That depends on how hard it has to work. The power supply is 12volt DC, rated at 12.5A but the aF4 typically does not need all that power. It uses the most electricity when bringing the reservoir to temperature, the rest of the time it uses significantly less power. On average in a 68F ambient temperature house, the aF4 consumes approximately 2 kWh per day. This number will decrease with cooler ambient temperatures and increase with warmer ambient temperatures.