Answer:
The same as stealing anything else.
With the exception that you're deeling with high voltages and power. To steal electricity, you'd need to modify or create a connection to the electric grid. Depending on the magnitude of power you pull, this can be detectible by the power company immediately, or will be detected given enough time. But the main point is you have to get close to the line, which is a bad idea to say the least, unless you are trained, and have the proper protective equipment.
Besides this, you face prosecution when (not if) caught.