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Using grid tie inverter with backup system - Page 2

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Posted by Ron Rosenfeld on July 20, 2008, 5:40 pm
On Sun, 20 Jul 2008 09:01:12 -0700 (PDT), Russ in San Diego

Hey, if your T105's are working well for you, that's fine.  And I'd agree
they might be appropriate for light usage during a power outage if
sufficient solar resource will be available.

But so far as the OP's request for backup power during a grid outage, I
don't think either of us can make an intelligent recommendation without
more information.  Some of the things we don't know are power usage, length
of blackout, weather conditions at the time (is the sun likely to be
shining during the blackout), how long a blackout he wants to prepare for,

For example, we have a 2nd home in NH where we spend a good deal of the
winter.  A winter power outage might be due to widespread blizzard and
icing conditions and might last four or five days or longer.  Depending on
how many of our kids are visiting at the time, we might be using as much as
50 kWh/day!  And this would be at a time when the solar resource is
virutally nil.  Battery backup with solar panels is not very practical in
that situation.

Posted by Ed on July 22, 2008, 2:25 am

Try looking at it from a different angle... Grid tied with battery backup

hook up the 24 hours needed circuits to the auto switch over circuitry
(fridge, a few outlets, computer, etc)

As a back up for longer use... hook up the generator to the other circuits
you may need while you are home (early evening stuff... hot plates, hot
water if youdon't have solar thermal, etc with another set of switch over
circuits) Also hook it up to a battery charger that can charge your solar
batteries if you don't get enough sun.
Hooking up a generator directly to grid tied inverter will not be a good
idea for 2 reasons... one: the freq will not be stable enough and worse,
generators don't like to be fed excess power

Posted by Ron Rosenfeld on July 22, 2008, 10:50 am

You probably should be addressing this to the OP.  For our setup in NH,
we'll just use a propane generator for backup.  We don't even have the
necessary exposure to set up for grid-tied PV (or the space/zoning/etc for

In our Maine home, everything runs from PV and/or wind.

Posted by busdweller on July 23, 2008, 10:55 am
 Hi Gang, New kid here

Do a google for a unit called Sunny Island I have'nt installed one yet
but im sure it on one of my future jobs.

Peace along the way
Dennis the bus dweller N.Y.

Posted by jb on August 3, 2008, 12:26 am
recently we installed just what you are thinking of (i think). in the
caribbean power generation goes offline frequently and the businesses
need to stay online. so a battery backup works fine. rule of thumb
outback is good for 4000watts or less and then xantrex does well for
above that. if you need a really big system you can parrallel these
units. at any rate when the power is on the batteries get charged,
then when the power drops off the batteries are routed to a subpanel
that power the critical loads. critical is key, because when u go to
bats you can drain them real quickly. power outages in the states last
say 5 minutes in the islands they last 30mins to hours so the setup we
just did is set up for 3 hours of runtime for computers phones fax and
lights&fans, basically keeps them in business.
and lastly always make sure you set up for a possible future renewable
upgrade, ie wind/solar or a non renewable generator this way if the
power company drops offline for days you just plug in that upgrade.

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