The Truth About AC & DC Coupling

A few years ago I was invited to an “energy roundtable” hosted by the South Australian government. The idea was we’d all sit around and hatch some big ideas to push the energy transition forward. This was peak excitement in SA – Elon had recently cut the ribbon on the Big Battery in Jamestown.

All I remember from it was the state energy minister in charge at the time telling us of one idea suggested to his department that we might consider: changing the default electricity supply from AC to DC. The logic? We’d save the conversion losses when running modern electronics off DC.

This was probably the stupidest thing I’ve ever heard in all my 25 years as an electrical engineer. Yes, converting AC to DC does cost you some efficiency. In the real world about 5-15%. But ripping up an entire state’s grid to claw back that sliver, while simultaneously losing efficiency in all the appliances that actually run on AC, is brain-meltingly dumb. I walked out at the first smoko and never went back.

A flashback of this experience hit me between the eyes while browsing the comments on our latest YouTube video about home batteries.

Critical Comments

We got a few viewers pointing out that the video didn’t mention how AC-coupling “has a LOT more loss” than DC-coupling, because of the extra conversion step.

It’s worth tackling this furphy of inefficient AC-coupled batteries, because this is a favourite trick of some battery salespeople who only sell DC-coupled solutions. They’ll bash AC-coupled systems to steer you toward the model they are set up to sell. The pitch sounds technical, it’s factually correct, and seems important. But just like the Minister’s big DC dream, scratch just below the surface and it’s readily apparent it doesn’t matter in the real world.

A Quick Explainer: AC & DC Coupling

• Solar panels generate DC electricity.
• Batteries use DC electricity.

DC-Coupling

If you DC-couple, the solar panels are connected directly to the battery¹.

It’s DC → DC. No conversion to AC required in order to charge the battery.

AC-Coupling

If you AC-couple, you connect the solar panels to the battery via your AC switchboard.

It’s DC → AC then AC → DC.

The solar inverter converts the solar first to AC , so it can be connected into your switchboard. Then the battery inverter takes that AC from the switchboard and converts it back to DC for the battery to charge. Two conversions.

So yes, AC-coupling is less efficient.

Why It Doesn’t Matter

1. The energy you’re “wasting” is often worthless. Daytime surplus solar in Australia is practically free. It’s worth maybe 2–10c per kWh (whatever your FiT is), or zero if you’re hitting an export-limit. If your AC-coupled battery needs a little more solar energy to charge than a DC-coupled battery would, you haven’t lost anything meaningful.

2. The losses are small. With two conversions at ~95% each, you lose ~10% charging a battery with AC-coupling. On a 20 kWh battery, that’s an extra 2 kWh of solar required.

The cost:

• $0 if you’re hitting export limits. 20c if you’re still hanging on to a 10c FiT. Chump change.

3. Other factors are far more important. AC-coupling makes retrofits easy, avoids warranty dramas, provides redundancy (2 inverters) and often delivers stronger backup power. These things genuinely affect your system’s usefulness.

Make A Smart Decision

If you’re building a new system, DC-coupling is usually neatest and cheapest. If you’re retrofitting, AC-coupling is often the smartest.

Don’t be fooled by the “AC is inefficient!” sales pitch. It’s just the home-battery version of the “convert the whole state to DC” brainfart – spectacularly irrelevant.

Next time someone bangs on about AC-coupling being so inefficient, just nod politely. Then remind yourself they’re asking you to choose their battery over a competitor’s to save an extra 20c a day.

Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.