First define the system boundary
The cost at the compressor outlet is not the same as the cost of dry air actually used in production. Treatment, regeneration, leaks and auxiliary drives affect the result. State whether maintenance, investment costs and other operating items are included. Do not count the same consumption once on the electricity meter and again as an accessory estimate.
Include all operating modes
If continuous measurements are unavailable, a balance can be built from power consumption and hours in each operating mode. Idle operation counts even when no air is delivered. Do not confuse motor output with electrical input, or catalogue flow with annual volume without accounting for actual running time.
What to check in practice
- Use the same period for energy, volume and maintenance.
- Record volume reference conditions and measurement location.
- Separate an indicative balance from a documented operating result.
Example scenario
Annual electricity costs are CZK 400,000 and maintenance CZK 50,000. With delivery of 900,000 m³, this defined operating cost is CZK 0.50/m³. Investment and any costs omitted from the balance are not included; without flow data, only CZK 450,000 per year could be stated.
What to watch out for
- Copying a price per m³ from another plant.
- Ignoring energy consumed at idle.
- Comparing the prices of reference and operating volumes as identical quantities.
Why can unit cost rise when production falls?
Some costs remain, and the compressor may spend more time unloaded. A lower annual volume then carries similar maintenance and standby costs. Assessment should use a consumption profile, not just electricity bill comparisons.