[RE-wrenches] DC to AC derate

[Maverick Brown [Maverick Solar]]

Please use the 90% of the CEC or PTC rating of the PV module at most.
 
Among several systems that I monitor daily, I have an example 11.760kw (STC)
system that has a 500Ah battery bank and it peaks at 77-80% of the STC
rating each day.
 
Peak is defined as the top of  the bell curve that forms from the graphing
watts output versus nameplate watts.
 
If I use the CEC rating (10.54kw), it peaks at 86-90% of the CEC rating each
day.
 
If I go to the CEC list and copy & paste into a spreadsheet and enter the
values for the STC versus PTC, I might  find I would loose 10% (or more) in
the PV rating.
 
http://www.gosolarcalifornia.org/equipment/pv_modules.php (go ahead, find
your modules and divide PTC by Nameplate).
 
I also have a grid-interactive system that has a very large battery bank
(2600Ah @ 48V, with 9kw STC Array) and it only has a peak efficiency rating
of 69% because of the load of the batteries. The larger the battery bank,
the happier customers are for outage situations, but the larger the battery
bank, the more current goes into "Floating" the batteries during selling.
For that system, I may lower "Grid Support/SellRE" and use Auto EQ to keep
the batteries fresh...
 
Anyway, keep in mind that you cannot simply assume:
 
12kw Array x 6 hours of sun =  72kwh of generation each day. The brand of
PV, the size of the battery bank and even the charge controller's Absorb &
Float settings can greatly reduce what the inverter thinks it can sell. With
multiple inverters and charge controllers it gets even more complicated
(worse).
 
Good luck,
 
Maverick
 
 

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of
boB at midnitesolar.com
Sent: Wednesday, January 04, 2012 5:14 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC to AC derate


On 1/4/2012 3:01 PM, All Solar, Inc. wrote: 

Wrenches,
Once again I am trying to size a grid tie w/battery system and I struggle a
little with the efficiencies of them.
With PV Watts (v.1), is changing the DC to AC derate an accurate method for
getting the output?
 
Thanks in advance!
 
Jeremy



If you keep the batteries charged  to a voltage just above their resting
voltage,
say, 52 volts for a 48V system, and the battery bank isn't too large, (~225
A-hours,
maybe a bit more), about 90%  PV to grid AC efficiency is in the ball park.

That's what we figured in the early days of GFX inverters at OB.  That's
giving
5% for the electronics and 5% for keeping up the batteries.
Different and/or more accurate information may have come along since
that time.

boB










  
All Solar Inc.
Jeremy and Amy Rodriguez
1463 M St
Penrose, CO 81240
www.asolarelectric.com
Phone 719-372-3808
Fax 719-372-3804
Email allsolar at scswifi.net
Email allsolar at live.com

 

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