The power density and variable compliance in pneumatic actuators makes them an attractive option for actuation in human assistive devices. Interaction safety in these devices can be robustly achieved through energetically passive controllers. Efficacy of these controllers depends on appropriate definition of actuator energy function. In previous works, the energy function was defined by assuming the thermodynamic process in the actuator to be either isothermal or adiabatic. In the current paper an estimate of work potential suitable for passivity analysis of a single chambered pneumatic actuator with finite heat transfer is reported. The energy function is developed by maximizing the work done on the actuator to reach an equilibrium position. Optimal conditions show that the maximal solution is attained if the thermodynamic process is a combination of adiabatic and isothermal processes. Through this storage function it is shown that the heat transfer has dissipative affect on the power flow in the pneumatic actuator, irrespective of the chamber air temperature.