A simple explosive-driven flux compression device is described which generates magnetic fields up to 4 MG. Results from experiments with these devices as well as from other laboratories are analysed from the point of view of energy transfer. It is possible to arrive at realistic estimates of the peak field that can be achieved in any given flux compression experiment in which radiative energy transfer may be neglected. This involves the efficiency of energy transfer from the explosive to the liner, the diffusion of magnetic flux, and the rapid final deceleration of the inner liner surface due to the formation of a shock wave. Also discussed are the plastic deformation energy, the influence of the initial field, and the effects of convergence thickening and gradual extraction of energy from the liner; these are found to have little influence on the peak field. In particular, the last two effects have a tendency to cancel each other out.