Coal's conventional use causes environmental problems and threatens future energy security. This research presents a photothermal method for coal use, integrating a granular coal photothermal absorber into a solar thermal collector for solar water heater. This method offers a novelty in utilizing heat energy from coal, which is usually burned coal, but this photothermal method converts light into heat. Therefore, the photothermal method will not produce emissions and can be used repeatedly. The solar thermal collector combines a Fresnel lens and a parabolic reflector. Scanning electron microscope and energy dispersive spectroscopy analysis shows the granular coal photothermal absorber’s light-trapping carbon structure, with optimal sizes of 10-20 mesh generating temperatures of 378.15 K and an absorbance of 1.04. Thermogravimetric analysis and differential scanning calorimetry reveal a working temperature range of 336.31–684.31 K, with an optimal light-to-heat conversion temperature of 399.25 K, achieving 86.64% energy efficiency and 21.29% exergy efficiency. Performance tests show optimal solar water heater storage temperatures of 317.88 K and 316.75 K. The water temperature in the storage corresponds to the application of warm water bathing. The solar thermal collector’s maximum energy and exergy efficiencies are 33.06% and 5.51%, respectively, with average efficiencies of 22.90% and 3.44%. The solar water heater system peaks at 27.85% energy and 5.90% exergy efficiency, with average efficiencies of 20% and 3.09%. This approach highlights potential sustainable energy use and reduced environmental impacts of coal utilization.