More than six million customers lost power Monday as Hurricane Sandy felled trees, downed power lines and flooded substations. The storm led to power failures in at least 17 states, including more than a million customers in Pennsylvania and New Jersey and about 660,000 in New York City. Roughly a quarter million customers lost power in Manhattan after a fiery explosion at a substation on East 14th Street darkened most of the island south of 34th Street. Con Edison officials called the power failures “the largest storm related outage in our history.” In an update Tuesday morning, New York Gov. Andrew M. Cuomo said that about 2 million families in the region were still without power, nearly half of them on Long Island.