Components / Storage and backup hardware / AC-coupled battery architecture

C-012·Components / Storage and backup hardware

AC-coupled battery architecture

Retrofit storage using a separate battery inverter.

An AC-coupled battery system connects the battery to the home through its own inverter/charger, wired on the AC side of the electrical installation alongside the existing solar inverter. The solar inverter and the battery inverter are separate units that share the same AC bus, and each runs its own power conversion independently. It is the usual way to add storage to a solar array that was installed without a battery, because the existing panels, wiring and solar inverter all stay in place.

In a DC-coupled system, by contrast, a single hybrid inverter handles both the panels and the battery, and the battery connects on the DC side before the electricity is converted to AC. Because AC coupling works with any existing solar inverter regardless of make, model or age, it is the common approach for retrofits, put by one industry estimate at 60 to 70% of retrofit projects.

The three-stage conversion and its efficiency cost

Storing solar energy in an AC-coupled system passes the electricity through three conversions. The panels produce DC. The solar inverter converts that DC to AC for the house. Any surplus AC is then converted back to DC by the battery inverter for storage, and converted to AC again when the battery discharges.

Each conversion loses roughly 2 to 5% of the energy passing through it, so the round trip from panel to battery to load loses somewhere around 6 to 15% overall. This is sometimes called the triple conversion penalty, because the same energy is converted three times.

Round-trip efficiency for AC-coupled systems clusters around 88 to 94%, against roughly 92 to 98% for DC-coupled systems that store the panels’ DC directly. The exact figure depends on the equipment and how it is measured, so the ranges are indicative rather than a fixed number. A single inverter’s efficiency is a different measure again: GivEnergy quotes 96.5% Euro efficiency for its AC Coupled Inverter, but that is the inverter’s own conversion efficiency, not the whole system’s round trip, which also carries the solar inverter’s losses and the battery’s internal resistance.

The energy lost to these conversions is daytime surplus solar, which is low-value: worth a few pence per kWh under the SEG, or nothing at all if the system is export-limited. For a household storing 5 to 10 kWh a day, the efficiency gap works out at roughly 0.25 to 0.8 kWh a day. In absolute terms the gap is small, though DC coupling’s higher round-trip efficiency is still a real advantage.

When AC coupling fits and when DC coupling wins

AC coupling’s main advantage is fit. It works with any existing solar inverter, needs no change to the existing solar wiring and often goes in as a one-day retrofit. The two inverters also operate independently, so if one fails the other keeps working, and each can be replaced or upgraded on its own. The battery can sit some distance from the panels, because it connects to the AC circuit rather than the panel strings. Charging the battery from the grid to arbitrage a cheap overnight tariff is standard on AC-coupled inverters.

Against that sit the lower round-trip efficiency above, a second inverter to buy and house, plus a limit on how much solar the battery inverter can be paired with (below). A DC-coupled hybrid inverter can also capture solar that would otherwise be clipped when the array generates more than the inverter’s AC rating; an AC-coupled battery sits downstream of the solar inverter and cannot recover that clipped energy.

Parts of an AC-coupled system

The battery inverter/charger is bi-directional: it converts AC to DC to charge the battery and DC to AC to discharge. Residential UK units are typically 3 to 5 kW.

A current transformer (CT) clamp clips around the main incoming supply cable and measures import and export. The battery inverter reads it to decide when to charge and when to discharge. A CT clamp fitted in the wrong position or the wrong orientation makes the battery charge or discharge at the wrong times.

A gateway or interface unit handles the system’s communication with the grid connection, the battery inverter and the owner’s monitoring app, and may also do the metering, export limiting and tariff scheduling.

The existing solar inverter stays in place and runs as before. Its type, whether a string inverter, microinverter or optimiser-based system, affects whether solar can keep generating during a power cut.

Battery modules in current UK installations are usually LFP and come in stackable increments. Historical GivEnergy systems used 2.6, 5.2 and 9.5 kWh modules, while Fox ESS is one current example using 2.88 kWh modules.

How the two inverters coordinate without talking to each other

In normal grid-tied operation the coordination is simple. The battery inverter watches net import and export through its CT clamp: it charges from surplus solar when the panels are producing more than the house needs and discharges to cover the shortfall when they are not.

During a power cut the arrangement is different. The battery inverter, or its gateway, first disconnects the home from the grid, then switches to grid-forming mode: its electronics generate a local AC waveform at 50 Hz, the UK mains frequency. To the solar inverter this synthetic supply looks like the grid, so it wakes up and starts generating again.

The battery inverter has no digital link to a third-party solar inverter, so once the battery is full it needs another way to tell the solar inverter to ease off. It uses the frequency itself, a technique called Frequency Shift Power Control. Below about 95% state of charge, the battery inverter holds a steady 50.0 Hz and the solar inverter runs at full output. As the battery approaches full and house demand drops, the battery inverter deliberately drifts the frequency upward, to somewhere around 50.2 to 51.0 Hz; a solar inverter that follows a frequency-watt curve reads this and throttles its output in proportion. If generation still overwhelms the system, the frequency is pushed past a hard limit, around 52 to 53 Hz, which the solar inverter reads as a grid fault and shuts down entirely.

A solar inverter that does not follow the battery inverter’s intended frequency-watt behaviour may disconnect instead of reducing output smoothly. It can then wait for its reconnect delay, restart and disconnect again, an oscillation often called hunting. Compatibility in island mode must therefore come from the battery-system manufacturer’s approved equipment and design rules, not from the fact that both inverters work independently on-grid.

