An inverter compressor varies its rotational speed through a variable-frequency drive, so a heat pump modulates output continuously instead of switching on and off at full power. Typical modulation range is 25–100% of rated capacity. Modulation cuts start-stop cycling, softens starting currents, and is the main reason modern air-to-water heat pumps reach SCOP values of 3.5–5.0.
How does an inverter compressor work?
An inverter compressor is driven by a variable-frequency drive: incoming mains power is rectified to DC, then electronically reconstructed at whatever frequency the controller requests, typically spinning the compressor anywhere between about 20 and 120 Hz. A brushless DC motor and a rotary or scroll compression mechanism convert that speed directly into refrigerant mass flow — half the speed, roughly half the heat output.
The controller continuously matches compressor speed to the building's momentary heat demand, using flow temperature targets from weather-compensated control. A fixed-speed compressor knows only two states, 0% and 100%; an inverter unit lives on the whole scale between them.
Why does modulation improve efficiency?
Modulation improves efficiency because buildings almost never need full power: a central European heating season spends the large majority of its hours below half of design load. The EN 14825 standard behind SCOP reflects this by weighting part-load test points, which is where inverter machines shine.
Two mechanisms do the work:
- Fewer starts. Every compressor start costs energy before useful heat flows and adds mechanical wear. Long, low-speed runs replace dozens of daily starts.
- Oversized exchangers at part load. At 40% speed, the same evaporator and condenser serve far less refrigerant, so temperature differences shrink, condensing pressure falls and the COP rises — the compressor works across a smaller temperature lift.
As a rule of thumb, every degree of reduced flow temperature improves efficiency 2–3%, and part-load operation is what lets the controller actually hold those lower temperatures.
What changes in practice versus a fixed-speed compressor?
| Criterion | Inverter compressor | Fixed-speed (on/off) |
|---|---|---|
| Output control | Continuous, ~25–100% | Full power or off |
| Starting current | Soft start, low inrush | Inrush several times running current |
| Cycling at part load | Rare above minimum modulation | Constant |
| Flow temperature stability | Steady | Sawtooth |
| Noise profile | Low speed most of the time | Full speed whenever running |
The soft start also matters for grid connections: no flicker, no oversized fuses, simpler approval for single-phase homes.
Does an inverter compressor remove the need for a buffer tank?
No — it weakens one argument for the buffer, not all three. When the unit can modulate below the building's minimum load, anti-cycling volume matters less. But a buffer tank also guarantees defrost energy and provides hydraulic separation for zoned circuits, and modulation does nothing for either. Zoned radiator systems and cold climates still specify a buffer; the full reasoning is in Do I need a buffer tank with a heat pump?.
Below its minimum modulation step — often 25–30% of rated output — even an inverter unit cycles. Oversizing a heat pump therefore still hurts: a 16 kW unit on a 6 kW house spends spring and autumn below its modulation floor. Correct sizing plus modulation is the combination that delivers the catalogue SCOP.
Inverter technology is standard across the European market's growth — heat pump sales rose 13% in 2025 to 2.88 million units (EHPA) — and in Solimpeks' Varm Up Series, whose EVI DC inverter units reach COP up to 4.92 at A7/W35.
Frequently asked questions
What is the difference between an inverter and an on/off heat pump?
An on/off unit always runs at full power and regulates by starting and stopping; an inverter unit varies compressor speed to match demand, typically between 25% and 100% of rated output. The inverter version cycles less, starts softly and achieves higher seasonal efficiency.
Do inverter heat pumps use less electricity?
Yes, over a season. Part-load operation with smaller temperature lifts and far fewer starts raises SCOP measurably compared with fixed-speed operation of the same nominal capacity — which is why EN 14825 rates heat pumps at part-load points rather than full power only.
Why does my inverter heat pump run almost continuously?
By design. Long, low-speed operation is its most efficient mode; a unit trickling along at 30% output is healthier and cheaper to run than one blasting at 100% and resting. Continuous running in cold weather is normal behaviour, not a fault.
Can an inverter heat pump still short-cycle?
Yes — whenever demand falls below its minimum modulation step, typically 25–30% of rated output. Oversized units and small zoned circuits cause this; correct sizing and adequate system volume, often via a buffer tank, prevent it.