FOX ESS KA9.9 - single-phase AC-coupled inverter 9.9 kW.

FOX ESS KA9.9 is a single-phase AC-coupled inverter rated at 9.9 kW for installation alongside an existing solar plant. High-voltage LiFePO₄ battery 85–480 V, charge and discharge current 50 A, efficiency 97.8%, output current 43.0 A, generator support. Wi-Fi and LAN, 5-year warranty.

How to match storage to an existing installation

The main question when choosing an AC-coupled inverter is this: what capacity should it have relative to the grid-tied unit already installed? The answer is not obvious, and an error here costs real energy.

The logic is straightforward. To take the whole daytime surplus into the battery, the storage inverter must be able to accept as much as the plant delivers at peak, less the house's current consumption.

For a typical 10-kilowatt plant where the house draws 1–2 kilowatts during the day, the midday surplus reaches eight or nine. The KA9.9, at 9.9 kilowatts, accepts that flow in full and restricts nothing.

Had you chosen a unit half the size, part of the energy at sunny midday would simply flow to the grid past the battery. For a house aimed at autonomy that is a daily loss, invisible in the reports but plain enough in the evening from the state of charge.

Nine point nine: approval without complications

The 9.9-kilowatt rating was not chosen at random. Many network operators apply a simplified procedure to installations up to ten kilowatts, and the manufacturer deliberately keeps the model under that line.

In the case of an AC-coupled unit this carries additional weight. You are not changing generation capacity — it stays as it was — but you are adding a new converter to the system, and its parameters also enter the paperwork.

By staying under the threshold you avoid a review of the connection terms: no new design documentation, no additional approvals. In practice that saves weeks.

Technical specifications

• rated power 9.9 kW, single phase;

• type: AC-coupled, connected on the AC side;

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

• charge and discharge current 50 A;

• efficiency 97.8% peak, 97.4% European weighted;

• output current 43.0 A nominal and maximum;

• generator support;

• Wi-Fi and LAN interfaces;

• 5-year manufacturer warranty.

Wired Ethernet, not only Wi-Fi

The unit has both communication channels, and the wired one matters more here than it first appears. An AC-coupled inverter is installed next to the consumer unit — in a boiler room, a cellar, a technical annexe, in other words where the router's signal arrives poorly.

For a storage system, a dropped link means more than gaps in the charts. Scheduled charging scenarios, remote threshold changes, remote diagnosis on a warranty call — all of it requires a stable channel.

An Ethernet cable settles the matter for good. If running twisted pair is awkward, a powerline adapter over the existing wiring is a workable alternative — but a wired link all the same, not Wi-Fi through two concrete walls.

A steady 43 amperes

Nominal and maximum output currents in this model are identical — 43.0 amperes. The unit is designed for continuous operation at exactly that level, without reliance on short-term excursions.

For a storage system this approach is logical. The bulk of a backup group's load is pumps, lighting, refrigerators, electronics and heating circuits — steady consumers without sharp surges.

If something with a heavy start is planned for the backup group — a deep borehole pump, a compressor or a machine tool — it is worth discussing at the design stage. A soft starter costing a few per cent of the system removes the question entirely.

Correctly sized storage takes your entire daytime surplus, not part of it. Send us your plant's capacity and your average daytime consumption: we will check whether this rating is the optimum and size the battery to your evening profile.