The short answer
Both are good batteries and we fit both. Choose Sigenergy SigenStor if you drive an electric car (or will soon), want the whole house to stay on in a power cut without even a flicker, or need a lot of capacity: it has EV charging built in and 0 ms whole-home backup with its gateway. Choose Fox ESS if you want proven, widely installed kit at a lower cost, or want to add a battery while keeping your existing solar inverter.
Sigenergy is one of our two top picks. Fox ESS is one of the best-known names in UK home batteries, and Fox says industry data puts its share of the UK home solar and storage market at more than a third. Both are modular LFP systems with their own inverters and apps. Where they differ is in backup, EV charging, retrofits and how far each one can grow.
Sigenergy vs Fox ESS at a glance
Battery sizes
Sigenergy: 6 kWh and 9 kWh modules
Fox ESS: EP6 (5.76 kWh) and EP12 (11.52 kWh), or the EVO all-in-one (10.24 kWh)
Most storage
Sigenergy: Up to 6 modules on one controller, about 54 kWh
Fox ESS: Up to 4 EP batteries: 23 kWh with EP6, 46 kWh with EP12
Inverter sizes
Sigenergy: 3 to 12 kW single phase, 5 to 30 kW three phase
Fox ESS: 3 to 6 kW (H1-G2), 7 to 10 kW (KH), EVO 5, 8 or 10 kW; three-phase models too
Battery chemistry
Sigenergy: LFP (lithium iron phosphate)
Fox ESS: LFP (lithium iron phosphate)
Rated cycles
Sigenergy: 10,000
Fox ESS: 6,000 on the EVO; not stated on the UK EP12 datasheet
Warranty
Sigenergy: 10 years, paid extension to 15
Fox ESS: 10 years on EP batteries, 12 on the EVO
Power-cut backup
Sigenergy: 0 ms whole-home, with the Sigen Energy Gateway
Fox ESS: Backed-up circuits in milliseconds; whole home with the EPS-BOX-SP gateway, after a 2 to 3 second break
Keeping your old solar inverter
Sigenergy: Usually replaced by the Sigenergy controller
Fox ESS: Yes, with the AC1-G2 AC-coupled inverter
EV charging
Sigenergy: Built-in DC charging module, 12.5 or 25 kW
Fox ESS: Not built in (separate Fox AC charger available)
Indoor or outdoor
Sigenergy: Either, IP66
Fox ESS: Either, EP batteries IP65, EVO IP66
App
Sigenergy: mySigen, with AI energy management
Fox ESS: FoxCloud and the Fox ESS app
Checked against Sigenergy’s and Fox ESS’s product pages, datasheets and warranty documents in October 2026. Manufacturers update their ranges, so we confirm the exact model and warranty in your quote.
What they have in common
- Safe battery chemistry. Both use lithium iron phosphate (LFP) cells, the stable, long-lasting chemistry most home batteries now use.
- Modular. Start with what you need and add batteries later without replacing the system.
- Their own hybrid inverters. Panels and battery run through one inverter, so solar charges the battery directly.
- Inside or outside. Both are weatherproof, so a garage wall or an outside wall both work.
- Octopus-friendly. Octopus lists both brands as compatible with its smart battery products, including its Charge Pack add-on.
- Warranties that need the system online. Both makers link their full warranty to the system being registered and connected for monitoring.
Where Sigenergy SigenStor is stronger
EV charging in the same stack
Sigenergy’s DC charging module (12.5 or 25 kW) slots into the battery stack, so your car can charge straight from the roof and the battery without a separate wall charger. It is also built for vehicle-to-home and vehicle-to-grid, which depends on your car supporting it. Fox makes good AC chargers, but none is built into the battery.
Backup with no break at all
With the Sigen Energy Gateway fitted, the whole house switches to the battery in 0 ms when the grid drops. Fox can back up the whole house too, using its EPS-BOX-SP gateway, but Fox says there is a short break, typically 2 to 3 seconds, while it switches over. Fox’s inverters can keep chosen circuits running with only a few milliseconds’ gap. For most homes a couple of seconds is fine; if you work from home or run anything that mustn’t reset, it matters.
More room to grow
One Sigenergy controller takes up to six modules, around 54 kWh, with single-phase inverters up to 12 kW. If you have a heat pump, an EV and a big roof, Sigenergy scales further on one system.
Longer cell rating and a 15-year option
Sigenergy rates its cells for 10,000 cycles and offers a paid warranty extension to 15 years (it has to be bought within 12 months of the system going live).
Where Fox ESS is stronger
Proven and widely installed
Fox ESS has been fitted in a great many UK homes, which means mature firmware, plenty of installers who know it well, and a UK service centre in Coventry handling technical support and warranty work.
Keeps your existing solar inverter
Fox’s AC1-G2 is an AC-coupled inverter made for adding a battery to solar you already have. Your panels and their inverter stay as they are, and the battery sits alongside. With Sigenergy, the usual route is to replace your old inverter with Sigenergy’s own controller.
A big battery in one box
The EP12 holds 11.52 kWh in a single wall- or floor-mounted unit, enough for most homes’ evenings, and up to four can be linked. Prefer one tidy unit? The EVO puts a 5, 8 or 10 kW inverter and a 10.24 kWh battery in one case, with a 12-year warranty.
Lower cost
A Fox system usually costs less than a Sigenergy one, particularly once you add Sigenergy’s backup gateway or EV module. If you don’t need those, Fox gives you solid storage without paying for features you won’t use.
Which one suits your home?
- You have an EV, or one is coming: Sigenergy. Built-in DC charging is the deciding feature.
- Power cuts must be invisible: Sigenergy with its gateway. If a 2 to 3 second break is fine, Fox does whole-home backup too.
- You already have solar and want to keep your inverter: Fox ESS, with the AC1-G2.
- Big usage, a heat pump or a three-phase supply: Sigenergy has the most headroom, though Fox has three-phase models as well.
- You want proven kit at a sensible price: Fox ESS.
Still weighing it up? Read is Sigenergy any good? and is Fox ESS any good? for the common questions on each, or see how Sigenergy compares with our other top pick in Sigenergy vs EcoFlow.
What about cost?
We don’t quote shelf prices, because the right size depends on your roof, your usage and your tariff. With Sigenergy, the backup gateway and EV module are optional extras; with Fox, so is the whole-home backup gateway. We design the system remotely and confirm the price after a free survey. A battery added to existing panels is currently 0% VAT, until 31 March 2027.
Our verdict
If an electric car, seamless backup or a big system is on your list, Sigenergy is worth the extra. If you want a well-proven battery that does the everyday job well, or you’re adding storage to panels you already have, Fox ESS is a strong, sensible choice. Our technical solar designer can design both for your home and show you the numbers side by side on a video call.
More detail on each: Sigenergy SigenStor and Fox ESS.
