DC power through a female socket
Never try using a regular AC extension cords with plugs intended for primary line power for anything other than their intended use. Likewise do not ever try using AC mains fixture outlets and plugs for non-mains applications. There is strong reasoning here that rests in the interests of safety to you and others nearby. It also helps protect equipment from getting the wrong voltages applied to them.

Think about the huge safety problem and potential damage that could occur if you start cobbling up your low voltage DC solar panel with AC electrical plugs and then someone decides to take the other end of your cords and plug it into live AC mains. 

There are a plethora of various types of connectors made that would be suitable for your DC extensions. Take a look at a web site such as Mouser.com or DigiKey.com and be overwhelmed you how many connector choices there are for DC applications.