How much does a 12w downlight use per hour running on a transformet?

A 12W downlight uses 12 watts of power per hour when it's running. To convert this to kilowatt-hours (kWh), which is how electricity is typically billed, you divide by 1,000, resulting in 0.012 kWh per hour. Therefore, if you run the downlight for one hour, it will consume 0.012 kWh of electricity.