Plug that draws X watt amount from/with solarpanel

A water heater is the easiest solution to do what you are trying to do. It can easily pull any excess and provides some utility rather than dumping it into waste energy like a resistor. You can look up Dump Load Water Heater or watch this YouTube video which describes the whole process in detail.

Adding a battery bank is also a useful alternative and there is plenty of info online to do whatever you would like.