An AC-coupled battery has its own inverter and connects to the home's AC supply alongside the solar inverter, so it can be added to almost any existing system. A DC-coupled battery shares a hybrid inverter with the panels, so solar can charge it without an extra conversion. AC coupling is the usual retrofit; DC coupling is usual for new solar and battery systems. Enphase quotes 90% round-trip efficiency on AC for its IQ Battery 5P and 96% on DC.
Two ways to connect a battery
AC coupling. The solar panels feed their own inverter, which turns their direct current (DC) into alternating current (AC) for the home. The battery has a second, separate inverter. When the panels make more than the home uses, the battery inverter converts some of that AC back to DC to charge the battery, and converts it to AC again when the home needs it. Nothing about the existing solar system changes, which is why most retrofits are AC coupled. MoneySavingExpert makes the same point: adding a battery to existing panels this way needs an extra inverter.
DC coupling. A single hybrid inverter is connected to both the panels and the battery. Solar DC can charge the battery directly, and the inverter converts to AC only for what the home uses or exports. One inverter does two jobs, and solar energy is converted once rather than twice on its way into storage.
How they compare
| AC coupled | DC coupled (hybrid inverter) | |
|---|---|---|
| Best for | Adding a battery to existing solar | New solar and battery systems |
| Existing solar inverter | Stays in place | Replaced by the hybrid inverter in a retrofit |
| Inverters | Two (solar and battery) | One |
| Solar-to-battery efficiency | Lower: two conversions | Higher: one conversion |
| Grid charging | Works | Works |
| Network notification | Both inverters count towards the G98 limit | One inverter’s rating counts |
Enphase’s UK datasheet for its AC-coupled IQ Battery 5P illustrates the efficiency gap: 90% round-trip on the AC side against 96% on the DC side, at the start of the battery’s life. Tesla quotes 97.5% solar-to-home and 89% solar-to-battery-to-home efficiency for Powerwall 3. This site’s savings model assumes 90% round-trip for every battery, which is conservative for DC-coupled systems.
What the makers publish, and what the gap is worth
| System | How it connects | Efficiency the maker publishes |
|---|---|---|
| Enphase IQ Battery 5P | AC coupled, six built-in microinverters | 90% round trip on the AC side; 96% on the DC side, at the start of life |
| Tesla Powerwall 3 | DC coupled when panels are wired to its built-in solar inverter; can also sit alongside an existing inverter | 97.5% solar to home; 89% solar to battery to home |
| Fox ESS EVO 10 | DC coupled: battery and hybrid inverter in one unit | 97.3% solar to battery; 96.4% battery to AC, at full load |
The makers measure different things, so the numbers do not line up row by row. Enphase’s two figures are the cleanest comparison because one maker gives both: about six percentage points between AC and DC on energy that goes through the battery. In this site’s model a London home with 4 kWp of solar and a 10 kWh battery on Octopus Flux stores about 2,500 kWh of solar a year, so six points is about 150 kWh, a difference worth about £39 a year at 26p a unit. For 3 kWp with a 5 kWh battery it is about £20, and for 6 kWp with 10 kWh about £48. That is roughly a tenth of the modelled saving in each case: real, but smaller than the price difference between two quotes.
Which batteries are which
- AC coupled: Enphase’s IQ Battery 5P, and the GivEnergy All in One, which comes as an AC-coupled 3.6 kW or 6.0 kW unit; the GivEnergy All in One page explains why a buyer should treat its warranty with care since the administration.
- DC coupled: Fox ESS’s EVO, which has the hybrid inverter built in, and Tesla’s Powerwall 3 when new panels are wired to it.
- Either: Tesla says a Powerwall 3 can also be installed alongside an existing solar inverter, in which case the existing panels are coupled to it on the AC side.
Coupling did not set Octopus’s published battery-only prices. A Fox EVO (DC) or an Enphase battery (AC) costs the same at each size on its list, from £3,947 for about 5 kWh and £5,336 for about 10 kWh at April 2026 prices. What moves a retrofit price is what the job needs around the battery: a new inverter, a G99 application, backup hardware or a consumer unit upgrade.
