One operating point versus a full year
From input power P in kW and flow Q in m³/min, specific energy is P/(60 × Q) in kWh/m³. Over a longer period, the correct relationship is total energy divided by total delivered volume. It is not a simple average of instantaneous ratios; for example, unloaded running uses energy without useful delivery.
Comparisons must be fair
Higher outlet pressure generally means more compression work. Do not therefore judge the difference in specific consumption between two machines without knowing the pressure. For multi-compressor systems, also consider sequencing, brief demand periods and night-time operation. The best catalogue point may not match where the machine will operate for most of the year.
What to check in practice
- Match energy and flow measurements from the same period.
- Record the common flow reference and meter location.
- State whether you are assessing the compressor alone, the entire generation system or air delivered to consumers.
Example scenario
Over a year, 120,000 kWh was measured and 1,000,000 m³ delivered at the agreed reference conditions. Specific consumption is 0.12 kWh/m³. With electricity at CZK 4/kWh, the energy component of the price is CZK 0.48/m³; this excludes servicing and investment.
What to watch out for
- Adding or averaging specific consumption without weighting by delivery.
- Excluding energy used at idle from the annual assessment.
- Comparing volume before and after air treatment without accounting for regeneration.
Can a system perform worse than the compressor alone?
Yes. Leaks, regeneration air, unloaded running and other consumption increase the energy per amount actually used in production. Always state where the assessment boundary starts and ends.