Fox ESS KH and KA hybrid inverters: models, specifications and warranty

The Fox ESS KH and KA are single-phase hybrid inverters, sold in 7, 8, 9 and 10kW options¹. A hybrid inverter does two jobs in one box: it converts the direct current from a solar array into mains alternating current, and it manages a high-voltage battery on the same DC bus, so the battery charges and discharges without a second inverter. Fox ESS lists the KH and KA as pairing with its own high-voltage batteries, including the EP, EQ and CQ ranges¹.

For a household, the practical questions are how much solar the unit can take, what it costs, what the warranty covers, and what happens in a power cut. The answers sit in the maker's datasheets and product pages, and they are specific to each rating in the range.

KH and KA at a glance: single-phase hybrids from 7 to 10kW

The KH and KA series is described by Fox ESS as a single-phase hybrid inverter, available in 7, 8, 9 and 10kW options¹. The KA models sit alongside the KH at the same nominal ratings, and the two share a datasheet, so the differences between them are in the detail of the specification rather than in the power class.

The ratings matter because they set what the inverter can deliver to the house and to the grid. The KH10.5 and KA10.5 are listed with a nominal AC power of 10500 VA, and the same figure for maximum EPS power³. That is the ceiling on what the unit can supply when running on battery alone during an outage.

Single-phase is the ordinary domestic arrangement in the UK, and it is the segment most hybrid inverters are built for. Fox ESS sells three-phase hybrid equipment separately, in the H3 range, which its datasheet labels a three-phase hybrid and AC inverter⁵. A home with a three-phase supply is therefore looking at a different product family, not a variant of the KH.

The wider market for single-phase hybrids runs to similar ratings. Sungrow lists an 8 to 10kW single-phase hybrid in its SH8.0/10RS⁶, and SolaX offers single-phase hybrids up to 7.5kW⁷. The KH and KA sit at the upper end of that domestic band, which is where a household with a large array and a sizeable battery tends to land.

The KH range: models, array sizes and input power

The KH range is best read as one platform at several array sizes. The KH7 is listed with a maximum input power of 10500 W and a maximum array power of 15000 Wp². The KH8 takes 16000 Wp, the KH9 takes 18000 Wp, and the KH10 and KH10.5 take 20000 Wp and 21000 Wp respectively².

Two different figures are in play here, and they are not the same thing. Maximum input power is the DC power the inverter's input stage will accept. Maximum array power is the nameplate capacity of panels that can be connected, which is deliberately allowed to exceed the input rating so the array still produces useful output in less than perfect conditions. On the KH9.9, KH10 and KH10.5 the maximum input power is 15000 W, while the array power runs higher².

The shared electrical envelope is consistent across the range. The rated grid frequency is 50 or 60 Hz with a tolerance of plus or minus 5², and the EPS rated voltage is 220, 230 or 240 V at 50 or 60 Hz³. Displacement power factor runs from 0.8 leading to 0.8 lagging, and parallel operation is supported up to a maximum of ten units³.

EPS peak power differs by model, which matters if backup is the reason for choosing a particular rating. The KH7 and KA7 are listed at 10000 W for 60 seconds, the KH8 and KA8 at 10000 W, and the KH9, KA9, KH10, KA10, KH10.5 and KA10.5 at 12000 W³. That short-term headroom is what lets a motor, such as a pump or a fridge compressor, start on backup power.

What the KH costs in the UK

There is no manufacturer list price for a Fox ESS KH inverter. What exists is distributor pricing for the unit alone, and it varies between suppliers and between the ex-VAT and inc-VAT figures they publish. For the KH7, one distributor lists £960.00 plus VAT while another lists £1,055.88 including VAT¹. For the KH8, the figures are £975.00 plus VAT and £911.28 ex VAT¹. For the KH9, £1,040.00 plus VAT against £1,111.27 including VAT¹. For the KH10, £1,075.00 plus VAT against £1,040.00 plus VAT¹.

Those pairs do not reconcile, and the figures differ between listings. They are also not like for like: some are quoted ex VAT and some inclusive, and they come from different distributors on different dates. Read them as an indication of the order of magnitude for the hardware, not as a price a household will pay.

The price a household actually sees is the installed price, which is quoted by the installer after a survey and includes the inverter, mounting, DC and AC cabling, isolation, commissioning, and the notification to the distribution network operator. None of that is in the hardware figures above. Prices for a full hybrid inverter installation are installer-quoted, and no published installed price for a Fox ESS KH system is available.

