A kettle has a very high power, often around 2 to 3 kW, but it normally runs for only a few minutes. A games console or computer may use a lower power but operate for much longer. Energy transferred depends on both power and time: E = P × t. That means the highest-power device does not automatically use the most energy overall. For example, a 3 kW kettle running for three minutes uses about 0.15 kWh. A 300 W gaming setup running for one hour uses about 0.30 kWh. Real devices vary, but the comparison shows why both the power rating and the duration of use matter.

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.