Your 250 kW screw air compressor is not actually a 250 kW load. In a Panrui energy audit, the calculation started with 250 × 1.2 because the motor carried a 1.2 service factor. If you ignore the screw air compressor service factor, you can undercount electricity use by enough to destroy a project’s payback calculation.

Most buyers quote by nameplate kW and rated FAD. They miss the real input because service factor and unloaded running are hidden in the fine print. The result is a payback model that looks good on day one and disappoints after the first utility bill.

What Service Factor Means on a Screw Compressor

Service factor is the margin between a motor’s rated output and the maximum continuous load it is designed to handle. On a screw compressor, that margin must be applied before you estimate input kW. In the Panrui example, the old 250 kW machine used a 1.2 service factor, so the calculation started from a 300 kW maximum load base.

That does not mean the unit always draws 300 kW. It means the energy formula must use 250 × 1.2, then reduce that figure by load rate and pressure correction to match real operating conditions.

Why Ignoring Service Factor Understates the Bill

The error shows up most on fixed-speed machines running below rated pressure. In the local Panrui case, the old compressor was rated 250 kW at 8.6 bar but was actually running at 4.9 bar with 34.7 m³/min of demand and an 85.7% load rate.

• Loaded input: 250 × 1.2 × 85.7% × (1 − (8.6 − 4.9) × 7%) = 190.51 kW
• Unloaded input: 250 × 1.2 × 14.3% × 40% = 17.16 kW
• Total measured input: 207.67 kW

An estimate based only on 250 kW would miss the service factor and unloaded power. The real input was 207.67 kW for only 34.7 m³/min of air demand.

The 30-Minute Service Factor Audit

• Record motor nameplate kW and service factor.
• Record rated pressure and rated FAD.
• Measure actual pressure, actual air demand, and loaded time or frequency.
• Calculate loaded kW: rated kW × service factor × load rate × pressure correction.
• Calculate unloaded kW: rated kW × service factor × (1 − load rate) × 40%.
• Compare total input against a right-sized replacement.

Use 7% per bar as the pressure correction for fixed-speed equipment in the Panrui calculation model. Actual motor performance should be confirmed from nameplate data and logged measurements.

What the Correction Changes

A 160 kW variable-speed screw compressor selected at 5 bar met the same 34.7 m³/min demand with 154.43 kW of input. The difference was 53.24 kW.

• Daily saving: 53.24 × 24 = 1,277.76 kWh
• Daily cost saving at 0.8 CNY/kWh: 1,022.21 CNY
• Ten-year saving: 1,277.76 × 365 × 10 × 0.8 = 3,731,059.2 CNY

That is a purchase decision driven by operating cost, not by nameplate kW.

In a separate Panrui VSD example, two old 37 kW units used 75.1 kW total for 9.9 m³/min. One 55 kW VSD screw compressor produced the same flow with 56.4 kW, saving 18.7 kW, 449 kWh per day, and 134,700 kWh per year. At 0.8 CNY/kWh, that is 107,760 CNY a year.

What Buyers Get Wrong

• Comparing compressor quotes by nameplate kW without checking service factor.
• Using rated FAD at full pressure instead of actual demand.
• Running an 8.6 bar machine at 4.9 bar and accepting the pressure-correction loss.
• Ignoring unloaded power, which the Panrui model counts at 40% of service-factor-adjusted load.

How to Apply the Correction

Start with a rated-versus-actual audit, then compare a correctly sized VSD unit. The rotary screw air compressor guide covers sizing and system design, while the variable speed screw air compressor page explains how load-following reduces unloaded losses.

If you have nameplate data, pressure readings, and run-hour logs, request a Panrui energy calculation before you replace or quote a unit.