What's Arthur power level when he uses the Excalibur sword?

Excalibur is the legendary sword believed to hold ultimate power, which can only be wielded fully by its true master. Whomever wields this sword will becomes near invincible. However, those who use it and are not destined for it, will be corrupted and ultimately destroyed by an all-consuming lust for power.

In the legends, Exalibur was given to King Arthur by the first Lady of the Lake. After the death of King Arthur, Sir Bedivere threw Excalibur into the lake. A hand then rose from the water and took it into the lake. Another legend says Excalibur was drawn from a stone by Arthur to prove his royalty.