Metering Grid-tie w/ Batteries [RE-wrenches]

Ray Walters walters at taosnet.com
Tue May 13 20:58:57 PDT 2008


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Hi Fellow Wrenches;

I am taking another look at how to properly meter an Outback grid  
tied system. The Trimetric setups for off grid do not really do much  
in grid tie. KWh meters on the AC grid connection only capture the  
net to the grid
(power generated - power to backup loads), and with several backup  
circuits and a small array, the meter would read zero, or even  
negative. The best I can figure is to put a DC shunt on the inverter.  
Even this would only read PV to grid until the grid went down, then  
it would read battery to backup loads, then when the grid returned it  
would read minus as it recharged the battery from the grid. If we  
could forget about the outages, or assume that backup load energy  
balanced with grid battery charging, I think the shunt on DC side of  
the inverter would be a more accurate number for REC credits. You  
could apply a simple efficiency deduction.  We've also considered  
just using the onboard meter on the charge controller, but this  
doesn't account for the inefficiency of the inverter or the batteries.
For RECs, if you disregarded losses, you could meter right at the  
modules, definitely sell more credits that way.
Any other states paying for RECs yet? PNM gives 13 cents/ kwh on top  
of net metering.

Thanks,

R. Walters
ray at solarray.com
Solar Engineer





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