Power, efficiency and reducing waste

Power is the rate of energy transfer: P = E ÷ t, measured in watts (W), where 1 W = 1 J/s. A 2000 W kettle transfers 2000 J every second. Two motors may lift the same load to the same height, but the more powerful one does it faster.

Efficiency is the fraction of input energy that is usefully transferred: efficiency = useful output ÷ total input, often given as a percentage. Wasted energy is usually dissipated as heat to the surroundings, making it less useful. Lubrication reduces friction in machines, and thermal insulation such as cavity walls and loft insulation reduces heat loss from buildings. Materials with low thermal conductivity transfer heat more slowly; thicker walls also lose energy more slowly.