AC vs DC-Coupled Batteries With Commercial Solar PV
Updated 23 October 2026 · SEO Dons Editorial
What is the difference between AC-coupled and DC-coupled storage?
It is where the battery connects. A DC-coupled battery sits on the direct-current side, sharing an inverter with the array; an AC-coupled battery sits on the alternating-current side, behind its own battery inverter. Fronius puts it this way for its range: DC-coupled storage devices "are located in the circuit upstream of the PV inverter and can be charged with solar power directly from the roof", while AC-coupled ones "are connected downstream of the PV inverter on the AC side". The US energy laboratory NLR describes a DC-coupled system as one in which the PV and the battery "share a single bidirectional inverter".
The energy takes different routes in each design:
AC-coupled path, array to battery
- The array produces direct current.
- The existing PV inverter converts it to alternating current for the site.
- Surplus alternating current reaches the battery inverter, which converts it back to direct current to charge the battery.
- On discharge, the battery inverter converts to alternating current again for the site.
DC-coupled path, array to battery
- The array produces direct current.
- It reaches the hybrid inverter, which either converts it for the site or passes it to the battery port as direct current.
- On discharge, the same inverter converts the battery's output to alternating current.
The hybrid inverter guide covers the DC-coupled hardware and its power ranges.
Where do the conversion losses arise?
Each conversion between direct and alternating current costs a little energy, and the AC-coupled path has one more than the DC-coupled path when the battery is charged from the array. Fronius states that in the AC-coupled arrangement "two conversions are necessary to store energy: from DC to AC in the PV inverter and then back again in the battery inverter", and that "around two percent of the PV surplus is lost with each conversion". That is a maker's illustration in guidance for home systems, not a measured result for any business site. NLR reaches the same direction for DC coupling, noting "increased roundtrip efficiency when the battery charges from the coupled PV".
For a commercial decision, the figure to request is the battery system's round-trip efficiency measured at its AC terminals, plus the inverter efficiency curves for the units involved. The battery storage page explains how storage economics are tested.
Which design can capture clipped energy?
Only a DC-coupled battery can use energy that the PV inverter would clip, because of where it sits. When the array's output exceeds what the inverter can convert, the excess is clipped before it becomes alternating current. A battery behind a separate battery inverter never sees it. NLR lists as a DC-coupling benefit "the ability to capture energy that would otherwise be clipped by the inverter or lost because of low-voltage conditions".
The benefit depends on how far the array is oversized against the inverter. In NLR's representative utility-scale case, with a DC to AC ratio of 1.3, clipped energy was "0.0%-0.5% of the DC electricity produced by the PV array in most of the contiguous United States", which NLR treats as negligible. That is US modelling, not a UK commercial roof, so the clipping loss in a site's own yield report is the number to use. The inverter types guide explains clipping and the DC to AC ratio.
Generation held back by an export limit is a separate matter from clipping. National Grid Electricity Distribution says a limitation scheme can either restrict output "or increase the customer demand", and a battery that is charging is added demand, whichever way it is coupled.
Which suits a retrofit to an existing array?
AC coupling is the route that leaves the existing inverter alone. SMA says its AC-coupled solution lets users "integrate battery-storage systems into both new and existing PV systems easily and flexibly and expand the storage capacity at any time without having to interfere with the PV system". Fronius calls it the biggest advantage of AC coupling that "there is no need to replace the inverter in existing PV systems".
DC coupling needs an inverter that has a battery port. Fronius lists the two options for a system without one: an AC-coupled device, or, for owners of an existing GEN24 inverter, a software upgrade that "transforms your inverter into a Fronius GEN24 Plus hybrid inverter". Whether an existing commercial inverter can be upgraded this way is a model-specific question for its maker.
