this is the most common term used in the US. There is usually no indication of whether it is "public" or "private". I think "the power grid" refers to the aggregate of local and regional power lines, which is usually an interconnected system of both private and public systems, and generally controlled by some sort of quasi public "power authority". It is often shortened to "the grid" as the meaning is normally apparent from the text. The "electrical grid" is also an option.