The joule is the SI unit of energy, but household appliances transfer millions of joules. Using joules on an electricity bill would create inconveniently large numbers. Electricity suppliers therefore use the kilowatt-hour, or kWh. One kilowatt-hour is the energy transferred by a 1 kW device operating for one hour. Because 1 kW is 1000 watts and one hour is 3600 seconds, 1 kWh equals 3.6 million joules. The unit makes bills easier to read while still measuring energy. Power tells you how quickly energy is transferred; kWh records how much energy was used overall.

Designers have to balance performance, safety and wasted energy. That is why phones manage battery temperature, chargers regulate current, power networks use particular voltages, and appliances carry power ratings. Looking at the label on a real device turns the idea into something measurable rather than abstract.

Power is the rate of energy transfer: a high-power device transfers a lot of energy each second. Electrical power can be found from P = IV, while energy transferred is E = Pt. Efficiency compares the useful energy output with the total input, and the remainder is not destroyed—it is transferred to less useful stores, often by heating or sound.