The core idea
No cooling system needs full capacity all day — the heat load swings with sun, occupancy and outdoor temperature. Two strategies exist for matching capacity to load. On/off cycling: a fixed-speed compressor runs flat out until the thermostat's hysteresis band cuts it off, then lets the room drift back — full power or nothing, with room temperature sawtoothing between the band edges.
Modulation: an inverter drive — the compressor's own variable frequency drive — lets the motor run anywhere from ~25 % to 100 % speed. Capacity rides on refrigerant mass flow: Q̇ = ṁ·Δh, where Q̇ = cooling capacity (kW), ṁ = refrigerant mass flow (kg/s), Δh = enthalpy gained across the evaporator (kJ/kg). Halve the speed, halve the flow, halve the capacity — continuously, with no starts and stops.
Modulation is also more efficient, not merely smoother. At reduced speed the heat exchangers become relatively oversized for the flow: evaporating pressure drifts up, condensing pressure falls, and the temperature lift shrinks — a smaller lift means less work per kilowatt of cooling. A good inverter unit at half capacity draws well under half its rated power, and holds room temperature within a fraction of a degree instead of a ±1–2 K sawtooth.
Real-world example
A family home with an inverter split AC on a hot afternoon: 100 % speed at the 3 pm peak, then as the evening load halves the unit eases to ~50 % and cruises there — never switching off. An equally sized on/off unit would instead cycle: start inrush, coil re-stabilisation, overcool past the setpoint, stop, drift, repeat — a start-tax paid dozens of times a day. Part-load hours outnumber peak hours almost everywhere, which is why inverter minisplits and VRF systems dominate the market.
Common pitfall
Sizing for the hottest hour "to be safe." An oversized on/off unit short-cycles: its runs are too brief for the coil to condense much moisture, so it cools the air sensibly while humidity stays high — the classic cold-clammy room — and the start transients that wear out compressors stack up. An oversized inverter unit fares better (it simply cruises very slowly), but you paid for capacity that never gets used. Size to the load's typical band, not its once-a-year peak.