Hi Everyone,
My name is Dan
I live in Portland, Oregon
I have a basic understanding of electricity.
It is easy to charge a 12 volt battery with house current and a transformer.

Keep in mind this is hypothetical.
(The only thing that can NOT be changed is correct input to return power to the power grid)

My grid service is set up for net metering (Power in/ out )

I have a rechargeable battery unit (size?? ) 1- 20 battery Voltage 12/ 24/ 48??
The storage unit is fully charged / and is disconnect from its input power supply
How can I transfer stored power back to the power grid

Well, I know you can do that with solar panels or wind turbines. If I had to guess, I would say use an inverter unit from one of those setups to convert the DC battery to the AC grid. I don't think it would be that effective though, and if you want to create this setup, I would hire a professional electrician, definitely not do it yourself.

As a final thought, I don't think it would be a very efficient way to turn your meter back.

You need a grid-tie inverter. See Xantrex Technology Inc.
What happens is that the inverter synchonizes it's voltage and frequency output with the incoming power and the current produced will subtract from the incoming current. If the utility disconnects, the inverter must too.