AC-Coupled vs DC-Coupled BESS

When a battery is added to a solar plant, the first design question is where it connects. In an AC-coupled system the battery has its own inverter on the AC side; in a DC-coupled system it shares the PV inverter’s DC bus. The Battery Energy Storage Systems hub covers this briefly. This page goes further: topologies, conversion losses, clipped energy, grid capacity and retrofit.

The two topologies

• AC-coupled. PV array to PV inverter to AC bus; battery to its own bidirectional inverter to the same AC bus. The two systems can operate independently and each can reach its full AC power [1].
• DC-coupled. The battery connects on the DC side of the PV inverter, either directly on the DC bus or through a DC-DC converter. One inverter serves both sources [1].

Conversion losses on each path

Each power conversion stage loses a few percent. The vendor source we read gave no stage efficiencies [1], so the figures here are our assumption, 98% for every stage, only to show how the paths compare.

• AC-coupled, PV energy stored and later exported: PV inverter (DC to AC), battery inverter (AC to DC), battery inverter again (DC to AC) is 0.98 x 0.98 x 0.98 = about 94.1% before battery losses.
• DC-coupled with a DC-DC converter, same energy: DC-DC converter, then the shared inverter on export is 0.98 x 0.98 = about 96.0% before battery losses.
• Energy that goes straight from PV to grid is unaffected by the battery in either case.

The gap is around two percentage points on the stored fraction under these assumptions. Real values depend on loading, since efficiency curves fall at low power. Dynapower, an inverter maker, states that DC coupling is more efficient but does not publish numbers in the page we read, so treat that as a vendor claim [1]. Compare with the system-level Round Trip Efficiency definition.

Recovering clipped solar energy

PV arrays are often built with more DC capacity than inverter AC capacity, so the inverter clips output at midday. The vendor gives an example of a 14 MWdc array behind a 10 MWac inverter and says typical utility-scale ratios are 1.25:1 to 1.5:1 [1]. In that example the ratio is 14/10 = 1.4:1 (our arithmetic). A DC-coupled battery can absorb the clipped energy before it reaches the inverter and release it later. An AC-coupled battery can only charge from energy that has already passed through the PV inverter, so the clipped part is lost unless the PV inverter limit is raised. This is the main reason DC coupling is chosen for new large solar sites.

The DC-DC converter

A DC-DC converter lets the battery run at its own voltage and be controlled independently from the PV string voltage. The vendor notes it can also allow charging at times when the array voltage is below the PV inverter’s start threshold, such as early morning, evening and cloud cover, and while the PV inverter is offline [1]. It adds another conversion stage and another item to maintain, see Power Electronics.

Metering and grid connection capacity

In a DC-coupled plant the shared inverter limits the maximum export, so the connection capacity is the inverter rating. In an AC-coupled plant, PV and battery inverters can together exceed the agreed export limit unless a site controller limits them. Separate metering of battery charging from the grid is simpler to arrange in an AC-coupled plant; some incentive and tariff rules depend on where the battery charges from. See Grid Connection Requirements for BESS and Grid-Forming vs. Grid-Following Inverters.

Retrofit versus new build

Adding a battery to an existing plant is usually simpler to do as AC-coupled because the existing PV equipment is untouched and the battery gets its own inverter and metering. DC-coupled retrofits can work, as the vendor notes DC coupling suits existing and new plants [1], but they need access to the DC bus and compatibility with the existing inverter. This is our reading, not a measured comparison. For new builds the shared inverter can lower balance-of-plant cost [1] and recover clipping, so DC coupling is commonly preferred where the site has high clipping.

Summary table (our reading; vendor claims where marked):

References

1. AC vs. DC Coupled Solar + Storage, Dynapower (vendor)

See also: Grid Battery Energy Storage Systems, BESS Sizing, UL 1741