The role of the plasma frequency ωp of conductors in their use for various solar energy and energy efficiency tasks, especially in transparent solar control window coatings, is analysed for a range of materials including noble and other metals, transparent compound conductors and the metallic phase of VO2. Ways of adjusting ωp for improved functionality are considered, including novel mesoporous metals and composites that can have an "apparent" or effective plasma frequency. While high ωp is needed for high thermal infra-red (IR) reflectance and strong surface plasmon resonant absorption, it is not the only requirement. The location of inter-band terms relative to ωp and the solar infra-red, effective bandwidth, and a high relaxation frequency can each alter these responses substantially. Two materials with elevated carrier relaxation rates, in one case when intrinsic, and in another due to mesostructure, are used to demonstrate this impact. Solar control and visible performance of a mesoporous gold film is analysed.