Hey Jcd, Ecko has some good advice here, but I think you both might be missing the central point here. Using a straight Grid Tie inverter to take battery power and feed it into the grid is like trying to tow a car with a boat. Even if you have water nearby the road, the equipment was never designed to do what you are planning, and the chance that something will get damaged in the end goes up exponentially. A grid tie inverter is not programmable, and that is really the achilles heal to your plan. It will come on and drain the battery power at whatever maximum wattage the inverter is designed to operate at until the battery is drained far enough for its voltage to fall below the inverters minumum operating voltage, and then it will quit.
There are three basic kinds of inverters in the world. The stand alone, which hooks to a battery and creates AC power to run its own loads. The intertie, which takes DC power from a continuous source, such as a solar array, and creates AC power that is synchronized with another power source, such as the grid, or a generator. And the third and least common, the Utility Interactive Inverter. This type of inverter is basically a hybrid. It can work as a stand alone, or can be connected to the grid to parallel power out, but it has to be programmed with commands like battery float voltage, periodic equalization specs, and so on. This type inverter is designed to parallel with the grid, but must have a battery connected to operate at all. A good example of this is the one we've been using for 12 years now to power our home and tie with the grid, a Schneider Electric (formerly Xantrex) model SW-4024. How it works is you have your battery bank which is charged by whatever source you have, such as a wind turbine, solar array, micro hydro turbine and so on. The inverter takes whatever power from the battery it needs to in order to keep the battery voltage at the "set" point. In our case, 26.5 VDC. It converts this power to AC to match the grid and feeds the power out at whatever rate it needs to to keep the battery at the programmed set voltage. As the solar array, or turbine add power to the battery, the voltage tries to climb, and the inverter pulls more out of the battery, seeking to bring it back down to set point voltage.
Only these "interactive" type inverters can accomplish these things correctly and safely. In your setup, it will always be impossible to have the inverter take only the power it should from the battery bank to sell back to the grid, because it doesn't know any better. This is akin to trying to heat your home with a very large furnace that has no thermostat, and you are trying to tell it how many hours to operate each day to keep the house at the desired temperature. This is why we have utility interactive inverters, and thermostats, they keep the batteries from dieing, and houses from overheating. It might be possible to use a voltage sensative switch to monitor the battery state of charge, and tell the inverter when to come on and off, but you'll also need some way to keep the inverter from draining the battery power too quickly, since a battery will deliver whatever it can when any load is connected. When a solar array is connected to an intertie inverter, the solar array decides what amperage it can deliver, and the inverter takes what the array can deliver. A battery bank can deliver whatever it takes to melt the cables, and an intertie inverter does not know how to say "no." Even if you were successful at controlling the intertie inverter with a voltage sensative switch, you would be turning the inverter completely on and off hundreds of times a day, and by the end of the month, you would be buying a new inverter. Use a truck to tow that broken down car Jcd, and a thernostat to keep the house warm. And while you're at it, get an interactive inverter to connect your batteries to the grid. Good luck with the project by the way, and take care, Rudydoo.
Xantrex/Schneider Electric Systems
Home Power Magazine, homepower.com
Solar Energy International, solarenergy.org
Midwest Renewable Energy Association MREA.ORG