How do export limitation, monitoring and warranties differ?
| Factor | AC-coupled | DC-coupled |
|---|---|---|
| Conversions, array to battery | Two, per Fronius | A DC-to-DC step at the hybrid inverter; NLR notes higher round-trip efficiency when charging from the coupled PV |
| Clipped energy | Cannot reach a battery behind the PV inverter | Can be captured, to the extent the array is oversized |
| Existing PV inverter | Stays; SMA describes no need "to interfere with the PV system" | Needs a battery-capable inverter, new or upgraded |
| Export limitation | Every exporting inverter must respond; SMA says that where limitation is required, "the selected inverters must ensure the limitation" | The hybrid inverter controls array and battery together, though other string inverters on the site still need to respond |
| Monitoring | Fronius states its AC-coupled storage is "incompatible with monitoring software"; other makers' products may differ | Fronius says a DC-coupled store with a compatible hybrid inverter enables "comprehensive system monitoring"; check the model |
| Warranty documents | Battery inverter and battery added to the existing PV inverter's terms | Hybrid inverter and battery terms; SolaX's UK page lists inverter and battery periods separately |
| Grid notification | The battery inverter is itself covered by G98 or G99 as applicable | IET: a DC-side-only battery with no extra AC inverter makes a new notification "unlikely" |
Export limitation deserves the closest read. National Grid Electricity Distribution describes a scheme as one that measures at the exit point and then "restrict[s] generation/energy storage output or increase[s] the customer demand" to keep the agreed export capacity from being exceeded, and says "Generation, including energy storage systems, also needs to adhere to ER G98 and G99 as applicable". The ENA's fast-track conditions likewise count "generating units (including electricity storage devices)". The G100 guide explains how schemes work and fail safe. The notification point is the network operator's to confirm for each site.
How should the choice be made?
- Is the PV inverter staying? If it is recent and a battery is a possible later addition, AC coupling keeps it untouched.
- Is the array oversized against its inverters? Ask for the modelled clipping loss, which sets the size of any DC-coupling benefit.
- Is an inverter replacement already planned? A hybrid inverter then adds a battery port to a purchase already being made.
- Is an export limit in the connection agreement? Ask how the scheme controls each inverter.
- Which warranty documents and monitoring platform apply to each component?
Sources
- Utility-Scale PV-Plus-Battery, Annual Technology Baseline 2025, NLR (US energy laboratory), checked 22 October 2026.
- Correctly dimensioning and retrofitting a battery for PV systems, Fronius (a maker's own guidance, used for statements about its products), checked 22 October 2026.
- Sunny Boy Storage becomes even more flexible, SMA newsroom, and Frequently asked questions, SMA manuals (statements about SMA's own products), checked 22 October 2026.
- Battery storage webinar questions and answers (webinar of 2 November 2021), IET, checked 22 October 2026.
- Connecting generation to the electricity networks, Energy Networks Association, checked 22 October 2026.
- Export Limitation, National Grid Electricity Distribution, checked 22 October 2026.
- Warranty, SolaX Power UK (a statement about the layout of SolaX's own warranty page), checked 22 October 2026.
Common questions
Can an AC-coupled battery work with a PV inverter from another maker?
For SMA's own battery inverters, SMA says that where active power limitation is not required, PV inverters from any manufacturer can be used when retrofitting an existing system. Where limitation is required, the PV inverters selected must ensure it. Other makers publish their own compatibility lists.
Is the two percent per conversion figure a measured loss?
No. It is Fronius's own statement in guidance for home systems. A business should ask for round-trip efficiency at the AC terminals from the datasheet of the specific battery system, and the efficiency curves of the inverters involved.
Does DC coupling always recover clipped energy?
Only energy that actually clips. NLR's representative case, with a DC to AC ratio of 1.3, put clipped energy at 0.0% to 0.5% of DC output in most of the contiguous United States. A site's own yield report should state its modelled clipping loss.
Do both designs need a new grid notification?
Not necessarily in the same way. The IET says that a battery added on the DC side only, with no additional AC inverter and no island-mode components, is unlikely to need a new notification or application. An AC-coupled battery adds an inverter. The network operator confirms the position for each site.