As far as I know all grid tie inverters sync to the grid and therefore shut down when the grid goes down. However, the safe-off methodology used in ebay (inexpensive) inverters may not meet NFPA70 requirements. And the cost of certification precludes any chance that these devices have even been submitted for certification.
The last machine I built for a customer required EN-954 class 4 safety controls. Two safety rated contactors had to be wired in series and monitored in such a way that if any of one 6 contacts failed (open or closed) the entire machine lost motive power. The contactors alone were $280 each. A code-compliant grid tie inverter will have similar requirements, though the topology may be implemented in solid state. Either way it is going to be expensive.
Keep in mind that the people selling "NEC/UL/CE certified" inverters on ebay are the same people who sell Gibson Les Paul guitars for $200.
The DHW device I mentioned powers the lower element in the tank and is completely separated from the grid. DHW storage is the closest you can get to batteries without the cost and trouble of batteries. Heat is the lowest form of energy but that is what makes it so easy to store. And most of us use a lot of it.
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