Solarwatt Battery flex: AC-Coupled Home Battery

The Solarwatt Battery flex AC-1 is a modularly expandable lithium-ion battery storage system for the home, offering 4.8 to 19.2 kWh of usable energy and connecting to the household on the AC side, at a single phase of 230 V and 50 Hz¹. It is built from three kinds of unit: a base module that contains the inverter and control electronics, and middle and top battery packs that stack onto it, each holding 2.4 kWh¹. The modules plug together without any cabling between them, and the whole system is certified to the safety guidelines for Li-ion household battery systems².

The battery was developed in cooperation with the BMW Group, whose battery cells, also used in its electric cars, are assembled by Solarwatt in Dresden⁴. It is designed as an addition to any photovoltaic system, new or existing, which is what makes AC coupling interesting for households that already have panels: the battery does not need the solar inverter to be replaced¹.

For energy independence, the Battery flex shifts more of your solar generation into evening use, with the maker stating self-sufficiency with PV energy of up to 80% when paired with a Solarwatt Manager¹. What remains is a dependence on the grid connection itself, on Solarwatt's Manager and update servers for smart operation, and on a certified installer for supply and fitting.

A modular AC battery, built from three parts

The system's architecture is worth understanding before comparing prices or capacities, because the headline figures come from combining the right units. The Battery flex base AC-1 1.3 is the electronics and control module: a battery inverter with a maximum rated real power of 6.0 kW, a DC voltage range of 25 to 350 V and an AC rated current of 30 A¹. It is the part that talks to the grid and to the household wiring.

The storage itself sits in the middle pack and top pack modules. Each carries 2.4 kWh of usable energy, a rated capacity of 93 Ah and a nominal voltage of 29.2 V². A complete product consists of at least one base module, at least one middle pack and a top pack, and the base accepts 2 to 8 battery modules in series¹. The top pack closes the stack, which is why a system always needs one.

The chemistry is lithium-ion, with a ceramic-coated cell separator and an aluminium housing with an integrated battery fuse¹. The battery management system is described in the product documentation as a UIT supervisor, and the modules connect with a plug-in system that needs no cabling between units².

Capacity: 4.8 to 19.2 kWh, in 2.4 kWh steps

The usable energy of a complete system runs from 4.8 kWh with two modules to 19.2 kWh with eight, and the maker states the system "can be extended in 2,4 kWh-steps"¹. That step size is unusually fine. RCT Power's Battery Premium scales in 2.5 kWh steps⁶, its smaller battery expands in 1.9 kWh steps up to a maximum of 11.5 kWh⁷, and SMA's home storage module scales from 3.2 kWh to 16.4 kWh⁸. The Battery flex's 2.4 kWh increments mean a household can add a single module at a time rather than jumping in larger blocks.

Each module stores 2.4 kWh of usable energy against a total energy content of 2.7 kWh⁵. The distinction between usable and total energy is the same one that applies to any home battery: the usable figure is what the household can actually draw on, and it is the one the warranty is written against. How usable capacity is defined across the market is explained in battery capacity and usable capacity.

For sizing, the range covers a modest household at the bottom end and a large solar home at the top. The maker's own positioning is that the system can raise self-sufficiency with PV energy up to 80% when managed by a Solarwatt Manager¹. What that figure means in practice, and how self-sufficiency is calculated, is set out in solar battery self-sufficiency.

Power, connection and housing

The base unit's maximum rated real power is 6.0 kW, and the delivered power depends on how many modules are fitted². With two modules the system manages a maximum of 1,900 W discharge and 1,700 W charge, at a nominal 1,400 W. With eight modules it reaches 6,000 W maximum on both charge and discharge, with a nominal discharge of 4,800 W⁹. Power ratings and what they mean for running household appliances are covered in battery power ratings.

Communications run through 2x RS485 (RJ11), 1x CAN (RJ45), 2x Ethernet (RJ45), Bluetooth (BTLE) and an LED status display⁹. The base unit is compatible with the EnergyManager pro and the Solarwatt Manager flex for energy management⁹. The maximum efficiency of the top pack is stated at 97.5%², though the round-trip efficiency of the whole system, including the AC conversion, is the more meaningful household figure, as explained in round-trip efficiency.

