Convert between household energy quantities, calculate energy efficiency, and describe the general shape of the current global and US energy mix.
Energy shows up in several everyday units that all measure the same underlying quantity: the kilowatt-hour (kWh, the unit on a home electric bill), the BTU (British thermal unit, common in heating and appliance ratings), and the kilocalorie (kcal, the "Calorie" on a food label, which you have already used since Week 3). Efficiency is useful energy output divided by total energy input, expressed as a percent; no real energy conversion reaches 100% efficiency, because the second law of thermodynamics (Week 3) guarantees some energy is always lost as waste heat at every conversion step.
The current global and US energy mix is still dominated by fossil fuels (oil, coal, and natural gas together), with nuclear and a growing renewable share (hydropower, wind, solar, biomass, and geothermal) making up the rest. This mix has been shifting over recent decades -- renewables' share has been growing and coal's share has been shrinking in many countries, including the US -- and the pace and endpoint of that shift is itself one of the policy questions explored later in this unit.
Mastery looks like: They can calculate daily kWh use from wattage and hours, calculate percent efficiency from input and output energy, and explain why 100% efficiency is physically impossible.
Common stumbles: Treating efficiency as if it could reach 100% with good enough engineering; confusing energy units as measuring different things rather than the same underlying quantity.