We had this problem last winter. Our bill for 6x60 watt bulbs in the grounds of our complex was 189TL for 1 month.The lights are on a timer, on at sunset and off at sunrise. We queried it and Aydem sent a team out to check our supply (checking if anyone was tapping in to it).They reported that the supply to our building was inadequate (if all the owners were out at the same time, the electricity supply would have cut out) and fixed the problem, but we still had to pay the bill.
Maybe you need to get Aydem out to check your supply.