Well, I think you already got it right. You need to close the circuit and add an electrical load. The PZT beam behaves as a voltage generator, for what concerns the electrical part.

I think you don't need to model the circuit in Abaqus. You can run the FEM simulations in Abaqus and measure the voltage output of the system. For this purpose, I suggest you create both a voltage vs time graph and a voltage vs load graph. Then, you draw you electrical circuit. It could just be a voltage generator (the PZT beam) and a resistance. You may also design something more detailed, where you model the presence of a wattmeter. In this way you can compute a value of the harvested power closer to what you might see in a real measurement. See for example the following link for a circuit representing the presence of a wattmeter:

http://www.allaboutcircuits.com/textbook/direct-current/chpt-8/wattmeter-design/

Such circuits can be solved analytically, and you should be able to derive an expression for the power like W = W(dV, R) in explicit form. You can then proceed to the last step. You write a function or subroutine in your language of choice (C++, Python, Matlab, Mathematica, Maple, ... they're all suited for this purpose) that takes as inputs the voltage history and the parameters of the circuit and provide the power as output. 

It seems a long job while explaining it, but I think it should take you just 1 or 2 hours of work (depending on your background on circuit theory).

I hope this answer might have been useful.

Good luck with your work!

Thanks a lot Luca for your elaborated answer. I am trying as per your inputs, I have few questions, 

Is that , You mean to say I have to take voltage vs time and voltage vs load graph for each node from the Abaqus ? as the domain as many nodes and voltages are generated at each node.

And how will I get the voltage vs load graph ?

You take the voltage vs time for each node, and I think you should sum them up together to get the total response. You can do that directly in Abaqus Viewer, where you have a few options to post-process the output data in simple ways (sum, substract, plot one field vs another). In Abaqus Viewer you can also create a custom plot of voltage vs load.

Thanks a lot again.

I could not found the field output for the Load. What should I choose for load as a field output. 

I might think two options. The first is just using the input load that you assign. It might be constant, time-dependent, sinusoidal,... as long as it's a design input, you know it. The second one is to get the reaction force at the constraint(s) that keep(s) the beam in place. It might be interesting to see if there is any relationship between the type of constraints and the piezoelectric behaviour.

Actually I got confused with the load (External electrical load resistance). Thanks, I will try this one.

 This is not answer, but more like a follow up question. I have a similar situation as Swapnil was describing. Im using a bimorph PZT beam in Abaqus, where I apply a dynamic (random) load and I want to obtain the electrical energy generated from it. I used a voltage boundary condition to close the circuit and I have the Voltage values from the system. Now, I have to add it to the circuit for the quantification. I was planning to do as Luca first suggested and work on it separately from Abaqus, solving analytically, but thinking further on the circuit, it seems weird that the only output necessary from the generator from Abaqus is the voltage.

I agree with Swapnil first question suggesting it should be a power output. Cause otherwise I could theoretically add any resistance value to the circuit and could get any power from the PZT generator, which does not make sense, because there is a limited power generated from it. The traditional generators we have are usually characterized by their power information, not only voltage.

Am I missing something here? How do you deal with this?

Thanks, Julia 

I think summing up of all potential differences seems to be the wrong idea. Instead its better to summing up all charges & from that we can calculate Power.