With the biblical rains and extended power outages experienced this winter it is probably not a good idea to trust a battery backup sump pump system to protect your home from rising water.  Most battery systems will only operate for a few hours before running out of juice.  Two other options may be a better choice. First, a water powered sump pump run by the water, under pressure, supplied from your water company which can be installed in the existing sump pit and can pump out two gallons of rising water for every gallon of "city" water it uses. Second, a generator, and there are two choices here, depending on your budget. Either a whole house, totally automatic, generator powered by propane or natural gas which will provide power for your sump pump and HVAC, refrigeration, lights, Internet....pretty much everything you need to resume a normal life, or a portable generator which you set up outside (NOT in the garage) and configure to run the essentials to protect your basement, keep your food cold and possibly provide heat.  Cost is directly proportional to what things you need to run. It's a good idea to plan ahead and make your purchase before you need it.  Running to Home Depot during a storm may have you coming home empty-handed or with an under-powered unit. 

GOOD LUCK!