How much electrical energy is required for intracellular stimulation of a neuron?
I think this question is somewhat ill-posed, but I'll try to answer it:

None.

All you have to do is to short the inside of the cell to the outside/ground. Of course in reality this means expending some energy somewhere, but at that point it's an engineering problem, not one of biology, and it has nothing to do with the neuron itself.

Action potential firing is a completely passive process. The only part that requires energy (besides protein synthesis) is for the sodium-potassium ATPase that maintains sodium and potassium gradients.