I have been looking at the same set of questions you are asking for some time. My sump pump runs on average every half hour (underground springs) so if the power were to go out or pump failure I would have a wet basement.
I have come down to this, a $5.00 alarm if the water reaches the floor. Know its only good if I am home. I then added sump pump failure insurance to my policy. I figured if power went out it would be for a short time, thus no need for back-up. If pump goes out guess i will return to wet basement.
If I really wanted protection I would invest in the generator that will start when and if the power goes off. This would cover all aspects of the house.
