Richard Bull, 32, was charging his phone next to the tub when it accidentally fell into the water.
He was found by his wife Tanya, who thought her husband had been attacked because his burns were so severe.
She made a distraught call to the police, who found that Mr Bull had used an extension lead from the hallway of his Ealing home so he could use the phone in the bath.
He was getting ready to go out meet family members to exchange Christmas presents when the tragedy happened on December 11.
Coroner Dr Sean Cummings said: "This was a tragic accident and I have no reason to believe at all that there anything other than it being completely accidental.
"These seem like innocuous devices, but they can be as dangerous as a hairdryer in a bathroom. They should attach warnings.
"I intend to write a report later to the makers of the phone."
Richard's mother Carole said: "I have worried that so many people and especially teenagers, that can't be separated from their phones, don't know how dangerous it is."
His brother Andrew said: "When you are younger you are taught about electricity and the bath, but you don't think about this.
"I still find it hard to believe that between the charger plug and the phone would be enough electricity to kill someone."
A keen rugby player, Richard captained amateur side Old Actonians RFC as well as playing for Ealing Exiles.
PC Craig Pattison told the inquest at West London Coroner's Court how he found the extension lead running from the hallway outside into the bathroom.
"We found an iPhone plugged into the extension cable and then the charger element in the bath," he explained.