f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

AC-coupled storage

AC-coupled storage Battery storage photovoltaics inverter retrofit storage Self-Consumption grid connection

An AC-coupled storage system is a battery storage system that is connected on the alternating-current side of an electrical installation. AC stands for alternating current. The battery stores energy electrochemically; its electrical terminals carry direct current. Bidirectional power electronics therefore convert alternating current to direct current during charging and back again during discharging. This function is usually performed by a dedicated battery inverter, which can also be integrated into the storage housing.

Integration and difference from DC coupling

In a system with photovoltaics, the PV system and the AC storage unit are connected via the local alternating-current network. The existing PV inverter continues to supply the building's electrical system. With DC-coupled storage, by contrast, the battery is on the direct-current side, often at a hybrid inverter. What matters is the electrical wiring, not the product designation: a suitable hybrid device can also be used as a battery inverter for an AC retrofit.

For self-consumption control, the power at the grid connection point is typically measured. If the controller detects a surplus, it can charge the storage unit; when electricity is needed, it can discharge it. The measurement captures the combined effect of generators and loads. In principle, a storage unit can also take up electricity from other local generation plants or from the public grid. Whether this is intended and permitted in a specific system is determined by device functions, operating agreements and the metering concept.

An illustration: if the PV system is currently supplying 5 kW of alternating current and the building needs 2 kW, 3 kW remain available for charging in arithmetic terms. If the storage unit absorbs all of it, the power balance at the grid connection point is approximately even. The energy actually stored in the battery is smaller because of charging losses. The example describes a momentary operating state, not a fixed design recommendation.

Conversion losses and system efficiency

With AC storage, solar power normally goes through three conversions: from module direct current to alternating current, then to battery direct current, and later back to alternating current. With DC coupling, the intermediate conversion via the alternating-current network is eliminated; however, this does not mean lossless storage. Besides the battery, power electronics, control and standby operation cause losses. Efficiency at low power levels is particularly important, for example during nighttime base-load supply. A reliable efficiency gap therefore cannot be derived from the coupling type alone.

An efficiency figure is only meaningful together with its measurement boundary. A peak inverter efficiency describes neither a complete storage cycle nor annual operation. When making comparisons, it should be clarified whether battery, conversion and standby losses are included. System metrics such as the System Performance Index assess the interaction under defined reference conditions; they are not equivalent to a simple charge-discharge efficiency.

Distinguishing capacity and power correctly

The storage capacity in kilowatt-hours (kWh) describes a quantity of energy. Charging and discharging power in kilowatts (kW), by contrast, describe how quickly energy can be absorbed or delivered. For quotations, the usable capacity and the measurement side are decisive: a battery energy content determined on the DC side is not fully available as alternating current because of the subsequent conversion losses. Likewise, short-term peak power and continuously available power must be kept apart.

A simplified calculation example: if a total of 10 kWh is available at the alternating-current output, five hours can be bridged in arithmetic terms at a constant 2 kW. If the inverter can supply at most 3 kW, it still does not fully cover a simultaneous demand of 5 kW. In grid-parallel operation, the missing 2 kW must come from elsewhere. High capacity therefore does not replace sufficiently high discharge power.

Retrofitting, economic viability and operating strategy

AC coupling is particularly interesting for retrofitting because the existing PV inverter can often continue to be operated. This avoids replacing equipment solely because of the battery. However, this does not imply complete manufacturer independence: battery, battery inverter, metering and control must be compatible. Communication with existing components must also be checked.

More self-consumption does not automatically mean more profit. The economic benefit results from avoided electricity purchases or price differences, minus forgone feed-in revenue, losses, investment, operation and ageing. Dynamic tariffs can open up additional uses but do not guarantee payback. A larger battery is not necessarily better: if its additional capacity mostly remains unused, it causes costs without a corresponding additional benefit.

Economical and grid-serving operation are also not the same thing. The Bundesnetzagentur (Federal Network Agency) points out that simultaneous charging of many storage units at low prices can overload local grids. An energy management system can coordinate storage units and other controllable devices within specified power limits. To do so, however, it needs suitable measured values, interfaces and operating rules.

Grid connection and legal framework in Germany

The following notes apply as of the review date of 11 November 2026 to stationary storage systems operated in parallel with the grid in Germany, in particular at low voltage. Connection, protection technology, permissible import and feed-in power and the metering concept must be agreed with the grid operator. The relevant connection rules and local connection conditions are decisive. Registration in the Marktstammdatenregister (core energy market data register) is independent of this. For medium-voltage installations and plug-in micro-systems, the connection concepts described here must not be adopted without verification.

Storage systems at low voltage that were newly commissioned on or after 1 January 2024 are in principle subject to the §14a regulations with regard to their possible grid consumption if the relevant grid connection capacity exceeds 4.2 kW. A mere software setting to PV-only charging does not automatically exempt them. The regulations concern power draw, not an obligation to feed in. Separate transitional and grandfathering rules apply to existing installations.

In the case of mixed charging with solar and grid electricity, the allocation under funding law is also important. The MiSpeL determination adopted on 1 October 2026 opens up new routes for proportional funding and levy privileges through a delimitation option and a flat-rate option. It is not a blanket release of every mode of operation. Until the end of September 2027, early use of both options requires the consent of the grid operator and the metering point operator. For the flat-rate option, the Bundesnetzagentur additionally cites a state-aid approval that remains to be awaited. The specific remuneration route must therefore be clarified in advance.

Backup power is an additional function

An AC storage unit does not automatically continue to supply the house during a grid outage. This requires safe disconnection from the public grid and a suitable grid-forming function. Changeover devices can be integrated or additionally required. The supplied circuits, phases, available power and switchover time must be checked. A backup power function is not automatically uninterrupted.

For selection, it follows: do not merely compare the AC or DC label, but define the specific task required. Should the storage unit shift existing solar power, optimize grid consumption over time, limit power peaks or supply selected loads during an outage? Only then can capacity, power, interfaces and additional components be sensibly assessed.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 13:58

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.