Sizing solar to the battery inverter: the Factor 1.0 rule

Victron sets out a sizing rule for AC-coupled systems it calls Factor 1.0: the connected solar power should be no greater than the VA rating of the battery inverter/charger. Its worked example pairs a 3,000 VA unit with 3,000 Wp of solar. The rule is applied to the solar inverter’s rating, not the installed panel capacity, so an oversized array where panel Wp exceeds the inverter rating is acceptable.

The constraint exists because of what happens in island mode. If a large load suddenly switches off, all the solar power has to be absorbed by the battery until the frequency shift can throttle the solar inverter. With the battery near full, the resulting voltage spike can damage the inverter and connected equipment. The rule is specific to Victron equipment, but the problem it addresses, a battery inverter having to soak up whatever the panels produce the instant a load drops, applies to any AC-coupled microgrid, even where a manufacturer manages it differently.

Backup power in a power cut

Backup is not automatic in an AC-coupled retrofit. Many battery systems provide no backup at all unless an EPS is specifically designed in, which needs a gateway or transfer switch in the product and a system design that specifies it.

Backup usually takes one of two forms. The common one is an essential-loads circuit: a separate protected circuit wired to lights, sockets, the router and the fridge, energised from the battery when the grid drops. Whole-home backup is more involved, needing a full transfer switch and enough inverter capacity to carry the whole installation.

When the grid fails, the gateway or transfer switch isolates the home, the battery inverter forms its 50 Hz microgrid, and if the solar inverter supports frequency-watt curves it can carry on generating into that microgrid, so solar and battery together supply the backed-up circuits. Whether the solar keeps running is product-specific.

Switchover speed varies. Some gateways transfer in 10 to 20 milliseconds, fast enough that sensitive electronics never notice; others take 2 to 3 seconds, long enough to reset clocks and drop equipment that cannot ride through the gap.

The Tesla Powerwall 3 provides whole-home backup through its Backup Gateway 2. The Fox ESS AC1 supports EPS through a separate backup circuit. The GivEnergy AC Coupled Inverter gives about 2.5 kW of backup, but with its non-hybrid inverter the solar array cannot generate during a power cut, so backup runs from the battery alone.

DNO notification: G98, G99 and export limiting

Adding a battery inverter changes what the installation can push onto the grid, so it changes the notification owed to the DNO. What counts is the combined export capacity of the solar inverter and the battery inverter.

G98 is the fit-and-inform route. It applies when combined export capacity is 3.68 kW per phase (16 A) or less: the install can go ahead and the DNO is notified within 28 days, normally free. G99 is prior approval, required when combined export capacity exceeds 3.68 kW per phase. It needs documentation, including the inverter’s ENA Type Test Certificate, and approval before the work, typically 4 to 8 weeks.

Many retrofits cross the threshold. A 3.6 kW solar inverter plus a 3 kW battery inverter has a combined rating above 3.68 kW, even though in practice the battery inverter cannot export more than the panels are generating at that moment; DNOs generally assess the worst-case combined rating. One way to stay under the limit is G100 export limitation, where the battery inverter is configured to cap total export and hold the system within the G98 or G99 threshold.

Feed-in Tariff generation payments continue

Adding a battery does not change the Feed-in Tariff (FiT) generation payments. The generation tariff is paid on total solar output, which the battery does not alter. In an AC-coupled install, Ofgem’s scenario 4.1, the battery sits after the generation meter with its own inverter, so the meter keeps measuring solar output alone and generation payments continue unchanged. A bi-directional meter is recommended but not always required.

A FiT system can move its export payments to the SEG while keeping the FiT generation payments. The FiT provider does need to be told about the battery. Some installers are wary of touching FiT systems, so finding one with the relevant experience can take effort.

Standards covering a UK battery install

The MCS battery standard, MIS 3012:2025, covers design and installation of electrical energy storage systems (EESS) up to 50 kW and sets four EESS classes by how integrated the system is. An AC-coupled system built from a separate inverter and battery, often from different manufacturers, typically falls in class 3 or 4.

PAS 63100:2024 is the fire-protection standard for domestic battery storage. It calls for installation outside living areas, avoiding lofts, keeping clearances and integrating fire detection.

Adding a battery is notifiable electrical work under Part P of the Building Regulations, covering the inverter, its wiring and the connection to the consumer unit. The IET (Institution of Engineering and Technology) Code of Practice for Electrical Energy Storage Systems is the industry design and installation guidance; its third edition covers electrochemical storage and both AC and DC low-voltage systems.

AC-coupled products on the UK market

The withdrawn GivEnergy AC Coupled Inverter was specified with a 3 kW AC output, 96.5% Euro efficiency, a 45 to 58 V battery range and an IP65 enclosure, paired with 2.6, 5.2 or 9.5 kWh LFP modules. Its original literature stated a 12-year warranty. GivEnergy Ltd ceased trading in April 2026 and its administrator says the company will honour no further hardware warranties, so that period is an archived product term rather than live cover.

The Fox ESS AC1 is a single-phase AC charger inverter, 5.0 kW in the first generation and 3 kW in the second (AC1 G2), IP65 rated and working with Fox ESS high-voltage battery modules, expandable across roughly 20 to 41 kWh. It ships with a CT clamp and a WiFi dongle.

The Tesla Powerwall 3 holds 13.5 kWh and has a built-in inverter that can be AC- or DC-coupled, with whole-home backup through Backup Gateway 2. It can be fitted as AC-coupled storage on its own, without solar.

Other AC-coupled products on the UK market include the Enphase IQ Battery 5P, Sigenergy SigenStor, Pylontech Force-H2, LuxPowerTek and Hanchu ESS.

Applies to

Solar, Battery

Last reviewed

22 Jul 2026