VAT treatment is not stated in the distributor figures, so they should be read as quoted rather than as confirmed VAT-inclusive prices. For the wider cost picture on a complete system, how much solar panels cost in the UK sets out the components and where the money goes.

Specifications that matter: communications, AFCI and smart tariffs

Fox ESS lists the KH and KA series with integrated WiFi and LAN¹. That is the connection the inverter uses to report generation, consumption and battery state to the monitoring platform, and it is built in rather than added as a separate dongle.

Arc fault protection is optional rather than standard. The datasheet lists AFCI protection as optional³. AFCI is the device that detects the signature of a series arc in DC wiring and shuts the system down, and it is a meaningful safety feature on a DC circuit that cannot be switched off at a socket. Whether a given installation includes it depends on the specification agreed with the installer and on the applicable wiring regulations.

Fox ESS describes the KH and KA series as smart tariff compatible¹. That is a statement about capability. Whether a particular time-of-use or export tariff can actually control the inverter depends on the tariff's own approved hardware list and on the battery and gateway in the system. The Energy Saving Trust has published work on combining clean technologies with smart tariffs, based on analysis of multiple tariff types using 2024 energy price data⁸. The Retail Energy Code Company describes a cross-sector programme connecting with the Smart Secure Electricity Systems Tariff Interoperability Standard, the National Energy System Operator's Data Sharing Infrastructure Pilot and Elexon's Flexibility Market Asset Register⁹. Those are the mechanisms through which tariff control of home hardware is being standardised, and they are still developing.

Where KH fits in the Fox ESS ecosystem

The KH and KA are the single-phase hybrid hub of a wider Fox ESS system. On the battery side, they pair with Fox ESS high-voltage batteries including the EP, EQ and CQ ranges¹. The EQ battery range is listed as compatible with Fox ESS hybrid inverters including the H1-G2 and K Series, the H3 Smart and the H3 Pro¹⁰.

On the backup side, the EPS-BOX-SP is a single-phase whole-home backup gateway designed to provide emergency power during grid outages⁴. Fox ESS lists it as working with the H1-G2, KH, EVO and PowerQ inverters, and states that it works with the entire range of single-phase storage solutions⁴. The company describes it as supporting full-home backup, subject to system sizing, and as isolating the property from the grid and, after a short transfer delay, restoring power using battery and solar⁴. A press release at launch said it automatically switches to backup power in the event of a grid failure and supports the latest firmware enhancements for smart switching¹¹.

The gateway is wall-mounted with natural cooling and communicates over an RS-485 meter connection⁴. That meter connection is how it knows the grid has failed and how it manages the changeover.

The KH and KA also sit alongside Fox ESS's other single-phase hybrid families. The company's own support material groups its single-phase hybrid inverters at 7kW, 8kW, 9kW and 10kW¹². For the wider picture of how the brand's residential products fit together, Fox ESS inverters: the UK residential range covers the other families, and Fox ESS vs Growatt compares two of the volume brands in this segment.

What owning a KH means for household energy independence

A hybrid inverter is the component that decides how much of a household's energy independence is real. The KH and KA generate from the roof, store in a battery, and, with the EPS-BOX-SP, keep selected circuits live during a grid outage⁴. That is a genuine step away from total reliance on the grid for the hours the battery can cover.

The dependence that remains is worth stating plainly. The system stays connected to the grid, and the inverter is grid-tied: it synchronises to the grid frequency, listed at 50 or 60 Hz with a tolerance of plus or minus 5², and it needs a grid reference to operate normally. Backup is a changeover to an isolated supply, not an off-grid capability, and it is subject to system sizing⁴. The battery is a Fox ESS high-voltage unit from the EP, EQ or CQ ranges¹, so the storage side of the system is tied to one manufacturer's products.

There is also a data dependence. Integrated WiFi and LAN¹ means monitoring and remote visibility run through a home broadband connection and the manufacturer's platform. Local operation continues without it, but the reporting and remote features do not.

On the fuel side, the household remains on mains electricity and on an electricity supplier. The KH does not change the fuel; it changes when and from where the electricity comes. For the broader picture of what a home can and cannot disconnect from, solar panels and household energy independence sets out the limits. Solar Energy UK's position is that more homegrown energy means greater energy independence¹³, and a hybrid inverter is one of the components that delivers it, within the boundaries above.