Piezo Loading

I have a 3D piezo model and the output is currently a floating potential. This tells me the theoretical unloaded voltage gain. I'm working on possibly using a global equation of a simple ohms law relation between voltage resistance and power. Since the resistive load can be purely numerical. But I haven't been able to figure out how to make this work. In the actual simulation. Since the voltage output is a boundary condition and the relationship need to be global.

Any feedback would be greatly appreciated, or any links to pages with similar information.