Take a look at online UPS systems. They convert incoming power to battery voltage, keep the battery charged, and output via an inverter that's made to run witho

Take a look at online UPS systems. They convert incoming power to battery voltage, keep the battery charged, and output via an inverter that's made to run without being tied to the grid. They shouldn't be that costly if you can find them without large batteries, instead just hooking them up to your own.