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Using grid tie inverter with backup system

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Posted by Norman Webb on July 16, 2008, 4:13 am
I intend installing a 240v AC  2500W grid tie inverter on a new house. The
inverter needs a supply voltage in order to operate so when the grid power
goes down you are left with all that power on the roof with no where to go.

If you have a back up power supply (either petrol generator or
solar/inverter) will the grid tie sync to these type of power supplies.

The main power grid can soak up any power generated by your grid tie system
but what about a small generator??

My electronics engineering 101 has long since left me, but my guess is that
the two systems should  sync.

Any thoughts out there?

Norman Webb

Posted by wmbjkREMOVE on July 16, 2008, 2:08 pm
On Wed, 16 Jul 2008 12:13:36 +0800, "Norman Webb"




Posted by Russ in San Diego on July 17, 2008, 4:43 pm
I don't think this plan will work out very well. It's possible that
some pure non-battery grid-tie inverters might eventually synch to a
generator (assuming a VERY stable generator), but this might not do
good things for that generator, particularly if your PV system is
producing more power than your loads are consuming.  The inverter will
try to push the excess out to "the grid", which is just your
generator.  I'm not sure, but I suspect the generator might not like
having its "load" provide a higher voltage than it is trying to

There are grid-interactive inverter/chargers out there, from Xantrex
and others, that can do what you want -- but you will need batteries
-- and probably not a whole lot of batteries, either.  I've got a
Xantrex SW-4048 running to a bank of 24 T-105 batteries, but you could
cut that down to 8 (or fewer, if you use higher voltage batteries, or
a lower DC voltage inverter).  The SW will happily push excess to the
grid, or it will just charge the batteries if the grid is down.  It
can even automatically fire up a backup generator (if you've got one)
when necessary.

Posted by Ron Rosenfeld on July 17, 2008, 7:25 pm
 On Thu, 17 Jul 2008 09:43:42 -0700 (PDT), Russ in San Diego

or same voltage but higher capacity batteries.

In your system, you could use something like 8 Surrette 6CS21P's and have
similar capacity, better tolerance to deep discharges, and only have to
maintain a single string instead of three strings.

Of course, the Surrette's will cost you over $,000 whereas you can
probably get 24 of the T105's for less than 1/2 that (or less if you are
getting them surplus, used).  But at some point in your life, the decreased
workload of the Surrette's (fewer cells to maintain, better longevity) may
make the extra cost worthwhile.

Posted by Russ in San Diego on July 20, 2008, 4:01 pm
The OP's question was about being able to continue operating his grid-
tie inverter when the grid is down.

I was trying to provide a minimal cost solution for him. Thus, the
suggestion about using fewer batteries -- all he needs is a battery
bank that can provide /some/ capacity at the requisite voltage.  It
would only give him a brief period of backup power, but would be
adequate to keep operating without the grid, while the sun is

As for me, my T-105 battery bank has worked very nicely over the last
7 years.  I rarely dip into it.  It was sized to allow me disconnect
from the grid long-term, if necessary -- but I've remained connected
the whole time.  The batteries only provide me with power during the
occasional grid disruption at night -- or allow my PV system to
continue operating during daytime disruptions.

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