A joule is an amount of energy, and a watt is a rate of using energy, defined as 1 W = 1 J / s. How many joules of energy are required to run a 100 W light bulb for one day?

Coal has about 30E6 J of energy per kg, but a power plant can only use 30% of this energy to make electricity (i.e., it is 30% efficient). How many kilograms of coal have to be burned to light a 100 W light bulb for one day?