AC-coupled vs DC-coupled battery storage: which suits a business?

AC-coupled vs DC-coupled battery storage is a choice about where a battery connects to a site’s solar. In an AC-coupled system the battery has its own inverter and connects on the alternating-current side, next to the solar inverter. In a DC-coupled system the battery connects on the direct-current side, to an inverter it shares with the solar panels, usually called a hybrid inverter. Which suits a business depends mainly on whether the array already exists, how large it is compared with its inverter, and what the installer and the network operator need to see.

AC-coupled and DC-coupled side by side

Both arrangements are covered by the same UK installation standard. The IET Code of Practice for Electrical Energy Storage Systems, 3rd edition, covers low-voltage systems that are AC- or DC-coupled, in commercial and industrial buildings as well as dwellings, so the choice is a design decision and not a compliance one.

How an AC-coupled system works

The solar panels produce direct current and the solar inverter converts it to alternating current for the site. The battery sits on the site’s AC wiring with its own inverter, which converts AC to DC to charge the battery and DC back to AC to discharge it. Nothing about the solar side has to change, so this is the option that fits an array that already works.

The cost of that independence is conversions. Solar energy that goes into the battery is converted from DC to AC at the solar inverter, from AC to DC to charge, and from DC to AC again when it is used. Each step loses a little energy.

How a DC-coupled system works

In a DC-coupled system the battery connects to the DC side of an inverter that also serves the solar panels. Solar energy can go into the battery without first being converted to AC: it passes through a DC-to-DC converter on the way in and is inverted once, on the way out, when the inverter supplies the site. One device then manages both the solar and the battery.

Fewer inverter stages does not mean a single conversion. In a 2019 worked example from the US Electric Power Research Institute (EPRI), at utility scale, the AC-coupled chain had three inverter stages and three transformer stages, and the DC-coupled chain had two DC-to-DC stages, one inverter and one transformer. With assumed efficiencies of 98% for each inverter and for the battery and 99% for each transformer and DC-to-DC converter, less 2% for the plant’s own load, the example gives a round-trip efficiency of about 0.87 for the AC-coupled chain and about 0.91 for the DC-coupled one. These are assumptions in an illustration, not measurements of real equipment. EPRI’s own text says only that DC-coupled systems may be slightly more efficient when storing solar energy, and that round-trip efficiency usually matters less to lifetime project economics than other costs and benefits.

So treat any single efficiency figure with caution, and ask each bidder for the measured round-trip efficiency at the power levels the battery will actually run at, and how it was measured. Our reading of six manufacturers’ datasheets, in round-trip efficiency on commercial battery datasheets, shows how differently the figure is stated: some give a battery-only figure, one includes all conversion and cooling losses, and two give none.

Adding a battery to solar you already have

If the array is installed and working, the question is whether the existing inverter can take a battery on its DC side. Some inverters have a battery port and some do not, and it is on the datasheet. If it cannot, there are two ways forward: AC-couple a separate battery system and leave the array alone, or replace the inverter with a hybrid unit. Replacing an inverter that is working, and may still be under warranty, is a larger job than adding a battery beside it, which is why AC coupling is the first option to price for a retrofit.

Two checks apply either way. First, the connection: the existing array has its own network agreement, and the network operator treats electricity storage as generation, so adding a battery means a new or updated application. Second, the inverter warranty: ask the manufacturer or installer whether adding a battery changes the terms of the solar inverter’s warranty.

Clipping, and why array size matters

Solar arrays are sometimes built with more panel capacity than the inverter can pass, because panels rarely produce their full rated output. When the panels produce more than the inverter can convert, the surplus is clipped and lost. The ratio of panel capacity (DC) to inverter rating (AC) is called the DC/AC ratio.

A DC-coupled battery can take energy that would have been clipped. A 2018 modelling study from the US National Renewable Energy Laboratory, using a hypothetical house in Honolulu, found that the DC-connected battery captured energy from the array that inverter limits would otherwise have clipped. At a DC/AC ratio of 1.23 the net present values of the two configurations were within 1% of each other, but at a ratio of 1.85 the DC-connected system increased the net present value by 13%. The study was of a house in a different market, so treat it as an illustration of the mechanism and not a forecast for a UK business. It shows that the advantage depends on how oversized the array is, and is small when the array is roughly matched to its inverter.

A 2021 NREL report gives a sense of scale. It simulated solar with single-axis tracking at about 55,000 locations across the United States and found that at a DC/AC ratio of 1.3 the average recoverable clipped energy was 0.2% of DC energy, with a maximum of about 2.1% at a small number of sites in the south-west, and more at higher ratios. Those are US utility-scale figures and not a UK forecast, but they show why the clipping benefit is small unless the array is well oversized for its inverter. NREL also notes a limit on the other side: a battery behind a shared inverter has its discharge restricted during hours of high solar output, which can reduce its value if those hours coincide with high demand or high prices.

What the network operator sees

The Energy Networks Association’s Distributed Generation Connection Guide is the UK reference, and this page cites the March 2026 edition (Version 3.0). It says electricity storage is classed in GB law as a form of generation, and that network operators treat storage devices as generation when they assess the impact on their network. Storage must meet EREC G98 or EREC G99 depending on its Registered Capacity, and storage devices commissioned after September 2022 need to be fully compliant with whichever applies.

Four points follow for a site with solar:

• Where storage is used with solar to offset consumption, the guide says the total connected generation is likely to be above 16 A per phase, and so G99 applies. Our G99 guide explains the process.
• The application asks whether the storage is on its own or combined with another technology, its technology, its rating and capacity, and its import and export capacities. The guide does not use the terms AC-coupled or DC-coupled and sets out no separate rules for them.
• Registered Capacity, which sets the Type and the requirements, is worked out from inverter ratings. Where units connect through inverters the guide treats the inverter rating as the unit rating, and a module’s Registered Capacity is the lesser of its inverter rating and the rating of its energy source. How the inverters are arranged can therefore affect the figure you declare. The guide’s text does not say how a hybrid inverter is treated, so ask your installer to agree the declared capacity with the network operator. Appendix A of the guide has worked examples.
• The fast-track route for some domestic-scale storage does not apply to storage designed to operate in island mode, for example during an outage, which goes through the standard G99 process. See backup power for what islanding involves. If the connection has an export or import limit, the guide says export and import limiting schemes need to comply with EREC G100.

The guide also sets a newer technical requirement. Storage commissioned on or after 1 March 2026 must switch from import to export mode if the system frequency falls below a defined threshold, while storage commissioned between September 2022 and February 2026 does not have to meet that particular requirement. Ask any bidder to confirm the equipment meets the requirements in force on its commissioning date.

Questions to ask an installer

• Is the array already installed, and does its inverter have a battery port?
• What are the quoted options for AC and DC coupling, and what is the whole-system price for each?
• What round-trip efficiency do you state, at what power, and how was it measured?
• How large is the array compared with its inverter, and how much energy is clipped today?
• What keeps running if the battery inverter fails, and what if the solar inverter does?
• Does adding a battery change the warranty on the existing solar equipment?
• Who submits the G99 application, what Registered Capacity will be declared, and what do you tell the network operator about how the storage is coupled?
• If the connection has an export limit, how is it enforced, and does the scheme comply with EREC G100?
• Which standards does the design follow, such as the IET Code of Practice and BS 7671 Chapter 57?

For the wider decision, read our buying guide to commercial battery storage, and for how solar and storage earn together see solar-plus-storage.