An IE class is useful for compliance, but purchasing value depends on hours, load and system control. Build the comparison with the same mechanical output requirement for every option.
For each operating point:
Annual input energy (kWh) = output power (kW) ÷ efficiency × annual hours
Then sum all operating points and add drive or auxiliary consumption inside the chosen boundary.
| Input | Source | Common error |
|---|---|---|
| Load and hours | Historian, meter or process model | Assuming full load all year |
| Efficiency curve | Tested or guaranteed supplier data | Using rated efficiency at low load |
| Drive/auxiliary loss | System quotation or test | Comparing motor-only with system input |
| Tariff | Site energy bill | Ignoring time-of-use demand structure |
Make the comparison auditable
State voltage, frequency, speed-control method, ambient and measurement period. Separate verified data from assumptions and test sensitivity to uncertain hours or load.
Annual cost = annual input energy × applicable energy price. Simple payback can be screened as price premium ÷ annual saving, but lifecycle review should also consider maintenance, availability and replacement risk.
Related reading: efficiency and project lifecycle selection and GB 18613 and IE efficiency classes.