What a Home Battery Warranty Actually Guarantees in 2026

A home battery warranty is sold as a single reassuring number. “Ten years.” Occasionally “twelve.” What you actually sign up to is three numbers and a page of conditions, and the three numbers only make sense together. This piece reads two current UK policies line by line — the Fox ESS UK & Ireland battery warranty and Tesla’s European Powerwall warranty — and shows you how to turn the figures on the certificate into the one that matters: how many years of cover your own household will really get. It is not a product comparison (we did GivEnergy against Fox ESS in March) and it is not about the panels, which have their own, rather different, warranty logic. It is about the battery document itself.

The three numbers on the certificate

Every battery warranty worth the name has a year count, a capacity retention percentage and, increasingly, an energy throughput cap. The year count is the one in the brochure. The percentage is the one in the datasheet. The throughput cap is the one in the PDF nobody opens — and it is the one that can quietly end your cover before the year count does.

Fox ESS’s UK & Ireland battery warranty policy, version 4.5 dated 11 September 2025 (the most recent UK version published, as far as we can find), sets it out neatly. For the EP5, EP6, EP11, EP12, CQ6 and CQ16 modules, the product is warranted free from defects for 120 months from installation, but no more than 125 months from the date of manufacture, whichever comes first. The EQ and EVO ranges get 144 months from installation, capped at 149 months from manufacture. Separately, the performance warranty says the product will retain at least 70% of its nominal energy for ten years (EP and CQ) or twelve years (EQ and EVO), or until a minimum energy throughput is reached — again, whichever comes first.

Tesla’s Powerwall warranty for the European region, revision 1.16 with an effective date of 3 December 2025, covers the Powerwall 3 for ten years from installation. For units installed in the UK it guarantees an energy capacity of 13.5 kWh on day one, 80% retention at ten years, and unlimited cycles for solar self-consumption, time-based control and backup. Any other application, or any mix that includes one, is limited to 37.8 MWh of aggregate throughput measured at the battery’s AC output.

Three numbers each. Now for what they mean.

What “70%” or “80%” actually promises

A retention percentage is a floor, not a forecast. The manufacturer is not saying your battery will hold 70% or 80% at the end; it is saying that if the battery falls below that line before the term ends, it owes you a remedy. Anything above the line — however disappointing — is contractually fine.

Two details matter more than the headline figure. The first is what the percentage is a percentage of. Fox defines nominal energy as the rated capacity printed on the label, so for a 10.36 kWh EP11 the guaranteed floor is 7.25 kWh. Tesla’s 80% is of the 13.5 kWh rated capacity, so the floor is 10.8 kWh. Neither figure is the usable number you see in your app, which will be lower because the system holds back a reserve; the Energy Saving Trust’s battery storage guide notes a system will stop drawing down at a set level, which it suggests might be around 20%, to protect the cells.

The second is that the percentage is a commercial choice, not a law of chemistry. The same Tesla document guarantees the same Powerwall 3 to 80% in the UK, Ireland, Germany and France, and to 70% in Italy, Spain, Malta, Portugal and Slovenia. Identical hardware, different number, because the contract is written per market. Fox’s 70% and Tesla’s 80% are not evidence that one battery degrades more slowly; they are evidence of what each company was prepared to promise in writing.

Then there are the test conditions. Fox’s capacity measurement is specified at an ambient temperature of 25 to 30°C, with the battery itself at 25 to 30°C according to its management system, measured on the DC side. If your battery lives in a cold garage, the number it shows you in January is not the number the warranty is judged on.

The throughput cap: the clause that ends a warranty early

The Fox policy includes a table that most buyers never see. Every model has a nominal energy figure and an energy throughput figure, and the performance warranty is only valid while the throughput stays below the table value. Here is the whole table, with two columns the policy does not print: the cap divided by the capacity (full-cycle equivalents), and that figure divided by 365 (years of cover at one full cycle a day).

The pattern is plain once it is laid out. The EP modules and most of the EQ range are set at about 3,750 cycles, which at one full cycle a day lands just beyond a ten-year term — presumably the intent. But the EQ4300 to EQ7000 PLUS carry a twelve-year term on the same 3,750 cycles, so a household cycling fully every day exhausts the performance warranty with about a year and three-quarters still on the certificate. The CQ pair come in fractionally under their ten years. Only the EVO units clear their term with room to spare.

