Piezoelectric Project
I'm assisting another engineer on a sensor project. With moderate taps on some round piezo, he observes 2volt PP across the piezo, with a 100Kohm resistor in parallel and with about 20nanoFarad capacitance.

Given the energy stored in a capacitor is

0.5 * C * V^2,

the energy is 0.5 * 20nF * 1^2 

The energy is 10 nanoJoule.