What % of electrical energy transmission in USA is DC

Power transmission in the US is essentially all AC. Thomas Edison was an advocate of DC power because he thought it was safer, and liked the idea of lots of small local generators. Westinghouse advocated AC power because it can efficiently be transmitted long distances with minimal loss.

DC power essentially has to be transmitted at the voltage at which it will be used. The higher the voltage, the lower the resistance to transmission. AC power can be generated at any voltage and stepped up or down as needed efficiently using magnetic transformers. Transformers only work where the voltage, and thus the magnetic field they use to pass energy, is constantly changing, otherwise they simply act as a short circuit.

About the only place you will see large scale use of DC power is in transportation, such as trolley cars. Even there, the DC substations are located every two miles or so because even at 600 or 1200 volts, it takes very fat and expensive wiring to avoid huge losses.

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Originally Posted by amircon

What % of electrical energy transmission in USA is DC

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As Catgita already indicated : DC can only be used over very short distances.
Also take into account that many electrical appliances standard to every household can not function on DC.
In the future DC possibly may return in homes, if fuel cell generators will be used to power individual houses. Of course electrical power suppliers are dead-against such plans...
;)