Electricity powers many of the most important devices in our lives, including our cell phones. There are many technologies for producing electricity, such as fossil-fuel power plants, solar panels, wind turbines, and nuclear power plants. Coal-fired power plants convert the chemical energy in coal to electricity that is then transmitted to users in the electrical power grid. The energy efficiency of a coal power plant is calculated as the amount of electrical energy produced divided by the chemical energy in coal. We can calculate the overall energy efficiency by compounding the individual energy efficiencies. Using these equations we can calculate the mass of coal required to power a typical cell phone each year.