AC or DC coupled: which battery arrangement, and when it matters

What the two arrangements are

Solar panels produce direct current. Your house runs on alternating current. A battery stores direct current. So somewhere between the roof and the socket there is a conversion, and the two arrangements differ in where the battery joins.

DC coupled. The battery sits on the DC side and shares an inverter with the panels — generally a hybrid inverter, which handles both. Solar charging the battery never becomes AC at all.

AC coupled. The battery has its own inverter and connects on the mains side of the existing system. Solar generation is converted to AC by the solar inverter, then converted back to DC by the battery’s inverter to charge, then back to AC to be used.

Ofgem’s co-location guidance describes the same distinction in the language that matters for payments — whether the storage sits before or after the generation meter — which is covered on will a battery affect my FIT or SEG payments?.

When each one wins

The efficiency argument, in proportion

Every DC-coupled sales pitch makes the same point: AC coupling converts the energy twice more, so it loses more.

That is true. It is also smaller than it sounds, for a reason people skip: the loss applies only to the energy that goes through the battery, not to the system’s output. Generation you use as it is produced never touches the battery under either arrangement. Exported generation never touches it either.

So the penalty is a few percent of a minority of your generation. Set that against the cost of replacing a working inverter, and it rarely decides anything. It is a good tie-breaker and a poor headline.

There is a second-order point worth knowing: an AC-coupled battery can be charged from the grid as readily as from the sun, which matters if you are on a time-of-use tariff — and which has consequences for export payments.

The grid connection consequence

This one does get missed, and it is not optional.

Storage counts towards the site’s registered capacity under the Energy Networks Association’s connection recommendations. Adding an AC-coupled battery with its own inverter to an existing array increases the total, and can push a site that was comfortably inside G98 past the 16 A per phase threshold — 3.68 kW single-phase.

That does not mean you cannot do it. G99 contains lighter Small Generation Installation procedures for exactly this case, and under SGI-1, where every unit is limited so the total registered capacity stays at or below 16 A per phase, you may install and commission first and notify afterwards. Export limitation under G100 is what buys you the simpler route where the equipment totals more than that.

The full picture, including which procedure applies and how long the operator has to respond, is on G98 and G99.

Questions to put in writing

• Which arrangement is proposed, and why that one for this house?
• If a hybrid inverter: what is the premium over a solar-only inverter of the same output?
• If AC-coupled: what is the battery inverter’s power rating, and what does that do to the site’s registered capacity?
• Which G98 or G99 procedure applies, and is the work notify-after or permission-first?
• Where will the generation meter sit relative to the battery, and will it be bi-directional?
• Can the battery charge from the grid, and is that configurable after installation?
• Is backup during a power cut included, excluded, or available at extra cost?

MIS 3002 requires the datasheet for any electrical energy storage system as part of the minimum technical information an installer gives you. Ask for it, and read the numbers on it rather than the ones in the brochure — what a battery specification should tell you.