitude, in the winter, your not much more than 20 
degrees off the horizon about now....that's a lot of atmosphere for those 
photons to penetrate to get to your array.  You may not have the irradiance 
you think.  It's a perfect time to check real world performance, with the 
solstice tomorow.

No doubt your cranking out the power with those cold temps, I know when we 
were down to -10 F here recently, my systems were putting it out. However, 
there's no need to worry about losing a few percent during the "best" 
conditions that may only last for a few hours during a few days of the 
shortest days of the year.  Of course if there is snow ON the PVs, no matter 
if your PV is oversized.

Bill

Feather
River
Solar
Electric



----- Original Message ----- 
From: "Solar Plexus" <solplex at montana.com>
To: <re-wrenches at lists.re-wrenches.org>
Sent: Sunday, December 20, 2009 3:22 PM
Subject: [RE-wrenches] inverter sizing and microinverters


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
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