Backup is a separate question from coupling. Both can supply it, but only with the island-mode isolator and earthing arrangement that MCS’s standard requires, and only if the maker supports it; see our backup guide for what each level keeps on.
The network rule that catches retrofits
The ENA’s G98 route, which lets an installer simply notify the network after commissioning, covers a total of 16 A per phase, about 3.68 kW on a single-phase home, counting every inverter that can export. An AC-coupled retrofit adds a second inverter to the one already there, so a 3 kW solar inverter plus a 3 kW battery inverter comes to 6 kW, which needs a G99 application instead, often with an export limit. A DC-coupled retrofit replaces the solar inverter, so only the hybrid inverter’s rating counts.
Which should you choose?
- You already have solar and the inverter is fairly new: AC coupling keeps it and avoids replacing working kit.
- Your solar inverter is old (inverters typically need replacing well before the panels): a hybrid inverter replacement can be the moment to go DC coupled.
- You are fitting new solar and a battery together: a hybrid inverter, or an all-in-one with a solar inverter built in such as Powerwall 3, is usually cheaper and more efficient.
- You have microinverters on each panel (for example Enphase): an AC-coupled battery designed for that system is the natural fit.
MCS’s battery installation standard classifies systems by how many enclosures and manufacturers are involved, from a single enclosure holding battery, battery management and inverter (Class 1) to mixed systems (Classes 2 to 4). Ask your installer which class your system is and who you would contact for a warranty claim on each part.
For the most common case, a battery added to panels you already have, see the retrofit solar battery guide, which models the payback by array size and tariff and explains what happens to Feed-in Tariff payments.
What to ask for in a quote
State the coupling, the inverter’s make and power in kW, and whether the existing inverter stays. Two quotes that look alike can differ by a whole inverter. Compare battery storage quotes on those lines, and read adding a battery to existing solar if you are retrofitting.
Questions people ask
What is an AC-coupled battery?
A battery with its own inverter that connects to the home's alternating-current supply, usually at the consumer unit, separately from the solar inverter. It can store solar surplus from any existing system and charge from the grid, without touching the panels or their inverter.
What is a hybrid inverter?
An inverter that handles both solar panels and a battery. The panels' direct current can charge the battery directly, and the inverter converts to alternating current for the home and the grid. It is the usual choice for a new solar and battery system, and it replaces the old inverter if used in a retrofit.
Is DC coupling more efficient than AC coupling?
For storing solar, usually yes, because the energy is converted once rather than twice. Enphase's UK datasheet for the IQ Battery 5P gives 90% round-trip efficiency on the AC side and 96% on the DC side. When a battery charges from the grid, both approaches involve similar conversions.
How much is the efficiency gap between AC and DC coupling worth?
About six percentage points on the solar energy that goes through the battery, going by Enphase's 90% AC and 96% DC round-trip figures. In this site's model a 4 kWp array with a 10 kWh battery in London stores about 2,500 kWh of solar a year, so six points is about 150 kWh, a difference worth about £39 a year at 26p a unit. For 3 kWp with 5 kWh it is about £20 and for 6 kWp with 10 kWh about £48, roughly a tenth of the modelled saving in each case.
Does AC or DC coupling matter for a power cut?
Not by itself. Either can supply backup, but only if the installation includes the island-mode hardware that MCS's battery standard requires, an isolator and a neutral-earth bond arrangement, and the maker supports it. Check the specific system: Enphase's Full Backup depends on the other Enphase equipment, and Tesla's Powerwall 3 needs its Backup Gateway 2.
Does Tesla Powerwall 3 count as AC or DC coupled?
Powerwall 3 has a built-in solar inverter with three solar inputs, so with new panels wired to it the solar is DC coupled to the battery. It can also be installed alongside an existing solar inverter. Tesla gives 97.5% solar-to-home and 89% solar-to-battery-to-home efficiency.