Man do I know why you need that. I spent 8 hours one night bailing water from a sump basket in Kansas after the power went out during a storm. I suggest the following options.
1. buy a gas powered generator. When the power goes out you can start this and plug it in. This is what I did. The generator can come in handy other times also. You will need about 15 amps max.
2. purchase a new sump pump with a battery back up.
3. Make your own battery backup system. You need a battery, a charger and a inverter. You need to convert the AC signal to DC.
Remember batteries will not last very long when pumping large quantities of water.