A DC coupled battery is wired on the solar side of the inverter, so the panels charge it directly as direct current. An AC coupled battery sits on the mains side and has its own inverter. The solar power is turned into alternating current first, then turned back for storage.
Both work. Which one suits you depends mostly on whether you already have solar, and whether the existing inverter is worth keeping. This guide sets out the difference in plain terms.
Key Points
- A DC coupled system uses one hybrid inverter for both the panels and the battery.
- An AC coupled system uses two inverters, the existing solar one and a separate battery one.
- Every conversion costs a little energy. DC coupling has fewer conversions on the way into the battery.
- AC coupling leaves the existing solar system alone, which matters if it is working and certificated.
- Manufacturers sell the two as a matched pair. Fox ESS lists the H1 as the hybrid and the AC1 as the AC coupled twin, on the same datasheet with the same ratings.
- Every battery system on our published work is DC coupled, running on a hybrid inverter.
- A hybrid inverter can be fitted now with the battery added later.
| Question | DC coupled | AC coupled |
|---|---|---|
| How many inverters | One hybrid unit | Two units |
| Where the battery sits | On the solar side | On the mains side |
| Conversions to store solar | Fewer | More |
| Best fit | New solar and battery together | Adding a battery to working solar |
| Existing solar inverter | Replaced by the hybrid | Kept as it is |
| Existing solar certificates | May need updating | Usually undisturbed |
| Wall space | One box plus battery | Two boxes plus battery |
| Charging from a cheap night tariff | Yes | Yes |
| Typical example we fit | Fox ESS H1 or KH, Anker SOLIX X1 | Fox ESS AC1 or KA |
Common Misconceptions
"AC coupled is worse." It is not worse. It has one extra conversion and a second box. In exchange it leaves a working system untouched.
"DC coupled is always cheaper." Not for a retrofit. If your solar inverter is healthy, replacing it to go DC coupled means throwing away something that works.
"You cannot charge an AC coupled battery from the grid." You can. Both routes can take cheap overnight power.
"A hybrid inverter is only worth it if you buy the battery now." No. Fitting a hybrid inverter now and adding the battery later is a normal way to spread the cost.
"The extra conversion loses a lot." It is a few percent, not a third. Fox ESS publishes a maximum efficiency of 97.08% for the H1 and AC1 range.
Real-World Advice
Our published 10 panel installation in Eastbourne is a 4.6 kWp array with a Fox ESS EP6 Plus battery. It runs on a Fox ESS H1-6.0-E-G2 hybrid inverter. See the Eastbourne case study. That is a DC coupled system, fitted as one job.
Our published 21 panel installation in Polegate uses a Fox ESS KH7 with a Fox ESS ECS4800-H2 battery. See the Polegate case study. Same idea, larger array.
Both were new installs where panels and battery went in together, which is the case DC coupling suits. Fox ESS publishes an AC coupled version of each of those inverters, the AC1 and the KA. Those are for the case where the solar is already there.
UK-Specific Considerations
Most East Sussex homes adding storage fall into one of two groups. Either it is a fresh install, where a hybrid inverter is the straightforward answer. Or the panels went in years ago under an older scheme.
If your panels are on the Feed in Tariff, the choice matters more. Changing the inverter can touch your accredited system, while an AC coupled battery leaves the generation side alone. Read upgrading existing solar systems on the Feed in Tariff before you decide, because that article covers what is and is not paid for.
Grid rules apply either way. A battery counts as a generating unit when the network operator works out which connection route your home needs. See UK Power Networks and G99 timescales.
Backup during a power cut is available on both routes, but only if the inverter supports it and you ask for it. The Fox ESS H1(G2) datasheet lists an emergency supply output, and so does the Anker SOLIX X1 datasheet.
We are in Eastbourne, so we see plenty of both. Around Polegate and Willingdon we have replaced whole systems where the original installer had gone. In Lewes and Seaford, where a lot of solar went in early, keeping the existing array and adding storage beside it is more common.
When This Isn't Right For You
Neither route helps if the roof or the array is the problem. If panels are failing or badly shaded, storing less power more cleverly does not fix the shortfall. Deal with the generation first.
An AC coupled retrofit also makes little sense if the existing inverter is at the end of its life. You would be adding a second box beside one you are about to replace.
Before You Spend Money
Find out how old your existing solar inverter is, and whether it is still in warranty.
Ask which route your quote uses, and why. The answer should be about your house, not about what the installer prefers.
Ask what happens to your existing paperwork. If your system is registered under an older scheme, changing the inverter is not a small detail.
Ask where both boxes will go if it is AC coupled. Two units need two places on a wall.
Ask whether backup is included, and which circuits it would cover.
If you are only replacing a failed inverter, read repair or replace the inverter first.
Frequently Asked Questions
Q: Which is more efficient?
A: DC coupling, slightly, because solar power reaches the battery with fewer conversions. The difference is small compared with getting the battery size right.
Q: I already have solar. What should I expect to be offered?
A: Usually an AC coupled battery, unless your inverter is old or faulty. Then replacing it with a hybrid can make more sense.
Q: Can I add a battery later to a new system?
A: Yes, if a hybrid inverter is fitted now. That is a common way to stage the work.
Q: Does the choice change what the network operator needs?
A: The battery counts either way, not the wiring style. G98 and G99 work from the current per phase and your supply arrangement, single or three phase. A G99 system needs the network operator's assessment before it is installed. Your installer confirms which route applies.
Q: Do both give me power in a power cut?
A: Only if the inverter has an emergency supply output and it is wired for it. Ask for it at the quote stage.
Summary
DC coupling means one hybrid inverter doing both jobs, and it is the natural choice when panels and battery go in together. AC coupling means a separate battery inverter beside your existing solar, and it is the natural choice for a retrofit where the current system works. The efficiency gap is small. The real questions are what you already own, what your paperwork says, and where the boxes will live. Every battery system on our published work is DC coupled, because those were all new installs.