While solar power availability depends on actual solar irradiance and power consumption is (usually) independent on that, reserve or back-up power capacity has to be to be installed.The related costs depend on the reliability of solar irradiance forecast (depends on region and actual weather conditions) and the ability of the back-up power plants to ramp up (or be throttled) within a short time - this is very depends very much on plant technology: hydro power stations can react within one minute, nuclear and lignite power plants need several hours to react.
A Dynamic Economic Dispatch Model Incorporating Wind Power Based on Chance Constrained Programming. Wushan Cheng and Haifeng Zhang. Energies. ISSN 1996-1073. 8, 233-256; doi:10.3390/en8010233. http://www.mdpi.com/journal/energies.