A phone charger often feels slightly warm while it is working. That warmth is evidence that some electrical energy is being transferred to the thermal energy store of the charger and its surroundings. Current passes through wires and electronic components that have resistance. When charges move through resistance, energy is dissipated as heating. A charger also changes the mains supply into the lower voltage needed by a device. Modern chargers are efficient, but no energy transfer is perfectly efficient, so some energy is wasted as heat. A little warmth can be normal. Excessive heat, damage or a burning smell is not, because high temperatures can indicate a fault.

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.