I need help on what formulas to use and how to start this physics problem. We estimate that there are 260 million plug-in electric clocks in the United States, approximately one clock for each person. The clocks convert energy at the average rate of 3.6W. To supply this energy, how many metric tons of coal are burned per hour in coal fired electrci-generating plants that are, on average, 13.4% efficient? The heat of combustion for coal is 33MJ/kg. Answer in units of metric tons. Another question, how is Watts a rate?