We show that a steady mean-field dynamo in astrophysical rotators leads to
an outflow of relative magnetic helicity and thus magnetic energy available
for particle and wind acceleration in a corona. The connection between ene
rgy and magnetic helicity arises because mean-field generation is linked to
an inverse cascade of magnetic helicity. To maintain a steady state in lar
ge magnetic Reynolds number rotators, there must then be an escape of relat
ive magnetic helicity associated with the mean field, accompanied by an equ
al and opposite contribution from the fluctuating field. From the helicity
flow, a lower limit on the magnetic energy deposited in the corona can be e
stimated. Steady coronal activity including the dissipation of magnetic ene
rgy, and formation of multi-scale helical structures therefore necessarily
accompanies an internal dynamo. This highlights the importance of boundary
conditions which allow this to occur for non-linear astrophysical dynamo si
mulations. Our theoretical estimate of the power delivered by a mean-field
dynamo is consistent with that inferred from observations to be delivered t
o the solar corona, the Galactic corona, and Seyfert 1 AGN coronae.