A modern charger with nothing connected usually draws much less power than it does while charging a phone, but the power is not always exactly zero. Small electronic circuits may remain active, so a tiny amount of energy can still be transferred over time. Energy used equals power multiplied by time. For one efficient charger the yearly waste may be small, but standby use across millions of devices can add up. Older or poor-quality chargers may also waste more. The wider lesson is that even a small power becomes an energy use if it continues for a long enough time.

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.