Electric Efficiency

Power efficiency

Power efficiency is defined as the ratio of the output power and the input power:

η = 100% · P_out / P_in

η is the efficiency in percent [%].

P_in is the input power in watt [W].

P_out is the output power or actual work in watt [W].

 

Example

Electric motor has input power consumption of 50 watts.

The motor was activated for 60 seconds and produced work of 2970 joules.

Find the efficiency of the motor.

Solution:

P_in = 50W

E = 2970J

t = 60s

P_out = E / t  = 2970J / 60s = 49.5W

η = 100% * P_out / P_in = 100 * 49.5W / 50W = 99%

Energy efficiency

Energy efficiency is defined as the ratio of the output energy and the input energy:

η = 100% · E_out / E_in

η is the efficiency in percent [%].

E_in is the input energy in joule [J].

E_out is the output energy or actual work in joule [J].

 

Example

Light bulb has input power consumption of 50 watts.

The light bulb was activated for 60 seconds and produced heat of 2400 joules.

Find the efficiency of the light bulb.

Solution:

P_in = 50W

E_heat = 2400J

t = 60s

E_in = P_in * t = 50W * 60s = 3000J

Since the light bulb should produce light and not heat:

E_out = E_in - E_heat = 3000J - 2400J = 600J

η = 100 * E_out / E_in = 100% * 600J / 3000J = 20%

 

---

See also

• Electric power
• Watt
• Electrical calculators