All energy transfers waste energy. The efficiency of a device is a measure of the amount of energy that is usefully transferred. The energy transfers of a device can be represented using a Sankey diagram. Efficiency can be calculated using the energy or power transferred by a device. Power is the energy transferred per second, and is measured in Watts (W) or kilowatts (kW).
Efficiency = useful energy out ÷ total energy in (× 100%)
Efficiency = useful power out ÷ total power in (× 100%)
