Generally, an average fan-assisted electric oven has a power rating of 2500W / 2.5 kilowatts (kW), meaning they consume 2500 watts (2.5 kWh) for every hour they are being used at medium-to-high heat. So, if we take an average electricity price of 14 cents/kWh in the US, then leaving an oven on for 12 hours would cost $4.2 in electricity.