Grid-Tie Inverters with Batteries [RE-wrenches]

[John Berdner]

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William:
 
You mentioned extra PV modules as a cost to make up for lower inverter
efficiency...
John Hill at Sierra Solar here in Grass Valley has matched arrays each
with 24 KC 120's.
One is connected to a Sunny Boy and the other is connected to a 48 Vdc
Outback Grid Tied system.
The battery back for the Outback consists of 4 Concord AGM batteries.
(~ 2.4 kWhrs of usable capacity at 50% DOD).
The Concord batteries are about as good as you can get in terms of self
discharge and this is a very small battery.
If these were T105's, L16's, or any other Antimony battery the losses
for self discharge could be many times higher.
The systems are data logged and Chistopher F. told me the net power
production for the Outback system is about 95% of that produced by the
Sunny Boy system.
One way of looking at would be to equate it to a grid tied inverter
that is only 89% efficient, i.e. 95% of 94%.
I don't think an 89% efficient inverter would be competitive in todays
grid tied market. 
 

The performance of the Outback system is certainly MUCH better than the
older grid tied SW based systems where the net number is more like 50%.
Even at that I still think it would is difficult for the average
customer to justify the extra losses / costs / maintenance for 99 plus %
of the time when the grid operates normally.
Of course there are places where the grid is much less reliable - like
out in the sticks where I live.
At my house the grid goes down several times every Winter. 
Last Winter it was down for 6 days then up for 2 then down again for
3.
9 days per year of outages translates into a grid reliability of 97.5%
which is WAY WAY below the service reliability of most utilities.
Backup power is nice to have when the power goes out but 4 small
Concord batteries are too small to be really useful for longer outages.
Of course you can add more batteries but then the self discharge will
go up making things worse for the other 97.5% of the time when the grid
operates without a hitch.
 
Best REgards,
 
John Berdner
 
-----Original Message-----
From: William Korthof [mailto:wkorthof at earthlink.net] 
Sent: Tuesday, August 17, 2004 10:50 PM
To: RE-wrenches at topica.com 
Subject: RE: Grid-Tie Inverters with Batteries [RE-wrenches]

Chris

>I don't think that the additional cost of the batteries, installation
>and additional switchgear HAS to end up being $4000 to $8000 - My
>figuring works out to be more in the $2000 range, assuming a
relatively
>small battery and only a few backed-up circuits in the house. We need
>to separate out providing some back-up power (for necessities like
>lights, tv and cold beer) from trying to powering the entire house.

The extra cost includes many components. For a typical 3-6 kW system:
-on a "maxed-out" system, the charge control+battery inverter cost is
close to break-even versus a PV direct inverter (thanks Outback!!)
-batteries add $500-$2000
-battery enclosure adds ~$200++
-heavier cable/wire runs to the roof/yard add ~$300
-extra panels (to overcome the higher inverter loss) $1000
-ancillary circuit breakers, disconnects, boxes, cable..... (~$1300)
-design time is *at least* twice as much (for us)
-ordering, stocking, and equipment handling is far more complicated
-installation & training time is at least 50-100% greater.... if
normal
installation costs are $1/watt for basic grid-pv, 75% extra labor on
a 4 kW system would represent an extra $3,000.
-the likelihood of inspector problems is much higher
-the warranty liability and service requirements (5 or 10 years) are
far greater for battery systems ( x 3 ? )
-the need for more interface/customer orientation on system operation
That's an extra equipment cost of about $3,000 alone. The labor and
other "soft" costs can easily add another $3,000. So we're up to $6k.

For a bigger system, or one with more inverter capacity than PV, the
premium can easily be greater---or a larger part of total system cost.
And my hunch is that to fairly count our own labor and O/H costs,
the installation cost premium would be even more than 100%.
***snip***





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