The installed price gap between the Tesla Powerwall 3, Sigenenergy SigenStor, and Fox Evo is under 4% when measured on cost per usable kilowatt-hour of storage. Spirit Energy has installed all three, including the first Powerwall 3 commissioned in the UK, and the honest conclusion is that price alone should not drive this decision.
What separates them is DNO flexibility, three-phase backup capability, thermal management, and warranty terms.
The common mistake is comparing headline installed costs without accounting for the usable storage you're actually getting. Do that, and the Fox Evo comes out slightly cheaper per kilowatt-hour, the Powerwall 3 sits around 3% higher, and the SigenStor around 4% above the Fox. The total spread across all three is less than 4%.
If your home doesn't need 13.5 kWh battery storage, the fixed unit size of the Powerwall 3, then a smaller Fox Evo or a SigenStor closer to your actual consumption may shift that comparison further in their favour. Paying for storage that rarely gets used is the most common sizing mistake we often see people make.
Your distribution network operator (DNO) sets a limit on how much power your inverter is permitted to export and generate. That limit can change over time as the local grid is upgraded or as more solar systems come online in your area.
This is where the Powerwall 3 has a clear practical advantage. Its inverter rating is fully software-configurable between 3.68 and 11.04 kW, with no hardware changes required. If your DNO tightens your generation allowance after installation, your installer adjusts it remotely. If the grid is upgraded and your allowance increases, the same applies.
The Sigenstor and Fox Evo both have fixed inverter ratings set in hardware. If your DNO allowance changes after the system is in, the only remedy is replacing the energy controller or inverter unit. That adds cost and a return visit.
If you are in an area where DNO restrictions are likely, or where grid upgrades are actively under way, the Powerwall 3's software flexibility has real financial value.
All three systems can protect your home during a power cut, but they do not all do it the same way.
The Powerwall 3 uses Tesla's Backup Gateway 2. It is not a true uninterruptible power supply. There is a brief switchover interruption when the grid goes down, though in most power cuts this is fast enough that appliances keep running. The more significant limitation for some properties: one phase can be backed up. three-phase backup is not currently available on the Tesla system in the UK.
The SigenStor is the only one of the three that can back up a three-phase supply. Its gateway specifies a 0-millisecond load-side disruption time. The smart port allows you to wire in specific loads and control which circuits are cut first if the battery runs low. There is also a generator input for sites that want an automatic backup charging source.
The Fox Evo's EPS box switches in under 4 milliseconds. Both the Fox and the SigenStor can also operate entirely off-grid with no network connection, which the Powerwall 3 cannot. For self-build properties or anyone pursuing full energy independence, that distinction matters.
Fox Evo and Sigenstor owners can join the Axle Energy virtual power plant scheme now. When grid stress events occur, which typically happen when demand spikes, participating batteries export electricity and receive £1 per kilowatt-hour, with a guaranteed minimum of £10 per month. You retain full control and can opt out of any individual event.
The Powerwall 3 does not currently have a UK virtual power plant option. Tesla is becoming a UK electricity provider, and when it does, Powerwall owners are expected to gain access to a VPP scheme. In the US, Tesla's equivalent programme has paid out up to $1,000 per Powerwall annually on the dynamic plan. UK terms have not been announced.
This is probably the starkest difference between the three systems.
The Powerwall 3 carries a 10-year warranty with 80% retained capacity guaranteed and no throughput limit. Tesla will replace the unit if anything goes wrong within that period, with no conditions.
The SigenStor has a 10-year warranty covering whichever comes first: 60% retained capacity or approximately 3,000 cycles. At daily cycling rates, the throughput limit is likely to be reached before the 10-year calendar term. A 15-year extension is available through your installer within 12 months of commissioning.
The Fox Evo has the longest calendar warranty of the three: 12 years on both battery and inverter, with 70% retained capacity and a throughput limit of 4,780 kWh. At normal cycling rates, the calendar term will typically be the binding limit rather than the throughput figure.
The Powerwall 3 is the most proven all-rounder, and the right choice for homes where DNO flexibility and long-term Tesla ecosystem integration matter. The SigenStor is the most flexible technically: modular sizing, four MPPTs on the larger inverters, three-phase backup, the most extensive fire safety engineering, and is well suited to complex installations or three-phase properties. The Fox Evo ticks most boxes, costs slightly less per kilowatt-hour of storage, and offers the longest standard warranty, making it a strong choice where the specification doesn't point clearly toward either of the other two.
None of these is a wrong answer. The right one depends on your roof, your supply, your usage pattern, and the specific parts of the above that are most relevant to your home.