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

C-013·Components / Storage and backup hardware

DC-coupled battery architecture

Shared solar inverter storage, conversion efficiency and backup constraints.

DC-coupled battery architecture connects the battery to the DC side of a solar system, sharing the same DC bus as the panels, before any conversion to AC. Solar electricity charges the battery as DC. It is converted to AC only when stored energy is drawn for the home or exported to the grid.

The power path runs from the panels through an MPPT input, then either into the battery for storage or through the inverter’s DC-to-AC stage for immediate use. In a home system this is usually handled by a single hybrid inverter that contains the MPPT inputs, a bidirectional DC-DC converter that steps solar voltage to the battery and back and the DC-to-AC stage that feeds the house and the grid. This is why DC coupling is often called a single-box arrangement, against the two-box arrangement of AC coupling with its separate solar inverter and battery inverter.

How the conversion count compares with AC coupling

Where the battery sits changes how many times the electricity is converted. In a DC-coupled system, solar charging the battery involves no DC-to-AC conversion; the energy stays as DC from panel to cell. One conversion happens on the way out, when the battery discharges to AC for the home. An AC-coupled system converts the same stored solar energy three times: DC to AC at the solar inverter, AC back to DC at the battery inverter to charge, then DC to AC again on discharge. Each conversion loses roughly 2 to 5%.

Not every DC-coupled design removes a conversion. Where the battery connects to the DC bus through a bidirectional DC-DC converter rather than sitting at the bus voltage directly, the conversion count is close to AC coupling. The efficiency advantage comes from the battery being charged and discharged at the shared bus voltage, which is how most residential hybrid inverters are built.

Round-trip efficiency and what it is worth

Round-trip efficiency is the share of energy put into the battery that comes back out. DC-coupled storage is generally quoted between 92% and 98%, against 85% to 94% for AC coupling. UK figures tend to sit narrower, at roughly 92 to 96% for DC coupling and 88 to 92% for AC. Real figures depend on the specific inverter and battery, so the ranges matter more than any single number.

Two different measurements circulate under the same name. A DC round-trip efficiency of 95 to 98% describes the battery cells alone and excludes inverter, cooling and standby losses. An AC round-trip efficiency of 85 to 92% measures the whole system as installed. Comparing a cell figure against a system figure overstates the gap, so it is worth knowing which one a specification quotes.

Whether the efficiency edge is worth much depends on what the recovered energy would otherwise earn. Where the alternative is exporting daytime surplus at a low rate, the saving from avoiding a few percent of conversion loss is modest. Where the stored energy displaces expensive imported electricity, it counts for more.

Clipping recapture on oversized arrays

When a solar array can produce more than the inverter’s AC output rating, the excess is normally clipped and lost. A DC-coupled system can divert that surplus into the battery instead, because power going to the battery is DC and does not count against the inverter’s AC output limit. On heavily oversized arrays this recapture can add around 8 to 12% to annual yield. AC-coupled systems cannot do this, because the battery sits on the AC side, after the solar inverter has already capped its output. DC coupling therefore suits arrays deliberately sized larger than the inverter.

High-voltage and low-voltage batteries

DC-coupled systems run the battery at either high or low voltage, set by the inverter’s battery voltage range. Most modern UK home hybrid inverters are high-voltage, with the battery running above about 300V. The GivEnergy Gen 3, for example, works across a 120 to 510V battery range with a nominal 340V. Higher voltage means lower current for the same power, which reduces resistive heat loss in the cables, needs fewer battery strings in parallel and allows thinner cabling.

Low-voltage designs, at 12, 24 or 48V, are cheaper for small systems and easier to work on, but reach a given capacity with more batteries in parallel and carry higher current, so they need thicker cable and lose more to resistance. Low voltage is more common in off-grid and self-build systems than in mainstream UK home installations.

Backup power and EPS behaviour

A DC-coupled hybrid inverter often includes an EPS function that keeps selected circuits running when the grid fails. When the inverter detects the outage it disconnects the home from the grid and runs it from the battery in island mode. Switch time varies by model; the GivEnergy Gen 3 quotes 10 milliseconds, fast enough that most electronics do not register the break.

