[RE-wrenches] Advice needed on upsizing a battery & grid interactive inverter stack

Doug Pratt dmpratt at sbcglobal.net
Sun Aug 8 09:43:16 PDT 2010


Hi Mick,

 

We're increasingly running into this "how big can you go with battery-based
grid-tie" question. Most of the hardware out there was designed with modest
residential systems in mind.  

 

Outback is limited to 2 inverters max in grid-tie mode. 7.5kW. (I'm really
curious what Darryl is doing with his 12-stack. I suspect it's functional
but not exactly in compliance with UL listings.)

 

With Xantrex we can get to 3 inverters, 18kW.

 

A Sunny Boy / Sunny Island stack can go to 4 Islands, 20kW.

 

Both Satcon and Princeton Inverters make 100kW battery-based, grid-tie
capable inverters.  The Satcon runs a 240vdc battery pack.exciting!  Not
sure about Princeton, but I believe it's the same.  Prices for these large,
more or less custom-built inverters is astronomical.  

 

And dammit, there's nothing in between 20kW and 100kW! (A least that I'm
aware of.) And we get requests in the 30-50kW range pretty regularly now.

 

The best advice I can offer:  First off, it's just crazy to build such large
battery packs for grid-tied systems.  Batteries have become very expensive
over the past few years, and even the best aren't going to last more than
15-20 years.  So build the client a modest 18-20kW battery backup system to
cover the really important stuff. (Security system, critical lighting,
communications, etc.) Put everything else on an auto-start backup generator.
The food isn't going to spoil in the two minutes it takes the backup system
to roll up to speed, and if the big screen goes blank for a couple
minutes.well life is just tough sometimes.  I usually recommend propane as a
backup fuel, it gets delivered and it doesn't spoil or evaporate with age.
But for something this big you'll probably need to go to diesel.

 

Good Luck!

Doug Pratt

DC Power Systems  

 

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mick Abraham
Sent: Friday, August 06, 2010 4:18 PM
To: RE-wrenches
Subject: [RE-wrenches] Advice needed on upsizing a battery & grid
interactive inverter stack

 

Yo, Mechanics~

 

A client is considering a "biggest ever for me" battery based power system.
They will probably scale down their ambitions once the cost realities settle
in but for now they've asked me to look for battery based inverter systems
which can scale bigger than a triple stack of Xantrex XW6048 inverters. Your
advice would be welcomed. 

 

* The system must be split phase 120/240, not 3 phase. 

 

* Outback seems "out" because the inverter group should handle backup
battery recharge using grid or generator AC...and we also want to sell
excess PV power to the grid.

 

* No cobbled together AC coupled systems would be considered without a
convincing way to feather the end of charge on the battery when the grid is
down...float & all. I don't want to go back to the days of clunky, coarse
battery "end of charge" care. 

 

* Sunny Island/Sunny Boy is in contention but I'm not keen on having to
stack in pairs to maintain the 120/240 three wire architecture. A single
Sunny Island failure would take down two of the yellow boxes. What is the
limit on stack-ability for Sunny Island?

 

* A battery sales rep suggested Satcon brand but I think that's bogus
advice. Satcon is pure grid intertie only, right?

 

***********************************************

 

So...there is your challenge de la semaine (challenge of the week). Who
builds an inverter system with Xantrex XW-like features only bigger...much
bigger? OR: who could supply the "Mother of Sunny Island/Sunny Boy" system?
Replies would be welcomed on the  List or off.

 

The Wrench List is the Bomb!

 

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675

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