At its simplest, this is a set of modular batteries used to store energy for when you don’t have any. For instance, if you have one charged and ready to go when a storm runs through and kills your power, you will have enough juice to run your lights. If you have two charged, you might have enough to run all of your appliances as well. And so on…

Pretty interesting idea for people that have frequent, but temporary power outages.

Details here.