The policy does not say whether it counts energy in, energy out, or both, so ask your installer how Fox’s monitoring records it. Not every installer will even tell you the cap exists; some do, and Home Comfort Enterprises’ breakdown of the Fox ESS UK warranty — the firm is a solar, battery and EV installer — prints the EP11’s 38.85 MWh figure and the version 4.5 date on its product page, which is the level of disclosure to hold your own quote to, whatever the brand.

Tesla’s cap works differently. For the three listed applications it does not apply at all: unlimited cycles. The document defines time-based control as storing grid or solar energy and using it to shift consumption into cheaper hours, so an ordinary overnight-tariff routine sits inside the unlimited column. The 37.8 MWh limit only bites on applications outside those three, or on any combination that includes one — and the document gives no examples of what those other applications are, which is the kind of gap that should prompt a written question before you enrol the battery in any scheme that pays you to export or discharge on command. Where the cap does apply: 37,800 kWh divided by 13.5 kWh is 2,800 full cycles, which is 7.7 years at one a day and 3.8 at two. Off the listed applications, it is an under-eight-year warranty wearing a ten-year label.

Work out your own number

Two full cycles a day is not an exotic scenario. The Energy Saving Trust’s guide describes a battery doing two jobs: storing surplus solar, and buying from a smart time-of-use tariff when electricity is cheap to avoid the peak rate. Do both on the same day — overnight charge, morning discharge, solar refill, evening discharge — and you are cycling twice. The more cleverly you use the battery, the faster you spend the cap.

You do not need to guess at your own figure. Here is the method, with an illustrative household to show the arithmetic; put your own numbers in.

1. Find your exact model in the manufacturer’s policy and write down the throughput cap in kWh (38,850 for an EP11).
2. Open your monitoring app and find the average energy discharged per day over a full year. A summer week will flatter you; a January week will not. If the system is new, estimate and revisit it after twelve months.
3. Multiply the daily figure by 365. Say the app shows 8 kWh a day: that is 2,920 kWh a year.
4. Divide the cap by the annual figure. 38,850 divided by 2,920 is 13.3 years, which is beyond the ten-year term — so for this household the calendar ends the warranty and the cap never does.
5. Change the input and watch the answer move. The same EP11 discharging 12 kWh a day on an aggressive overnight tariff reaches 4,380 kWh a year and hits the cap in 8.9 years. A Powerwall 3 on an unlisted application at 12 kWh a day gets 8.6 years.

Whichever is shorter — the year count or the cap — is your real warranty length. Write it on the certificate.

What “ten years” is worth once you have read the conditions

The year count itself has conditions attached, and they chip away at it from both ends.

The clock may start before you do. Fox’s 120 months from installation is capped at 125 months from manufacture, so a module that sat in a warehouse for eight months before it reached your wall carries 117 months of cover, not 120. The date of manufacture is on the label; check it on the day of installation, not when you need it.

The clock can be shortened by your broadband. Tesla’s warranty says that if the Powerwall is not connected to the internet for an extended period, or has not been registered with Tesla, the company may not be able to deliver remote firmware upgrades, and in that case may not honour the full ten years. It commits to honouring at least four years regardless. Four is the guaranteed figure; ten is conditional on staying online and registered.

The remedy is not the price you paid. Tesla may repair with new or refurbished parts, replace with an equivalent new or refurbished product, or refund what an equivalent product costs on the market at the time of the claim — if equivalent batteries are cheaper in year nine than when you bought, that is the figure. Fox limits its liability to replacement, repair, refund and compensation, and states that a replaced product is warranted for the remainder of the original term; the replacement does not restart the clock. (If you are a consumer who bought a Powerwall directly from Tesla, the UK appendix adds Consumer Rights Act 2015 rights, including the right to reject it for a full refund — but only within 30 days of installation.)

The environment can void it. Fox’s performance warranty requires an operating ambient between -10°C and 50°C, and its exclusions list refuses claims where less than 300 mm of clearance was left around the pack for cooling or where ventilation was inadequate. Tesla excludes damage from installation, modification or repair by anyone other than Tesla or a Tesla certified installer, and from removing and reinstalling the unit anywhere other than its original location without Tesla’s written consent — which matters if you were planning to take the battery with you when you move. (Tesla does allow a subsequent owner with proof of ownership to claim, so leaving it behind keeps the cover with the house.)

Set those against what the thing costs and how long it lasts. The Energy Saving Trust puts installed battery costs at £1,500 to £10,000, with a 5 kWh system around £4,600, a typical home system at about 10 kWh, and a typical lifespan of about 10 to 12 years. The warranty, in other words, is meant to span most of the battery’s working life. Every month knocked off it by a condition you did not read is a month of a £4,600 asset you are self-insuring.