AC or DC coupling: where the Battery flex fits

The Battery flex is an AC-coupled battery: it connects on the household side of the solar inverter, at 230 V AC, rather than on the DC side of the panels¹. The maker states it "is suitable for existing and new installations"². At the system's launch, the company also described the storage unit as able to be integrated into both the DC and AC circuit⁴, but the product as sold in the UK is specified with an AC grid connection, and the datasheets list it as the AC-1.

The practical difference matters most to households with older systems. DC-coupled batteries are, in the words of one energy supplier's guidance, "more complicated to integrate with a FIT system, but possible", particularly if the inverter is being replaced at the same time, and they require a hybrid inverter and a bi-directional meter¹⁰. An AC-coupled battery such as this one leaves a Feed-in Tariff metering arrangement alone, because generation is still measured where it always was. The full comparison is in AC and DC coupled battery systems.

Solarwatt's own Battery vision, by contrast, is specified as installable "as DC- or AC-battery"¹¹, so the company sells both approaches and the choice between them is a system design question, not a brand question.

Adding the battery to panels you already have

Retrofitting is where an AC-coupled design earns its place. The maker states the Battery flex is suitable for existing and new installations¹, and at launch described it as "suitable for retrofitting into any existing system"⁴. Independent guidance agrees that in most cases battery storage can be retrofitted to an existing solar PV system, though some setups may need an inverter upgrade¹².

For a household with a working solar installation, the significance is that the panels, their inverter and any generation metering stay as they are. The battery and its base unit are added on the AC side, and the existing system keeps generating whether or not the battery is working. The trade-off is an extra conversion step between the panels and the battery, which costs a little efficiency compared with DC coupling.

What a retrofit cannot do by itself is provide backup in a power cut; that depends on how the system is wired and on what changeover arrangements exist, as covered in battery backup power and backup interfaces.

German engineering: BMW battery cells built in Dresden

The Battery flex's origin story is unusual among home batteries. The battery modules are manufactured by Solarwatt in close cooperation with the BMW Group, and the BMW Group supplies Solarwatt with high-quality battery components that are also used in the group's current electric vehicles⁴. Solarwatt obtains original components, such as the battery cells, directly from BMW¹³, and the final assembly of the Battery flex takes place in Dresden¹³.

The datasheets carry the phrase "quality Made in Germany"¹, and the same claim appears on Solarwatt's solar panels sold in the UK¹⁴. For a household, the practical points are supply chain and service: cells sourced from a major automotive maker, and assembly within the company's own Dresden facilities rather than contracted out.

The chemistry is lithium-ion rather than the LFP chemistry several competitors now favour, which is a difference in cell behaviour worth understanding alongside the warranty terms. The chemistries sold in the UK are compared in home battery chemistries.

Warranty: 10 years, registration and exclusions

The Battery flex carries a 10-year product warranty after successful warranty activation, with a performance promise of at least 80% of the usable energy over the term¹. The cycle service life is stated as an unlimited number of full cycles during the warranty period¹. The base unit also includes five years of FullCoverage insurance, with an optional extension to ten years¹.

Registration is the condition that turns the paper promise into an active warranty. The warranty terms state the product "has been registered at www.solarwatt.com within three months of the initial operation date"³. Miss the deadline and the cover is at risk. This is not unique to Solarwatt: SolaX's warranty guidance states that if a system is not registered within 90 days, the warranty automatically starts 6 months after the battery left the factory¹⁵. The conditions across the market are compared in home battery warranties.

Two further points from the warranty document: the warranty transfers to a new owner to the extent of the remaining period if the house is sold³, and the Manager units carry a shorter, separate warranty of two years from the date of invoice, capped at two years and six months after shipment from the factory¹⁶. A 10-year battery paired with a 2-year Manager is a distinction worth holding onto.

