To realize a latent-heat type solid-state heat storage substances, electronic phases and their transition are investigated in the viewpoint of thermal (latent-heat) features. VO2 (vanadium dioxide) exhibits an insulator-metal transition at around 67 degrees Celsius, accompanied by release/absorption of latent heat of about 250 J cm−3. Such a large latent heat, which is comparable to those of organic PCMs, is explained from strong coulomb interaction, or strong (electron) correlation by accounting the enthalpy-entropy exchange relation at the first-order transition, although a liftup influence of primary excitations in the ground state on an internal energy substantially reduces an effective amount of the latent heat. Compared to the influence of “moderate” correlation in the case of magnetic transition in 3d metallic system (La(Fe,Si)13), the latent heat became apparently larger in the strong correlation system, while the controllability by external fields as the “active” heat-storage ability is limited. As an excellent example of the active heat storage function, heat storage features for the NiTi - based material are introduced.