For starters a good backup sump pump should not rely on electricity. The same storms that bring the kind of downpour that floods a basemen has also the potential to knock down power lines and cause long lasting power outages.
Every home that runs a sump pump should have a battery operated backup sump pump. Even if the home has a generator, its power is also need to keep the household running, food from spoiling in the fridge, etc...
The best backup sump pump relies on a battery specifically developed for that purpose as opposite to marine or automobile batteries.
Proprietary sump pump batteries, unlike the others, are made to withstand long periods of being unused without loosing power.
A good back up sump pump will also automatically work with the main pump, through a system of switches and alarm.