Hybrid organic-inorganic perovskites show great promise as photovoltaic materials. These compounds belong to a class of crystalline materials that can host molecular species in large interstitial cages and exhibit a wide range of atomic and molecular excitations already at room temperature. The various phases of hybrid perovskite CH3NH3PbI3, for example, are characterized by important molecular rotational/orientational excitations of the methyl ammonia molecules as well as large anharmonic vibrational excitations of the inorganic PbI3 perovskite host. Optimal photovoltaic properties are achieved by alloying on all three sublattices of the ABX3 perovskite crystal, leading to configurational disorder in addition to the molecular and vibrational excitations. A statistical mechanics approach is therefore essential to accurately predict the electronic, thermodynamic and kinetic properties of these materials.