What is the difference between AC-coupled and DC-coupled BESS?

In an AC-coupled system, the storage has its own bidirectional converter (PCS — Power Conversion System) and connects on the AC bus. It is independent of the solar inverter, can be retrofitted next to an existing PV system, and works even when the sun is not shining — which makes it suitable for both grid arbitrage and peak shaving. In a DC-coupled system, the storage connects on the DC side of the solar inverter and uses a shared inverter. It is cheaper and makes better use of clipped PV surplus, because the energy does not pass through the converter twice — but the PV and the storage share the inverter’s power, and the storage can only charge when the PV is generating. For industrial systems sized for self-consumption, AC coupling is more common, because flexibility is worth more than a gain of a few percent in efficiency. One terminological clarification worth keeping in mind: the storage uses a bidirectional converter, not an inverter — an inverter is unidirectional (DC→AC), while a PCS works in both directions.