DC-DC Step Down converter for Solar Panel

I am planning to power a 5V microcontroller through a solar panel. Solar panel's Vmax = 17.78 VDC and V(open circuit) = 21 VDC.

While selecting the appropriate buck converter, should it be rated at max input 18 V or 21 V? basically, I would like to know how does open circuit voltage affect my power circuit design.

• It should be rated at even higher than 21V. For example, an easy-to-find converter having a $V_{in}$ range of 9-36V would do the job. – Rohat Kılıç May 9 '17 at 5:57