Solar power depends on the sun. Nighttime is dead time for all methods of generating solar power. Variability of sunlight on cloudy days diminishes the energy that can be gathered by any solar method currently available. So what can be done to provide power to the grid during night or under cloudy conditions? There are several possible approaches to this challenge.
Baseload Supplemental Power
Supplemental power from steady, base-load sources (e.g. natural gas) is the most reliable and, up to now, the most often used method for insuring base load capabilities when solar energy is used as the main source of electrical power. This requires that a supplemental power plant be connected to the grid and always available to provide backup power during periods of low production from the solar power supply.
Power from Renewable Sources
Power from renewable sources (e.g. wind, hydropower, geothermal) can often supply backup power to a solar installation. However, wind, as with solar, is intermittent and needs supplemental power of its own. Hydropower could provide an answer by using excess solar energy generated during the day to pump water into a dam reservoir and using that power during peak load times. Geothermal and other renewable sources are likewise possibilities as supplemental sources, but none are as yet competitive with fossil fuels for that purpose.
Battery storage of electricity could conceivably be a sufficient backup to solar, but battery technology has not yet evolved to the point where they are capable of economically storing the amount of electricity that would be required to maintain a consistent power supply.
Heat storage in the form of molten salts can be used in conjunction with a concentrated solar power installation to provide a more consistent supply of power. The plant must be designed to produce more than enough heat to supply base load power on clear, sunny days. The excess heat is stored by using it to heat and melt salts, which can then provide the heat required to generate steam at night and run turbines to generate electricity.