  To understand the physics of solar flares, including the local reorganisation
of the magnetic field and the acceleration of energetic particles, we have
first to estimate the free magnetic energy available for such phenomena, which
can be converted into kinetic and thermal energy. The free magnetic energy is
the excess energy of a magnetic configuration compared to the minimum-energy
state, which is a linear force-free field if the magnetic helicity of the
configuration is conserved. We investigate the values of the free magnetic
energy estimated from either the excess energy in extrapolated fields or the
magnetic virial theorem. For four different active regions, we have
reconstructed the nonlinear force-free field and the linear force-free field
corresponding to the minimum-energy state. The free magnetic energies are then
computed. From the energy budget and the observed magnetic activity in the
active region, we conclude that the free energy above the minimum-energy state
gives a better estimate and more insights into the flare process than the free
energy above the potential field state.
