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An analytical approximation of a nonlinear force-free magnetic field (NLFFF)solution was developed in Paper I, while a numerical code that performs fastforward-fitting of this NLFFF approximation to a line-of-sight magnetogram andcoronal 3D loops has been described and tested in Paper II. Here we calculatethe free magnetic energy $E_{\rm free}=E_{\rm N}-E_{\rm P}$, i.e. thedifference of the magnetic energies between the nonpotential field and thepotential field. A second method to estimate the free energy is obtained fromthe mean misalignment angle change $\Delta\mu=\mu_{\rm P}-\mu_{\rm N}$ betweenthe potential and nonpotential field, which scales as $E_{\rm free}/E_{\rm P}\approx \tan^2{(\Delta\mu)}$. For four active regions observed with STEREO in2007 we find free energies in the range of $q_{\rm free}=(E_{\rm free}/E_{\rmP}) \approx 1%-10%$, with an uncertainty of less than $\pm 2%$ between the twomethods, while the free energies obtained from 11 other NLFFF codes exhibit alarger scatter of order $\approx\pm10%$. We find also a correlation between thefree magnetic energy and the GOES flux of the largest flare that occurredduring the observing period, which can be quantified by an exponentialrelationship, $F_{\rm GOES} \propto \exp{(q_{\rm free}/0.015)}$, implying anexponentiation of the dissipated currents.

A Nonlinear Force-Free Magnetic Field Approximation Suitable for Fast Forward-Fitting to Coronal Loops. III. The Free Energy