Sigenergy vs Fox ESS: which battery is right for your house?

Sigenergy SigenStor is the one-stack route. One product carries the solar inverter, the battery, the energy management and, if you want it, a DC car charger, and switches to backup in 0 ms with a Sigen gateway. Fox ESS is the volume route: a hybrid inverter plus a battery you size in modules, or an all-in-one unit, with whole-home backup through a separate box that changes over in about two to three seconds.

They are not chasing the same buyer. In our experience Fox comes in lower on a like-for-like quote; Sigenergy does more inside one product, and is the only one of the two that charges a car on DC from the same stack.

We are an early adopter of the Sigenergy SigenStor in the North East — we install and commission SigenStor systems and have two complete installs documented on our YouTube channel. Hubs: Sigenergy installer, Fox ESS installer.

Sigenergy sells one idea in sizes: a "5-in-One" unit holding PV inverter, EV DC charger, battery power conversion, battery pack and energy management, with modules that "auto plug once stacked" rather than being hand-wired. Fox sells a catalogue: three battery families, three all-in-one systems and five inverter families. Both use lithium iron phosphate cells.

This is where Sigenergy earns its price. One Energy Controller takes 1 to 6 modules, and every pack has its own DC-DC optimiser, which Sigenergy says allows "mixed use of old & new batteries" with different capacities and states of health — so you can add a module in three years without matching what is already there.

Fox is modular with rules. On a single-phase hybrid an EQ5000 runs from 2 modules (9.84 kWh) to 6 (29.52 kWh) and a CQ7 from 2 to 7, up to 49.14 kWh. Note that minimum of two: Sigenergy will run a single module. Two more to design around rather than discover on the day: battery warm-up is "Not available on EQ4800", and Fox’s UK battery policy wants 300 mm of clearance around the pack, which quietly rules out some cupboards. An Evo AIO bought now also expands only by adding whole Evo units, until the second generation Fox has said is due autumn 2026.

Sigenergy’s Energy Controller is the inverter: 3.0 to 12.0 kW single-phase taking 6,000 to 24,000 W of PV across two to four MPP trackers. The limitation to know before you commit is one string per MPPT at up to 16 A — fine on a roof split two or three ways, a design conversation on anything unusual.

Fox’s H1-G2 is dual-MPPT with a 200% PV oversize ratio and installs DC or AC coupled; the H1-5.0-E-G2 takes 10,000 Wp of array for 5,000 W out, and the 3.7 kW model is footnoted "3680 for G98". Larger roofs get the KH/KA at 7 to 10 kW with three or four MPPTs, and it can go in battery-ready with storage added later — see adding battery storage to existing solar, or solar panel installation if the array is going in too.

If DC car charging from the same stack is the point of the project, the comparison is over. Sigenergy’s EVDC module runs at up to 12.5 kW DC, or 25 kW on specific models with a purchased licence, on a CCS2 connector, and needs the Energy Controller. Sigenergy claims 25 kW gives "up to 150 km of driving range for EVs in just one hour". The modes matter more: scheduled, PV-surplus on 100% solar, battery-boost with a cut-off state of charge, and a fast mode drawing from grid, PV and battery at once.

Do not buy it for vehicle-to-grid yet: Sigenergy says V2X is limited by the EV’s capabilities and can be upgraded over the air once standards are published. Fox’s UK residential range is batteries, inverters and all-in-one systems, and no Fox UK page we have read puts EV charging inside FoxCloud, so plan on a separate AC charger — which is what most of our customers have anyway: EV charger installation.

Sigenergy quotes 0 ms disruption on the load side, footnoted that it needs the Sigen Energy Gateway working with the Energy Controller and battery, and that the controller’s nominal power must exceed the total backup load. The Gateway HomePro comes as SP (12 kW / 54.6 A) and SP-F (22 kW / 100 A), and Sigenergy says the SP-F "fully supports 100 amp single-phase backup power for the entire home" and can shed loads, its own example being cutting a heat pump below 20% battery.

Fox’s answer is the EPS-BOX-SP: on grid loss it isolates the property and, after what Fox calls a short transfer delay, restores power from battery and solar — typically about two to three seconds. It is rated 100 A in and 100 A out, with full-home backup "subject to system sizing" — the same caveat as Sigenergy’s in different words.

The honest read: two to three seconds is nothing for lights, heating and a fridge, and a reboot for a desktop PC. If you work from home on a desktop or run medical equipment, 0 ms is worth paying for. Otherwise it is a smaller difference than the spec sheets make it look.

Fox publishes UK and Ireland documents; Sigenergy does not. Sigenergy’s warranty page publishes by region and the European versions are the only English ones, so ask for the UK position in writing.

