The range of values for the energy density of ball lightning has been determined from the observational reports detailed enough to allow evaluation and from experimental measurements made during the laboratory production of a ball lightning-like phenomenon. Ball lightning is shown to be a phenomenon of relatively low energy density below the plasma limit within the range of 2¿10-3 J cm-3 to 2¿102 J cm-3 with a median value of the order of 1 J cm-3. Further analysis indicates ball lightning to be a single form phenomenon in contrast to previous assumptions.