Intermediate · 9 min
Efficiency and energy losses
Calculate useful energy output and compare the efficiency of devices.
Useful output is part of the input
Efficiency is useful output energy divided by total input energy. Multiply by 100 to express it as a percentage. You can also compare useful output power with input power over the same operating conditions. Energy is conserved even in an inefficient device: the rest is transferred in less useful ways, often as heating or sound. The useful output depends on the device's purpose.
Worked example
A motor receives 500 J and transfers 350 J into useful mechanical energy. Efficiency = 350 ÷ 500 × 100 = 70%
Calculate the output
To find useful output, multiply input by efficiency written as a decimal. A 75% efficiency is 0.75. To find the required input, divide the desired useful output by that decimal. Include all relevant inputs when defining the system. For a simple device with one energy input, useful output cannot exceed total input; a result above 100% signals an error in that accounting.
Worked example
A device has input power 80 W and efficiency 0.6. Useful output power = 80 × 0.6 = 48 W
Account for all the energy
- A device receives 1,000 J with an efficiency of 65%. Calculate its useful output and the energy transferred in other ways.
- Draw an energy-flow sketch whose output values add up to the input.