Who actually pays, and what happens if they are gone

A warranty is a promise from a company, and both policies route the promise through the installer rather than to you directly. Fox’s policy states that the only people eligible to claim are the installer and Fox ESS authorised personnel; if your installer has gone into administration, you or they may appoint a local installer to act — at your own discretion and expense, the policy says, not Fox’s. Tesla’s claims process starts with the certified installer who sold you the Powerwall, and the warranty only applies to units purchased from Tesla or a Tesla certified installer in the first place. A Powerwall bought through a non-certified route, or moved to another address without Tesla’s written consent, starts outside the document.

Then there is the manufacturer itself. GivEnergy Ltd, the British battery storage company, had an administrator appointed on 9 April 2026, per the Gazette notice recording the High Court appointment in Leeds. Companies House records the company as in administration, with the appointment filed on 21 April; the administrator’s proposals filed in May record that trading ceased on the day of appointment, and the most recent filing, the result of a creditors’ decision, is dated 10 June 2026. The Energy Storage Association’s guidance, last updated 17 April 2026, is blunt: GivEnergy warranties “can only be enforced against GivEnergy itself”, and the closure likely means owners will be unable to claim against them. GivEnergy Software Limited, which the ESA says runs the app and cloud portal, is a separate company and is recorded as active, so monitoring continues. GivEnergy Commercial Limited passed a resolution to wind up on 13 August 2026 and has a voluntary liquidator appointed. As of 6 October 2026 there is no filing on the register, and no announcement we could find elsewhere, of a buyer for the hardware business — so the position is unchanged since the spring: the batteries keep working, the warranties do not.

The lesson is not “avoid brand X”. It is that a ten-year promise from a manufacturer is worth exactly as much as the manufacturer’s solvency in year seven, and the only parts of your cover that survive its failure are the ones that never depended on it.

The cover that survives: the installer, the insurer and the law

Three things stand behind an installation when the manufacturer cannot, and none of them is in the battery PDF.

The first is the law. The Consumer Rights Act 2015 reads into every contract for a service a term that the trader must perform it with reasonable care and skill, and the Limitation Act 1980 gives you six years from the breach to bring a contract claim in England and Wales. That is a claim against the installer, for the installation, and it exists whether or not anyone wrote a warranty. If you paid any part of the price by credit card — the deposit will do — section 75 of the Consumer Credit Act 1974 gives you the same claim against the card issuer as against the supplier, for a single item with a cash price over £100 and not more than £30,000, which takes in nearly every domestic battery sold.

The second is the installer’s own workmanship warranty and the insurance behind it. HIES describes its insurance-backed guarantee as covering the product and workmanship in the installer’s written guarantee if the installer ceases trading or cannot honour it — with one line worth underlining: no cover is in place until you have received the certificate. Not promised. Received.

The third is the MCS scheme itself. Under its redeveloped installer scheme, MCS says every installer will need to purchase an MCS-approved financial protection product on behalf of their customer, funding remediation when the original installer cannot or will not, with at least six years’ cover. Ask whether your installer has moved onto the new scheme yet, and which product they will be buying for you.

None of these replaces the manufacturer. If a cell pack dies in year nine and the maker is gone, a workmanship guarantee does not owe you a new battery. But they cover the far more common failures — a bad crimp, a unit sited where it cooks itself, an inverter commissioned wrong — and they are the only cover that survives the GivEnergy scenario at all.

Six questions to put in writing before you sign

1. Which version and date of the battery warranty policy applies to my purchase, and can I have the PDF?
2. What is the throughput cap for my exact model, and does your monitoring count charge, discharge or both?
3. What is the date of manufacture on the modules you are fitting, and how many months of cover does that leave?
4. Is the performance test done at a specified temperature, and will you carry it out on site?
5. Who makes the claim, you or me, and what happens if your company is no longer trading?
6. Is there an insurance-backed guarantee or MCS financial protection product on the installation, who underwrites it, and when do I get the certificate?

A battery that keeps 80% after a decade is a good battery. A warranty that guarantees 70% for ten years, or 3,750 cycles, or 125 months from a date you have never checked — whichever comes first — is a different thing from the ten years on the brochure. Do the arithmetic, write the real number on the certificate, and buy with that figure in mind rather than the one in the headline.