Static,
George is correct in saying that you pay for all power used. There are however some utilities that will charge a penalty based on the peak usage during the billing period. Generally the meter will record the largest usage of power and the PoCo will charge based on that. This does not affect residential charges, however this does affect large scale commercial and industrial users. This encourages the consumer to balance their load since the highest usage is taken on a per leg basis. You still pay for all power used, but by balancing the load evenly, the penalty can be brought down dramatically. Again I only know of this being used in large commercial and industrial areas.
