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Watts is already a unit of energy over time. (J/s)

Aka it's a unit of power. Hence the OP asked over what time to figure out the amount of energy used..?

Surely that is the average usage over time? 28.8kWh per day?

Edit: The average household in the U.S. uses 10500kWh of power per year. 28.76kWh per day.