Enter your heater's wattage and room-to-tank temperature difference to estimate real annual running cost.
Duty cycle depends heavily on the gap between room and target tank temperature — a tank in a cool basement runs its heater far more than one in a warm living room.
An aquarium heater's thermostat cycles it on and off to hold a target temperature, so its real average power draw is far below its rated wattage most of the time — the duty cycle depends on how far room temperature is from the tank's target and how well-insulated the tank and its surroundings are. A larger temperature gap, common with tropical species in a cool room, raises duty cycle and cost accordingly.
A large gap between room and target tank temperature — common in unheated basements or during winter — pushes duty cycle much higher than the 20-40% typical of a heater in a warm room close to target temperature.
Larger tanks generally need higher-wattage heaters to maintain temperature but often have more thermal mass that resists rapid temperature swings, so duty cycle isn't purely proportional to tank size alone.