η = Puseful/Pin or η = Euseful/Ein
- η efficiency (no dimension)
- Puseful useful power out in W
- Pin total power in in W
- Euseful useful energy out in J
- Ein total energy in in J
In example 2 the water is heated by a an electric heating element with a power of 1000 W.
The water was heated for 1.5 minutes.
Calculate the efficiency of the process
Ein = P t = (1000)( 1.5 x 60) = 9.0 x 104 J
Euseful = Q = 6.39 x 104 J
η = (6.39 x 104)/(9.0 x 104) = 0.71 or 71 %