A power capacitor (7) is described for use in an RF power delivery system. The power capacitor comprises at least two RF electrodes (18, 19) separated by a capacitor dielectric (17) comprising a solid paraelectric dielectric material whose relative permittivity is controllable by varying a DC bias voltage applied across the dielectric (17) at DC bias electrodes (10, 26, 28). Composite capacitor configurations, an RF power system and a method of controlling the power capacitor are also described.