Hello Arrowspace.
It entirely depends on the device you are implementing the VHDL code in.
For example if it's a xilinx part you can use xilinx power estimator or if it's a newer part you can use Vivado power analysiser tool.
All you need to do is provide a synthesised netlist, ambient temperaturs, toggle rates of io pins etc etc.
In vivado you can add this information via the post implementation ".xdc"
Basically the tool will see how many lut/mux' or whatever are required. It will then calculate the power consumed with the components based on a specific duty cycle(toggle rate).