Not all hybrid inverters include EPS, and some need an external gateway unit to provide it. The backup is limited by the inverter’s rated power, so it covers selected circuits rather than the whole home, and that same rating caps both solar conversion and battery discharge. Where an AC-coupled system pairs a separate solar inverter and battery inverter, their outputs can combine, so its backup ceiling is not tied to a single unit’s rating in the same way. A DC-coupled hybrid inverter can usually keep generating solar during an outage, because the battery gives the MPPT the voltage reference it needs to run; AC-coupled systems need extra hardware to do the same.

The single point of failure

If the hybrid inverter fails, both solar generation and battery storage stop, because one unit does both jobs. An AC-coupled system splits the work between a solar inverter and a battery inverter, so a failure in one can leave the other running. The trade-off runs the other way too: two inverters mean two things that can fail and more to configure. Neither arrangement is simply more reliable than the other.

Charging the battery from the grid

Charging the battery from the grid, to fill it on a cheap overnight rate, depends on the hybrid inverter model rather than on DC coupling itself. Many current inverters support it; some are solar-charge-only or need particular firmware or settings. It is worth checking the specific product, because the claim that DC-coupled batteries cannot be grid-charged is common but wrong as a general rule. AC-coupled batteries all support grid charging, since they connect on the AC side where grid power is already available.

Matching the battery to the inverter

A DC-coupled battery has to match the inverter’s DC voltage range, which limits which batteries can pair with which inverter. AC-coupled batteries connect through a standard AC interface and are more interchangeable; DC-coupled batteries must be designed for the inverter’s high-voltage bus, commonly in the 300 to 600V region. In practice this often means buying the battery and inverter from the same manufacturer or from an approved compatibility list. A buyer cannot assume any battery will work with any hybrid inverter.

New installations versus retrofits

DC coupling is usually the neater choice for a new combined solar and battery installation, where a single hybrid inverter can be specified from the start, and for systems where efficiency or clipping recapture on an oversized array is worth chasing. Adding a battery to an existing solar system is the harder case: the existing solar inverter generally has to be removed and replaced with a hybrid unit, and the array rewired to suit. Most battery retrofits therefore use AC coupling, which leaves the working solar inverter in place and adds a separate battery inverter alongside it. On hardware alone a single hybrid inverter is usually cheaper than two separate inverters, though a retrofit can cost more overall because the swap scraps a working solar inverter.

UK notification and certification

Adding a DC-coupled battery changes the connection to the grid, so the DNO has to be told. For an inverter up to 3.68 kW per phase this is a G98 notification, the connect-and-notify route, submitted within 28 days of install. Above 3.68 kW per phase it is a G99 application, which needs approval before the work goes ahead; on a three-phase supply the combined limit before G99 applies is 11.04 kW. Swapping a string inverter for a hybrid one counts as a change and needs notifying either way. Thresholds and processing vary between operators, so confirm the route with your DNO.

Battery installation in the UK falls under the MCS Battery Installation Standard, MIS 3012, which covers systems up to 50 kW. It classifies systems by how the components are packaged: a hybrid inverter with matching battery modules from the same manufacturer, linked by a DC cable, is typically a Class 2 system, while the same arrangement using a battery and inverter from different manufacturers falls under Class 3. The work also creates new circuits in the home, so it is notifiable under Part P of the Building Regulations, usually self-certified through a Competent Person Scheme. Wiring sits under BS 7671, the IET (Institution of Engineering and Technology) Wiring Regulations, with the IET Code of Practice for Electrical Energy Storage Systems as the specific guidance.

UK hybrid inverter products

Several hybrid inverters sold in the UK use DC-coupled architecture, among them the SolaX X-Hybrid, GoodWe ET Plus, Sungrow SH, Fox ESS H1 and H3, Huawei SUN2000 paired with the Luna2000 battery, SolarEdge Home Hub and Tesla Powerwall 3. The withdrawn GivEnergy All-in-One and Gen 3 used the same architecture and remain relevant to installed systems. Some products need a specific accessory for backup, such as the SolarEdge Backup Interface or Fox ESS EPS-BOX gateway. Model line-ups change, so check the current specification and support position for any equipment being considered.

Applies to

Solar, Battery

Last reviewed

22 Jul 2026