Hello, in 33 or in 31 mode ?

33 mode; the equation I have been using is We=V/2*k33*s33*P^2

where: V=volume; P=pressure

I suggest you write down the piezoelectric constitutive equations and find the voltage/power output per input pressure. For the reference, you can look up this article:

http://www.sciencedirect.com/science/article/pii/S0924424714001952?via%3Dihub

There, you can find the equations. You will see that the energy equation can be simply obtained from the power output expression. 

Feel free to contact me if you had further questions,

Best,

Amir

The equations are:

for pressure out (as a transmitter) P=e*eo*V/(kt*d33*t)

for voltage out (as a receiver) V=P*kt*d33*t/(e*eo)

for resonant frequency f=c/(2*t) [or f=c/(4*t) if the piezo material is in contact with a soft material on one side and a hard material on the other].

P=pressure, e=relative dielectric permittiivity, eo=permittivity in vacuum, V= voltage, kt=coupling factor, d33=piezoelectric constant, t=thickness of piezoelectric layer; f=frequency; c=speed of sound