I beleive you are asking about "smart meters" used for measuring energy consumed (or produced). In that case, it depends on the meter and now it communicates. The ones we use here communicate by shorting the buss to ground, near the zero crossing of the AC - a kind of power line communication. In that case, monitoring the AC buss, by tying to the buss (pluggin into an outlet) or inductive pickup would allow access to the communication. You would then need inherent knowledge of the communicaiton protocol and any encryption to access the data being sent.
