The ultracapacitor is able to deliver frequent pulses of energy without any detrimental effects. Many batteries experience reduced life if exposed to frequent high power pulses. An ultracapacitor can be charged extremely quickly. Many batteries are damaged by fast charging. An ultracapacitor can be cycled hundreds of thousands of times.
Batteries are capable of only a few hundred cycles.
Sizing is based on the system's allowable voltage change for the required power and time of discharge. An ultracapacitor's output voltage drop has two components. One component is due to the ultracapacitor discharging its energy. The second component is due to the resistance of the ultracapacitor. Depending on the time-scale, one component may dominate the other.