A plumber caught on camera relieving himself in a customer's loft told a court he became overcome by "bright lights and the sound of running water".
Roy Williams, 46, from East Grinstead, Sussex, was captured urinating into a vase before pouring it into the water tank at a house in Leatherhead, Surrey.
Unknown to Mr Williams, he was being filmed as part of an undercover Trading Standards operation.
Mr Williams denies all charges against him at Guildford Crown Court.
He told the jury on Thursday that he had a medical condition dating back to his childhood which had left him needing the toilet regularly.
But Mr Williams said in his 11 years in the plumbing trade he had never found himself caught short before.
Surrey Trading Standards hid the camera in the duck
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However he revealed that his problem was so acute that he found it difficult to go for a drink at his local pub.
Mr Williams apologised and said he was "disgusted" by what he had done and "could only put it down to the bright lights and the sound of running water".
Jerome Wilcox, prosecuting, asked the defendant if he had the same problem in bright sunlight to which he replied: "No not really."
Mr Williams was called out to the house as part of an operation by Surrey Trading Standards, which had rented a house for six months and rigged it with hidden cameras.
An officer posed as a householder as tradesmen were called in to fix "faults".
Drinking water
On the morning of 4 November 2003 an expert had set up an apparent leak in the water tank by displacing the lid on the arm of the ballcock, causing an overflow through a pipe into the back garden.
The court heard how Mr Williams replaced two valves that were not faulty and charged a total of £173 for the work.
Jurors were also shown footage of the moment Mr Williams relieved himself into a vase before pouring the contents into the hot water tank and washing it out in the cold water tank, which provided the household's drinking water.
Mr Williams denies charges of attempted deception, making false trades description and making a reckless statement.
The case continues.