Six days after a sudden summer storm knocked large parts of US powerless, more than 5,80,000 homes and businesses in the American capital and eleven states were still without electricity with temperatures hovering around 37 degrees Celsius mark in many places.

The number of households without power dropped considerably Thursday from a peak of about four million from Ohio to New Jersey who lost power after the Friday night storm that also claimed 22 lives.

Many in Washington and affected states of Indiana, Ohio, West Virginia, Pennsylvania, Maryland, Virginia, Delaware, Kentucky, North Carolina, New Jersey and Illinois spent the July 4 Independence Day in darkness.

Thursday saw more sweltering temperatures, from Chicago to Washington to Atlanta and beyond with mercury touching 38.3 degrees Celsius in Baltimore and 40 degrees Celsius in St Louis.

"The heat will begin to expand eastward over the next few days, with high temperatures and humidity levels combining to create dangerous heat index values in the triple digits," the forecast said.