Battery vs no Battery grid tie systems - efficiency [RE-wrenches]

[Doug Pratt]

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If you tell customers that a battery-based system is going to cost them
about 5% (watts into the grid), that's fairly honest. That's also assuming
you're running an FX or a Sunny Island with sealed batteries and carefully
adjusted charging settings. 

The old SW units can easily eat 30% or more of your PV power. The default
factory battery charger settings are *very* aggressive, and will waste a
huge amount of power as heat. You must set them down to a gentle float
voltage, about 2.25v per cell is usually comfortable. (And then add the SW
Memory Kit from Right Hand Engineering for about $100 so the #*&% thing
won't forget all the setting every time someone does battery maintenance!)

Cheers,
Doug Pratt



-----Original Message-----
From: Warren Lauzon [mailto:windsun at wind-sun.com] 
Sent: Monday, April 18, 2005 9:22 AM
To: RE-Wrenches
Subject: Battery vs no Battery grid tie systems - efficiency [RE-wrenches]



Does anyone have any real world information on the overall system efficiency
of a battery based grid tie system, with backup (such as using Xantrex SWP
inverters), vs straight grid tie, such as the SMA inverters?

My calculations show that there "should be" about 25% less total conversion
efficiency from panel to grid with a battery based system, but those are
based on some assumptions that might not be correct, such as how much the
battery efficiency etc affects the overall system, and how the inverter
determines when the batteries are fully charged.

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