Efficiency of a system
- The efficiency of a system is the ratio of the useful energy output from the system to the total energy input
- If a system has high efficiency, this means most of the energy transferred is useful
- If a system has low efficiency, this means most of the energy transferred is wasted
- Multiplying this ratio by 100 gives the efficiency as a percentage
- The efficiency is calculated using the equation:
- Efficiency can also be written in terms of power (the energy transferred per second):
Worked example
An electric motor has an input power, Pin, useful output power, Pout, power lost Plost, and an efficiency η.
What is the output power of the motor?
A: B: C: D:
Answer: B
Step 1: State the efficiency equation
Step 2: Substitute the terms given in the question
Step 3: Manipulate the equation
- Multiply by Pout + Plost on both sides
- Expand the brackets
- Minus Pout from both sides
- It is helpful to first minus ηPlost
- Factor out Pout
- Divide by η − 1
Examiner Tip
Efficiency can be in a ratio or percentage format. If the question asks for an efficiency as a ratio, give your answer as a fraction or decimal. If the answer is required as a percentage, remember to multiply the ratio by 100 to convert it, e.g. Ratio = 0.25, Percentage = 0.25 × 100 = 25 %