Why inverter compressors save energy at part load — the hidden physics
By DigiEntropy Engineering · 2026-07-18 · 9 min read
A variable-speed compressor is only slightly more efficient than a fixed-speed one at any single operating point. The saving comes from the coils running a smaller lift at part load, and it only reaches the bill if the fans follow the load. Here is the physics, with real data.
About this post — Authored by an AI assistant using DigiEntropy's polynomial corpus, the universal compressor predictor, and the project's chart-generation tooling. Charts are produced by Python scripts that read the same database the live site queries; tables and formulas are pulled from the same engineering modules. Findings reflect the project's current dataset and methodology — send corrections or deeper questions to admin@digientropy.com.
A user sent us a sharp question. In our Operation Simulation tool they had put a variable-speed (inverter) compressor next to a fixed-speed one on the same cold room, and the inverter came out using more energy over the day. That is against everything the brochures promise, so either the brochures were wrong or the model was. We checked, as a refrigeration engineer would rather than a marketer, and the answer is a piece of physics most people get slightly wrong.
We have since checked this post against real part-load data and the System Simulator's COP Optimizer. Its first version got two things wrong: it said a compressor's COP does not change with capacity at all, and it left the fans out of the story. Both are corrected below, and the full list is at the end.
2.02 - inverter COP at 50 % - BITZER 4EES-4Y, compressor only; 1.98 at full capacity
On its inverter data the COP rises from 1.98 at full capacity to 2.02 at 50 % and falls to 1.95 at 140 %; on its VARISTEP data it drops to 1.83 at 50 %.
1.47 - system COP at half load, full fans - the same 1.47 as at full capacity
In the same circuit, with both fans still at full speed, the system COP does not rise at all at half load: 1.47 at full capacity, 1.47 at 50 %.
2.04 - system COP at half load, optimal fans - up from 1.64 at full capacity
Solved with the Part Load Optimizer's fan speeds it rises from 1.64 to 2.04.
16 % - less energy on the inverter - at 50 % average load; 10 % at 70 %, 3 % at 90 %
With the coil temperatures floating, the inverter uses 16 % less at 50 % average load, 10 % less at 70 % and 3 % less at 90 %.
17 % - less energy over a real day - BITZER 4PES-12Y, 62.4 kWh against 75.1 kWh
A fixed-speed BITZER 4PES-12Y uses 75.1 kWh in the day with 48 starts; the same model on an inverter uses 62.4 kWh with 1 start, 17 % less energy and EUR 12.70 against 15.28 a day.
2 % - drive loss - of what the compressor draws
A real inverter wastes a little power as heat, about 2 % of what the compressor draws.
Start at one fixed operating point
Hold the evaporating and condensing temperatures still and turn the compressor down. Does its own COP change?
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