A significant energy loss results from the poor thermal insulations of the commercial and public buildings. Windows diffuse a large fraction of building heating and cooling energy to the external environment, representing an annual impact of 4.1 quadrillion British thermal unit of primary energy in the US. The current technology for efficient windows relies upon the double-pane insulated glass unit with an insulating gas in between. A key challenge is to reduce thermal conductivity of the windows without relying on insulating materials. The photothermal effect can be possibly utilized for particular functionalities that can collect solar energy for reducing heat loss. The insulation efficiency is quantified through the U-factor, defined as the ratio of the heat flux (H) per unit area through the pane to the difference (Delta T) between the window interior surface and exterior temperatures. Upon solar irradiation, single-panes can "self-heat" via the photothermal effect from the nanoparticle coatings. This can effectively reduce Delta T for enhanced thermal insulation. In this study, the photothermal effect on Fe3O4 nanoparticles stimulated by solar light was investigated for nanoparticles in solutions and as thin films for energy-efficient windows. The Fe3O4 nanoparticles were surface-functionalized with different polymers to modulate colloidal stability and for the investigation of the photothermal effect. The photothermal heating efficiencies of Fe3O4 with different surface coatings were found to be much greater under the white-light irradiation than near infrared (NIR) in both aqueous suspension and as thin films. The mechanism for the photothermal effect of Fe3O4 was identified in terms of its band structure. Both Urbach energy and band gap were obtained based on absorption spectra of various Fe3O4 nanoparticles. The Urbach "tail" was found consistent with nanoparticle surface defect structures, while the band gap (similar to 3.1 eV) corresponded to the electronic transitions in the octahedral site of Fe3O4. We also discuss the absorption based photonic physics responsible for the much-enhanced photothermal heating by white-light as compared with NIR. Based on the photothermal heating, the U-factors were obtained with the nanoparticle coatings that show promise in producing energy efficient windows.