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inverter with dual (battery and utility) inputs?

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Posted by danny burstein on July 13, 2015, 5:10 am
Wondering if the following device is off-the-shelf
at some not too absurd cost:

I'm looking for an inverter/power supply that
would have to incoming feeds. The first would
be a connection to a battery bank; the second
to a grid utility (a wall plug...)

Concept: I'd have a solar charged battery
array. As long as the batteries were reasonably
"full", the inverter would draw power from
them and feed it out the outlets.

Once the batteries dropped to (for illustration)
about 30 percent, the inverter would switch
to the grid.

This would allow us to ease into the solar
option, starting off with, say, enough panels
and batteries to provide ten percent of  
our demand. Six months from now we could
pick up another couple of panels and get
to 25 percent. (numbers for illustration).

Any such beast exist?


Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by Vaughn on July 13, 2015, 11:42 am
On 7/13/2015 1:10 AM, danny burstein wrote:

The normal solution is to forget batteries altogether and use grid tie  
inverters.  That way, your panels automatically supply all the power  
they are capable of at any given moment.  If your house needs more than  
that, it comes from the grid.  Should your panels produce excess power,  
it feeds back to your utility to offset part of your power bill.

The prime disadvantage of this setup is that your system is useless as a  
source of emergency power.

We live in hurricane country, so I have taken a different tack.  For my  
little solar system, I don't use inverters at all.  Instead, I have  
created a limited 12-volt system.  It runs my yard lights and a few  
lights in my house that we use every day.  The battery is also the  
starting battery for our emergency generator.  (You can buy some very  
nice CFL or LED 12 volt light bulbs that are made for the marine and RV  

Posted by clare on July 13, 2015, 3:53 pm
 On Mon, 13 Jul 2015 05:10:32 +0000 (UTC), danny burstein

 Operating in reverse would beimple - just a dual conversion UPS - not
aware of an off-the-shelf "battery priority" system.

Posted by Jim Wilkins on July 13, 2015, 9:24 pm
You could run your lower power, intermittent use devices like TVs and  
computers from the inverter with an extension cord and monitor the  
batteries with low-cost voltmeters like these:
(Amazon.com product link shortened)36787598&sr=8-1&keywords.99+volt+meter
They are big and bright and can be read from a distance, though not  
from an angle greater than 45 degrees.

This is the DC equivalent of a Kill-A-Watt power meter:
(Amazon.com product link shortened)
It's not as accurate as it appears to be at low current, and reads  
current flow only from Source to Load, not in reverse.

When the batteries run down just move the plug to the wall. The meters  
will show you when the batteries have recharged. Leave the critical /  
higher-demand appliances like the refrigerator on the grid because you  
don't want it shutting off at 1 AM.


Posted by ads on July 13, 2015, 9:25 pm
 On Mon, 13 Jul 2015 05:10:32 +0000 (UTC), danny burstein

A "smart" battery charger with capacity to run your load and recharge
the battery at the same time would be the cheapest way to get what you
want.  Set it to start charging at whatever point you wish.  

If you're using a lead-acid battery (whether wet or SLA/AGM), be aware
that lead-acid battery life is affected by level of discharge and a
battery repeatedly discharged to 80% will last much longer than one
repeatedly discharged to 50% or less.

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