There are various choices for storage elements for backup power systems. In general, these can be batteries which are good for long run time, low to medium power applications, SuperCapacitors which are good for short run time, medium to high power systems, and electrolytic capacitors which are good for very short run time, high temperature capable applications. As to which one will be used, this will depend on system parameters such as backup time required, voltages required, and form factor.