Utility power in the United States is very reliable. But what is the quality of that power? It is not a digital product; it is an analog product, with continuous variations in voltage and phase, depending on load profiles and myriad variables across the transmission and distribution network. Voltage can sag to sometimes as much as half the nominal line voltage for brief periods. The utilities do an admirable job controlling what they can, but with the hundreds of thousands of local loads affecting power quality, local consumers with critical loads should consider what steps they can take on their own to harden their systems against poor power quality.

Ultracapacitors (UCs) are ideally suited as an energy storage solution for hardening sensitive equipment against voltage sag. They have extremely high energy density for capacitors, they can deliver very high power instantaneously, and they can sit at operating voltage for many years without any conditioning (unlike batteries).