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Grid Tie PV / Backup system Current readings

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Posted by Slick on March 15, 2009, 6:06 pm
Hi Guys  ,

I am running a  5KW Sunny Backup System on my home , I also have 2800W of
panels feeding into a SB2500 Grid Tie inverter .

Grid Guard has been removed from SB2500 and connected to input of Sunny
Backup which now handles the grid guard side of the system as per
manufacturers instructions.
Meaning that if grid fails ( does often ) the Sunny backup replaces the Grid
within 20ms , it is supplied by a 48V / 720AH Battery bank, so when Grid
does fail it also disconnects SB2500 from the external Grid Via internal
contactors in the backup system and then can be used to provide extra power
on top of what the backup system can supply or if not needed will charge the
batteries , if load it too low the SB2500 with be sent a freq out of range
that will shut it down until needed again.

Short explanation of wiring , is Grid comes into home meter as per normal ,
then goes straight to Sunny Back Up , ( via breaker of course )
output from Back Up goes back to switch board and is distributed to all
breakers that supply all house circuits.
So all house loads pass through backup inverter which basically remains idle
if the grid is ok , but I can see the load that is being used at all times
in the house.

The PV inverter is connected bacilli to input of the backup inverter via
contactor so it feeds back into grid when internal load is smaller than
power being produced.

All this works perfectly well has been run through it's paces several times
already , What I can not work out is the current readings I am seeing on the
For instance I can have things running in my house that draw exactly 500W ,
The meter on the backup system will confirm this load , but if I connect the
PV system which is producing say 2000W , the current draw on the meter
remains exactly the same , when I would assume it would say -1500W or
similar .
I just cant understand how the backup system can read so accurately the load
being drawn without being even slightly effected by what is being produced.,
Keeping in mind that input and output of backup system are basically tied
together via contactors until the grid fails.

Has anyone got any ideas ?

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