A piezo material like pzt produces voltage upon mechanical stress application. But how much power it can generate or how much current it can generate as a function of piezo characteristics and force applied?
Piezo devices produce hundreds of millivolts or more upon flexure, but the current they produce is far smaller. Think of it this way: the typical output impedance of a piezo transducer is of order ~megaohms. Even if it produces a volt of output, this means the current is of order ~1/million, and the power generated is correspondingly tiny.