An analysis of the energy balance in the Coaxial Slow Source Upgrade (CSSU) device is reported. The CSSU consists of two concentric coils carrying pulsed azimuthal currents only, which form an elongated plasma (an 'annular field reversed configuration (FRC)') in the space between the coils. The plasma contains no toroidal field, and is confined by poloidal fields only, resulting in a very high average beta. The CSSU, which operates at loop voltages of 2 kV or less and with risetimes of the order of 70 mu s, was developed to provide a low voltage, slow formation alternative to conventional FRC generation techniques that are based on fast theta pinch technology. It is found that the CSSU device does form annular FRCs, which persist for the duration of the inductive current drive, apparently free of MHD instability. Temperatures are low, however, and the transport is correspondingly poor. To analyse the energy balance, the power input to the plasma is calculated directly from external and internal magnetic field measurements. No assumptions about the resistivity profile have been made. A triple Langmuir probe located at the device end region was used to calculate the energy lost due to escaping particles. Electron temperature measurements from Thomson scattering and impurity estimates from doping studies are used in a time dependent corona model calculation to show that the CSSU plasma is impurity line radiation dominated. Time dependent coronal calculation simply that, with operation over much longer formation times (>100 to 200 mu s) at lower density (1014 cm-3), it may be possible to burn through the carbon and oxygen impurity radiation barriers and attain plasma conditions closer to those produced in conventional FRCs