This document compares piezoelectric, electromagnetic, and electrostatic devices. Piezoelectric devices have advantages like high output voltages and capacitances without needing precise gaps, but their materials are expensive. Electromagnetic devices can produce high output currents over long lifetimes but are inefficient at low frequencies and small sizes. Electrostatic devices allow for building low-cost systems and adjusting coupling coefficients, but have low capacitances and require controlling micrometer dimensions. Everyday applications of these principles include cell phones, printers, washing machines, induction cooktops, and fabric softener sheets.