Powell received a rapturous reception on his first time back to The Valley - and then watched man-of-the-match Watt tear his side apart.
Watt capped a fine display by netting two second-half goals to add to Johan Gudmundsson’s first-half free-kick.
Powell left for “footballing reasons” in March of last year amid suggestions he did not see eye to eye with the Belgian owner Roland Duchatelet.
And the home supporters left their feelings known whose side they were on.
Charlton, in the meantime, have since gone through three different managers while Powell ended up at Huddersfield.
Both of the these mid-table sides were closer to the relegation zone to the play-offs so they will know they are not out of the woods just yet.
With tickets priced at £5, the ground was packed out but there was little goal-mouth action to entertain them early on.
Watt and Igor Vetokele wasted half chances for the hosts.
While Huddersfield had the ball in the net in the 29th minute, only for Ishmael Miller to be penalised for hauling down home goalkeeper Stephen Henderson.
There was little between the two sides before Gudmundsson scored the latest wonder goal of his short Charlton career.
The Icelandic winger is a dead-ball specialist and he proved that by curling home a 25-yard free-kick past Alex Smithies in the 34th minute.
Huddersfield were now second best but Miller squandered a golden chance to haul themselves level just four minutes later.
The giant striker found himself with room on the edge of the area but he dragged his shot wide on his favoured left foot.
And it proved crucial as Charlton extended their advantage just three minutes into the second half. Vetokele’s break freed the Scottish striker who steadied himself before shooting beyond goalkeeper Smithies.
Huddersfield now faced an uphill task and Miller brought a routine 55th save out of Henderson as the visitors pushed men forward in numbers.
It left them exposed at the other end and Watt’s run and cross saw Vetokele come within inches away of killing the Terriers off for good.
Watt was causing Powell’s side a headache and another long-range drive was kept out by Smithies. It was all Charlton and substitute Lawrie Wilson was again kept out by Smithies.
Henderson had been largely redundant but he had to be alert to keep out Murray Wallace’s low drive. To rub salt into the wounds Charlton went up the other end to wrap up the win via Watt.
He found room to break forward before firing home in the 71st minute on afternoon to forget for Powell and his players.