  We analyze the statistical characteristics of the quasi-nonequilibrium
two-dimensional electron-hole plasma in graphene layers (GLs) and graphene
bilayers (GBLs) and evaluate their heat capacity.The GL heat capacity of the
weakly pumped intrinsic or weakly doped GLs normalized by the Boltzmann
constant is equal to $c_{GL} \simeq 6.58$. With varying carrier temperature the
intrinsic GBL carrier heat capacity $c_{GBL}$ changes from $c_{GBL} \simeq
2.37$ at $T \lesssim 300$~K to $c_{GBL} \simeq 6.58$ at elevated temperatures.
These values are markedly differentfrom the heat capacity of classical
two-dimensional carriers with $c = 1$. The obtained results can be useful for
the optimization of different GL- and GBL-based high-speed devices.
