Big honking capacitor bank that cleans up the current on the circuit that it's attached to. Helps to manage power factor inefficiencies, balance transient load over phase.

These are ubiquitous in industrial situations, and they are worthwhile on that scale. Yer not gonna see a whole lot of impact on your bill until you get up into the 10k range, unless there are already problems with your power distro (shit being out of phase, that kinda thing).

Trust me, if these things were viable @ anything like household consumption levels, every spec house built since the Carter administration would have one.