Thermal energy can be stored in a fluid-based thermal storage system for later use. The stored thermal energy may be derived from steam generated using insolation in a steam-based solar power system. The thermal storage system can store energy when insolation is generally available. Alternatively or additionally, the thermal energy may be derived from electricity from the electrical grid. For example, the thermal energy can store energy when the electrical grid has excess electricity available for storage. At a later time, the energy stored in the thermal storage system can be released to heat pressurized water or steam in addition to or in place of steam generated by the insolation. For example, the stored thermal energy may be used in preheating the solar power system during startup, in supplementing steam output of the solar power system, or to replace steam generation during low insolation periods.