You can size your solar field to collect much more over the day than the tanks can store. So if your field were 100MW and tanks were 200MWh, it would only take 2 hours to fill them (assuming no electricity production). But after those two hours, you better have turbine capacity to take up the excess heat. Baseload CSP designs usually have storage in the range of 6-10 hours (real storage hours vary based on the solar input).
