Energy conversion devices are commonly built from individual subunits in order to increase the force or flow that can be obtained from the device. Examples occur in both engineering and biology and include the cylinders of an internal combustion engine, the plates of a battery, the cross-bridges of muscle, and the active transport complexes in a cell membrane;This work describes the behavior of assemblies of individual energy converting subunits. The linear phenomenological laws of nonequilibrium thermodynamics are used as constitutional equations that describe the relationship between the forces and flows of a subunit. These relationships along with the restrictions imposed because of the organization of the system are used to derive equations relating the overall flows and forces. Two types of systems have been considered where the total input flow is the sum of the individual input flows, and the output flow is either also the sum of the subunit flows or is the same as each subunit flow. Most of the effort has been directed toward describing systems in which the subunits are not all phenomenologically identical and the fractions of subunit types vary. Systems containing two distinct types of subunit have been studied. Several properties are investigated, including limiting operating states and the input flows needed to support these states. An overall coupling coefficient is derived that represents an effectiveness factor for the system. More complex systems are briefly discussed;As an example, muscle contraction has been considered as a system where the output flow is the same for each subunit. Unfortunately, because it is not yet possible to measure the number of active subunits in muscle, applications of the theory is limited to describing properties that do not depend on the number of subunits. These include the maximum contraction velocity, the isometric rate of adenosine triphosphate hydrolysis, and the system coupling. The theory is applied to phosphorylation, calcium binding and isoenzymes variations that have been found to affect the mechanical and chemical properties of muscle.