The method
Before using the result
- Tariffs, operating hours and costs are user assumptions. Results exclude financing, taxes, demand charges and changes in equipment performance.
- Compare fixtures providing equivalent useful lighting. Include driver/ballast power in fixture input.
Worked examples
These original training examples illustrate the method for IFMA Facility Management Professional (FMP). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Screen a fixture replacement using actual fixture input and a consistent service schedule. Keep driver or ballast consumption inside the power boundary.
- Fixture count
- 50
- Existing power per fixture
- 60 W
- Proposed power per fixture
- 25 W
- Annual operating time
- 3,000 h
- Energy tariff (currency units/kWh)
- 0.15
- Project cost (currency units)
- 4,000
- Saving = 50 × (60 − 25) × 10800000 J.
- Annual cost saving = 5250 kWh × 0.15.
- Annual energy saving: 18,900,000,000 J
- Annual cost saving: 787.5 currency units
- Simple payback: 5.079365 years
2. Comparison example
Compare a different supplied condition using the same method. For Facility Cert Exam, the important check is that the input meaning, units and method assumptions still match the problem.
- Fixture count
- 75
- Existing power per fixture
- 60 W
- Proposed power per fixture
- 25 W
- Annual operating time
- 3,000 h
- Energy tariff (currency units/kWh)
- 0.15
- Project cost (currency units)
- 4,000
- Saving = 75 × (60 − 25) × 10800000 J.
- Annual cost saving = 7875 kWh × 0.15.
- Annual energy saving: 28,350,000,000 J
- Annual cost saving: 1,181.25 currency units
- Simple payback: 3.386243 years
Check your understanding
1. Which relationship is used?
Energy saving = count × (old watts − new watts) × time; payback = cost/annual cost saving.
2. Which assumption should you check first?
Tariffs, operating hours and costs are user assumptions. Results exclude financing, taxes, demand charges and changes in equipment performance.
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 IFMA Facility Management Professional (FMP) 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.