Xantrex makes two different types of inverters. Grid Tie such as the GT series and Battery Based such as the XW.
Grid Tie inverters are designed to take energy directly from the renewable energy source and convert it directly to AC. The energy is then put directly to the AC loads connected to the service panel that the inverter is connected to. If there is more energy being produced than is being consumed, the energy is "sold" to the grid. In most cases, what you would see would be your kilowatt meter turning backwards. This type of inverter does not use batteries and is not possible to be modified to use batteries. Due to Anti Islanding regulations, Grid Tie inverters cannot operate if the grid is not present and within tolerance as defined by UL1741.
Battery based inverters are DC powered. In order to operate, they must be connected to a battery bank. When connected to renewable energy sources, it is the battery bank that regulates the DC voltage from that source. These inverters will not turn on or operate if not connected to a battery bank. Likewise, it is not possible to modify these inverters to operate without batteries.
If you are wanting to reduce your utility bill and not worry about batteries, but have no need for backup, the Grid Tie inverters are what you are looking for. If you would like the inverter to operate in the event of a utility outage (or entirely off grid), then Battery Based inverters are what you are looking for. Keep in mind that the XW can both sell excess renewable energy to the grid when present and provide backup power in case of an outage.