Most models of solar water heater are designed to hold the heat inside for a longer time to keep the water hot. In case, if there is a little sunlight / during night time, the heat dissipates eventually and it won’t retain hotness. In winter or monsoons, they won’t stop working due to inadequate sunlight. They provide warm water (not hot water). In some special high-end models, they come with an electric heating option to retain heat.