Around 400 guests had to be evacuated from the building on Saturday night after the fire took hold at the five-star hotel.
More than 50 fire fighters from were brought in to deal with the fire.
Fire investigators believe the unit under a bedroom window on the sixth floor of the five-star seafront hotel was to blame for the alert.
Most guests were allowed back in to the hotel within an hour, but a small number had to be offered alternative accommodation due to water damage and problems with the electricity supply.
Group commander Peter Cox, of East Sussex Fire and Rescue Service, said: 'The most likely cause was a fault in an electrical appliance in the room.
'There has never been any suspicion. We have done a comprehensive investigation of the fire room and it left us with a fair amount of evidence that it was due to the combined air conditioning and heating unit under one of the bedroom windows.
“There was a young couple staying in the room but they were not in the room at the time. It had been empty for about 30 minutes. The unit was left running, blowing hot air into the room.'
The make and model of the unit has not been disclosed.
Photograph: Damon Hart-Davis/DHD Multimedia Gallery