Watt was a South Saxon whose existence is attested by three charters that he witnessed, in the reign of Noðhelm, as Wattus Rex. He probably would have ruled between about AD 692 and 725 and there is some suggestion that he may have been King of the Hæstingas.

Charter evidence

He witnessed a charter from Noðhelm in 692, without any indication of his territory, but it is likely that he reigned in Sussex

Kingdom of Sussex

The Kingdom of Sussex or Kingdom of the South Saxons was a Saxon colony and later independent kingdom of the Saxons, on the south coast of England. Its boundaries coincided in general with those of the earlier kingdom of the Regnenses and the later county of Sussex. A large part of its territory...

, because he also witnessed (again as Wattus rex) an undated charter (but before about 705) by "Bruny dux Suthsax" together with Nunna rex (Nunna being the short form of Noðhelm).

He is also listed as a witness (as Uuattus rex) of another charter, erroneously dated 775, which is believed to be a forgery.

King of the Hæstingas

The historian C. T. Chevallier suggested that Watt may have ruled the Hæstingas, a tribe found in east Sussex around what is now Hastings

Hastings

Hastings is a town and borough in the county of East Sussex on the south coast of England. The town is located east of the county town of Lewes and south east of London, and has an estimated population of 86,900....

, his explanation being that placenames with "Wat" or "What" in them, could be found in that area, but not in western Sussex. However, this hypothesis has been largely rejected by other historians, who suggest that it is based on the misinterpretation of the place-name evidence." However, Chevalliers suggestion that Watt may have been a sub-king to the South Saxons may be more plausible.