The method
Before using the result
- Input and useful output use the same system boundary and time period. This is thermal/energy efficiency, not heat-pump COP.
Worked examples
These original training examples illustrate the method for Certified Energy Manager (CEM). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Draw the boundary between useful output and energy input in an energy-management problem. Distinguish efficiency from a heat pump coefficient of performance.
- Energy input
- 0.2777778 kWh
- Useful energy output
- 0.2222222 kWh
- η = 800000 / 1000000 × 100%.
- Loss = 1000000 − 800000 J.
- Efficiency: 80 %
- Energy loss: 200,000 J
2. Comparison example
Compare a different supplied condition using the same method. For Energy Cert Exam, the important check is that the input meaning, units and method assumptions still match the problem.
- Energy input
- 0.2777778 kWh
- Useful energy output
- 0.25 kWh
- η = 900000 / 1000000 × 100%.
- Loss = 1000000 − 900000 J.
- Efficiency: 90 %
- Energy loss: 100,000 J
Check your understanding
1. Which relationship is used?
Efficiency = useful energy output / energy input; loss = input − output.
2. Which assumption should you check first?
Input and useful output use the same system boundary and time period. This is thermal/energy efficiency, not heat-pump COP.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
This tool connects to Certified Energy Manager (CEM) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.