Fox’s UK & Ireland battery policy v4.5, dated 11 September 2025, gives 12 years from installation on the EQ range and 10 years on the EP6, EP12, CQ6 and CQ16, warranting at least 70% of nominal energy for that period or a throughput cap of 18.45 MWh on an EQ5000. Claims are raised by the installer, not by you. Inverters get 60 months, plus 60 months free on the hybrid and AC series if the unit is registered through Fox’s website within 36 months of manufacture and sits on Fox Cloud with a Fox datalogger: 10 years if the paperwork gets done, five if it does not, and the all-in-one series is not eligible at all.

Sigenergy’s European warranty, dated 27 May 2025, gives 10 years on the BAT modules and Energy Controllers, 3 years on the EVDC module, and at least 60% of usable energy for 10 years or a throughput cap of 30.66 MWh on a BAT 10.0. The gateway is five years, and an extension adds five more if bought within 12 months of installation. Two conditions people miss: cover starts at the earlier of on-site activation or 180 days after the unit left the factory, and the system has to stay online, because more than a year offline in total cuts the 10 years to five.

mySigen is the better piece of software: a 3D energy-flow screen refreshed every 10 seconds, a Sankey diagram tracing each kWh across six energy sources and five load categories, SigenAgent taking plain-language instructions, AI Mode scheduling charge and discharge from weather forecasts and prices, and AI Insight explaining the last 24 hours. Sigenergy claims AI Mode returns "up to 50% higher average returns than traditional Self-Consumption mode" — its figure, not one we have measured. British users can select Octopus in the app’s dynamic-tariff function.

FoxCloud is plainer and, in one way, better value. It runs in a browser and syncs with the Fox ESS app: battery health, a charge and discharge schedule, a Manual Tariff of up to 96 slots and one-click Smart Tariff, with FoxCloud 2.0, announced in February 2026, adding a five-second refresh, an AI Mode and VPP control — though Fox said at the time that certain features would not be available in the initial release. Then there is Capture AI, which charges when electricity is cheapest: Fox says it works with all UK suppliers and tariffs, is included free with all Fox ESS systems, and that the savings are "100% yours". Optimisation thrown in with the hardware is the strongest argument for Fox that comparisons never mention. More in our Octopus Flux and battery tariff guide.

On the Fox side, we install Fox ESS battery and hybrid-inverter systems (including the Fox Cube battery) and have two complete installs documented on our YouTube channel: 9.6 kW of solar with a Fox Cube battery in Ingleby Barwick and a Fox Cube with JA Solar panels on a garage roof. On the Sigenergy side, a slate-roof solar and battery install and a SigenStor with full home backup.

Where the kit physically goes decides more than the specification does. A Sigenergy BAT 6.0 is 62 kg and a BAT 10.0 is 78 kg; a Fox EQ stack goes up module by module — Fox’s EQ4300 datasheet gives a 570 x 380 mm footprint — and a two-module CQ7 is 105.5 kg. That, plus the clearance rule, decides which wall or which corner of the garage, and occasionally which brand. Sigenergy claims a 15-minute stackable installation and five-minute commissioning, and the stacking honestly is that fast; what fills the day on either brand is the cable route, the consumer unit and the network operator’s paperwork.

One thing to be straight about: the "Fox Cube" in our films is Fox’s ECS, or Energy Cube, generation, which is not among the battery ranges on Fox’s current UK residential page nor in the current UK & Ireland battery policy. Those systems run well and hold the warranty in force when they went in, but a Fox quote from us today is an EQ, CQ7 or all-in-one system.

Fit the SigenStor if you want DC car charging in the same stack, a power cut has to be invisible, you intend to grow the battery over years without matching modules, or you want one product family across single and three phase. Fit Fox ESS if budget is the deciding factor, you are retrofitting or want a battery-ready install now, the space is awkward (EP batteries wall-mount), or you want optimisation included rather than bought.

And the concession neither manufacturer will make for you: with no solar, no electric car and no intention of moving your import into the cheap hours, a battery of either brand is a slow payback. Read the battery storage buying guide first, and let us talk you out of it if the numbers do not work.

Neither manufacturer publishes a UK price, and Fox does not sell direct: its own quote page pairs enquiries with a partner installer. The only honest number is a fixed price against your roof, your consumer unit and your usage. VAT helps either choice: HMRC’s notice 708/6 zero-rates the installation of specified energy-saving materials until 31 March 2027, reverting to the 5% reduced rate from 1 April 2027, and electrical storage batteries in homes have qualified since 1 February 2024.

Andy Pemberton surveys and specifies every system we quote. We are MCS Certified and NAPIT Registered with more than 500+ five-star reviews across Checkatrade and Google. We install across Teesside, County Durham and North Yorkshire from our base in Yarm. How far we travel depends on the job — just ask. From here: the Sigenergy installer and Fox ESS installer hubs, battery storage, or get in touch and we will price both.