Efficiency and Electrical Consumption

The efficiency of a device

An electrical device never converts 100 percent of the energy it receives into useful energy: some is always lost, often as heat. The efficiency eta (Greek letter) compares the useful energy obtained to the total energy consumed:

eta = E_useful / E_consumed (or equivalently eta = P_useful / P_consumed)

This efficiency, which has no unit, is often expressed as a percentage (eta x 100). It always satisfies 0 <= eta <= 1.

Concrete example

An LED bulb consumes an electrical power of 10 W. It gives back 8 W as useful light, the rest being dissipated as heat.

eta = 8 / 10 = 0.8, that is, an efficiency of 80 percent.

By comparison, an incandescent bulb of the same brightness consumes more (around 60 W) for a much lower light efficiency: almost all of the energy is lost as heat.

Reading an electricity bill

A household's bill is based directly on the energy consumed in kWh, multiplied by the price per kWh set by the supplier:

Cost = E (in kWh) x price per kWh

Device	Power	Duration per day	Energy per day
Refrigerator	150 W	24 h	3.6 kWh
LED lighting	40 W	5 h	0.2 kWh
Washing machine	2000 W	1 h	2 kWh

Common pitfall

An efficiency cannot exceed 1 (or 100 percent): if a calculation gives eta > 1, an error has crept in, often a swap between useful energy and consumed energy in the denominator. Also be careful not to confuse instantaneous power (in W or kW) with cumulative energy over a period (in kWh), which are the two separate quantities shown on devices and bills.