Is it possible how they generate electricity in the republic city powerplant? I know at least of a few things that have to be overcome for this to work.
First, how do you store the energy that comes from lightning, since it is a lot of energy? My answer would be: Firebenders can controll the intensity of the electricty (e.g. Azula striking Aang vs Mako who zips Amon with only a little movement, which just hurts him and doesnt kill him). So maybe they are trained and can somewhere see if they are shooting at the right Voltage.
Second, they need giant batteries (which they seem to have, how? Maybe they electrolyse water into hydrogen and thereby store the electricity?). These batteries produce direct current, which they then would have to transform into alternating current (they could do this by using a converter), or the battery could power a generator.
And probably there are many more problems that should be overcome. (im not that technical), but think it could be done.
I think a neat explanation is that they use their lightningbending for the electrolysis of water (in the "batteries"). This is a nice way to store the energy and water is not hard to come by. Would be cool if the Avatar Univese makes use of the Hydrogyn to power their city!