Installation, weight and siting

Each battery module weighs 25 kg, and the maker states single-person installation is possible at that weight¹. The base unit is lighter at 23 kg². The system is designed for wall installation, with optional floor mounting¹, and the modules plug together without cabling². For comparison, Growatt's APX battery set weighs about 250 kg in five 50 kg modules¹⁷, so the Battery flex's per-module weight is at the manageable end of the market.

The protection rating is IP54¹, which means the housing is protected against dust and splashed water. The datasheet lists outdoor installation as possible up to 2,000 m above sea level, according to the installation instructions¹. The Manager flex is only IP20¹, so that unit must stay indoors. Where a battery may legally and safely be placed, and how the UK nations differ on planning, is covered in battery siting and location and the planning pages for England, Scotland, Wales and Northern Ireland.

Installation itself must follow the standards governing home battery work, including grid notification under G98 and G99, which are explained in installation standards and grid connection rules.

The Manager, its AC Sensor Flex and over the air updates

The Solarwatt Manager is the energy management unit, and the maker's own positioning ties the battery's headline benefit to it: self-sufficiency with PV energy up to 80% with a Manager¹. Both the Manager flex 1.0 and the EnergyManager pro are listed as Battery flex ready¹⁸, and the Manager flex 1.0's supported storage list includes the Battery flex AC-1 alongside a VARTA battery¹⁸. The Manager flex is a light unit at 0.22 kg, wall-mounted, drawing a nominal 3 W¹.

The AC Sensor Flex is a measuring device, made in Germany, listed as supporting the Battery flex AC-1¹⁹, and it appears in the battery pack's own documentation among its supported devices⁵. Its role is to measure household energy flows so the Manager can decide when to charge and discharge.

Software updates arrive over the internet. The datasheets list the battery as remotely updatable¹, the Manager receives firmware and app updates via an update server²⁰, and the company has stated that "the latest software version and new functionalities are always automatically uploaded using an existing Internet connection"⁴. This is the clearest dependence the system creates beyond the grid: smart operation relies on home broadband and on Solarwatt's servers remaining available for the ten-year warranty term. The trade-offs are discussed in battery monitoring and apps.

UK availability and the installer route

The Battery flex was announced as available from Solarwatt-certified premium installers from autumn 2021⁴. Solarwatt established a UK subsidiary with an office and a warehouse in England to serve installers and customers²¹, so stock and support are routed through the company's own UK operation rather than through general distributors.

There is no published list price for the Battery flex, so costs are installer-quoted and depend on the number of modules and the siting work involved. How UK battery prices are built up, and what drives a quote, is set out in home battery cost; the VAT position on battery storage is covered separately in the VAT rate page.

For context on routes to market, UK buyers elsewhere can get some batteries direct: Which? notes the Tesla Powerwall is sold direct from Tesla, while the Enphase AC Battery is available via UK installers²². The Battery flex sits firmly in the installer channel.

How the Battery flex is positioned against rivals

Against the wider field, the Battery flex's distinguishing features are its 2.4 kWh expansion steps, its BMW-sourced cells and its German assembly. RCT Power sells a Battery Flex of its own with a 10-year warranty²² and a Battery Premium scaling in 2.5 kWh steps⁶; SMA's module scales from 3.2 kWh to 16.4 kWh⁸; and Solarwatt's own Battery vision offers 5.2 to 18.2 kWh in a single-phase version, installable as a DC or AC battery, with a 12-year performance warranty on its packs¹¹. Ten-year warranties are the market norm, with Tesla's Powerwall and SolaX systems both at 10 years²⁰.

For a household weighing energy independence, the honest summary is this: the Battery flex stores solar generation for evening use and can be added to an existing installation without touching the panels or their inverter, but it does not take a home off the grid. It depends on the grid connection, on Solarwatt's Manager and update servers for its smart functions, on a certified installer for supply, and on registration within three months for the warranty to bite. The broader picture of what a battery can and cannot do for a home is in home batteries and energy independence, and the Solarwatt range as a whole is covered in Solarwatt home batteries.