This all depends on the model you buy. Some pond heaters or de-icers are made to be highly energy efficient and can run all day and all night without really making a dent on your electricity bill. Others are less efficient. In general, the best pond heaters only use up to 500 watts of electricity. They're also equipped with thermostats to check when the temperature is sufficiently warm to turn the heating element off and save energy. When buying a pond heater, check out how much energy it uses and try to find an efficient one if you want to keep your costs low, like the Thermo-Pond Floating De-Icer