FOX ESS KH9 - single-phase hybrid inverter 9 kW.

FOX ESS KH9 is a single-phase hybrid inverter rated at 9 kW with four MPPT inputs and entirely silent operation. High-voltage LiFePO₄ battery 85–480 V, charge and discharge current 50 A, efficiency 97.8%, output current 39.1 A, generator support. IP65, mass 29 kg, 5-year warranty.

Nine kilowatts for a heat pump

A house with a heat pump has a fundamentally different consumption profile from one with a gas boiler. The pump runs around the clock through the whole heating season, and it is most often the reason a hybrid of this class is chosen.

A typical ground-source pump serving a 200-square-metre house draws 2–3 kilowatts continuously and produces a brief surge when the compressor starts. Nine kilowatts of rating gives enough margin for that surge not to upset the system even while an oven and a water heater are running at the same time.

In winter there is little sun, and hoping to cover the pump entirely from panels is unrealistic. But the hybrid solves a different problem: it stores daytime generation and releases it at night, and during an outage it keeps the pump running so the house does not cool down.

Absolute silence

The KH9 specification lists 'silent' as a separate line. This is not marketing: the unit is cooled solely by natural convection, with no fans inside at all.

At nine kilowatts that is a non-trivial engineering decision. The aluminium-alloy enclosure works as a single heatsink, dissipating heat across its whole area. Typical noise is below 30 decibels — quieter than a refrigerator.

The consequence for the owner is twofold. First, the inverter can hang in an entrance hall, on a stairwell or in a corridor next to a bedroom — wherever cabling is convenient. Second, the absence of moving parts removes the commonest cause of failure in power electronics: a seized fan and the overheating that follows.

Technical specifications

• rated power 9 kW, single-phase output;

• four independent MPPT inputs, tracking efficiency 99.9%;

• MPP range 80–500 V, start-up voltage 75 V;

• high-voltage LiFePO₄ battery, 85–480 V;

• charge and discharge current 50 A;

• efficiency 97.8% peak, 97.4% European weighted;

• output current 39.1 A nominal, 43.0 A maximum;

• generator support, THD below 3%;

• IP65, passive cooling, operation from -25 to +60 °C;

• dimensions 450 × 527 × 208 mm, mass 29 kg.

Current headroom: 39.1 against 43.0 amperes

The difference between nominal and maximum output current is nearly four amperes — about ten per cent. That reserve serves a thoroughly concrete purpose.

Inductive loads — compressors, pumps, electric motors — draw several times their rating at the moment of starting. An inverter without headroom goes into protection in that situation, and the refrigerator or borehole pump simply fails to start.

A ten-per-cent overload capability covers most domestic starting currents. For a house with a pump set, air conditioners and a workshop, this is not an abstract figure but the difference between 'it works' and 'it trips once a day'.

Autonomy all year round

The combination of a high-voltage battery and generator support covers the entire annual cycle. In summer the system feeds on sunlight and barely calls on the grid. In autumn and spring the battery smooths evening peaks.

In winter, when daylight is short, a third tier engages: the inverter starts the generator itself at a set battery charge level and stops it once the threshold is reached. No manual intervention is required.

A start-up voltage of 75 volts means conversion begins very early in the morning — in winter every hour of light counts, and a low starting threshold works in your favour.

Nine kilowatts, four MPPTs and complete silence — a configuration for a house with a heat pump or a great deal of equipment. Describe your heating circuit and allocated capacity: we will size the battery and check whether the supply will take the starting currents.