  We consider stationary driven systems in contact with a thermal equilibrium
bath. There is a constant (Joule) heat dissipated from the steady system to the
environment as long as all parameters are unchanged. As a natural
generalization from equilibrium thermodynamics, the nonequilibrium heat
capacity measures the excess in that dissipated heat when the temperature of
the thermal bath is changed. To improve experimental accessibility we show how
the heat capacity can also be obtained from the response of the instantaneous
heat flux to small periodic temperature variations.
