A 13.5kWh battery costs from £7,795 to £8,499.99 installed for a Tesla Powerwall 3 in four published prices read in October 2026 (£577 to £630 per usable kWh), against an official median of £630 per kWh for batteries of 11 kWh or more, about £8,500. In this site's model it saves about £569 a year with 4 kWp of solar on Octopus Flux in London, £155 more than 10 kWh, but with no solar the extra 3.5 kWh adds only £14 a year for a 3,000 kWh home and £197 for a 4,500 kWh one. A full battery runs an average home for about two days.
Who can actually use 13.5kWh
A battery earns only on energy it moves each day. To use 13.5 kWh daily, a home typically needs one of these:
- High evening and night use, around 4,000 kWh a year or more, with most of it after dark.
- An electric car charged at home. An overnight EV tariff charges car and battery cheaply, and the battery covers the house during the day.
- A heat pump. Winter heating adds several kWh a day of demand the battery can shift from peak to cheap hours.
- Whole-home backup, where the reserve kept for power cuts takes a share of the capacity.
- Export on Octopus Flux, where spare stored energy is sold in the 4pm to 7pm peak, up to the home’s export limit.
Without one of those, a 13.5 kWh battery spends much of the year partly empty.
What 13.5kWh costs
| Source | Price | Per usable kWh | What it covers |
|---|---|---|---|
| E.ON Next | From £7,795 | £577 | Tesla Powerwall 3, installed (read October 2026) |
| EDF | From £7,995 | £592 | Tesla Powerwall 3 (installation not itemised) |
| Octopus Energy | From £7,999 | £593 | Tesla Powerwall 3, battery only, installed (April 2026 prices) |
| Fresh Solar through Costco UK | £8,499.99 | £630 | Tesla Powerwall 3 retrofit installation (read 7 October 2026) |
| Tesla (list price) | £5,000 or £5,975 | n/a | Powerwall 3 without or with Backup Gateway 2, before VAT, delivery and installation |
| Octopus Energy | £3,773 | n/a | An extra 13.5 kWh Expansion Pack fitted at the same time as a new Powerwall 3 |
| DESNZ official median | £630 per kWh | £630 | MCS-certified installs of 11 kWh or more, 2025/26: about £8,500 for 13.5 kWh |
Across the whole market, bigger batteries cost less per kWh: the official median drops from £890 to £630 per kWh between the 6 to 11 kWh band and the 11 kWh and over band. Individual brands vary, though. Octopus’s Powerwall 3 price works out at about £593 per usable kWh, more than the £550 or so per usable kWh of its 10 kWh Fox ESS package. Either way, what matters is whether the extra capacity earns its keep.
The 11 kWh price cliff
DESNZ’s median per kWh falls at the edge of its bands, and that produces an oddity: at the medians a 10 kWh battery comes to £8,900 (10 × £890) while a 13.5 kWh one comes to £8,505 (13.5 × £630), £395 less for 35% more capacity. DESNZ’s figures also show that the average battery in the 11 kWh and over band was bigger than 13.5 kWh: its 12,176 MCS-certified installations in 2025/26 held 188,540 kWh in total, an average of 15.5 kWh, at a mean of £670 per kWh, which is about £10,400 for the average installation. So the median per kWh describes batteries a size or two above the Powerwall’s, and a quote for exactly 13.5 kWh should be tested against the median with that in mind. The cost guide sets out every band.
What it saves compared with 10kWh
This site’s model, London, with 4 kWp of solar on Octopus Flux and a home using 3,000 kWh a year:
| Battery | Modelled saving a year | Extra saving for the extra capacity |
|---|---|---|
| 10 kWh | £414 | |
| 13.5 kWh | £569 | £155 a year for 3.5 kWh more |
| 20 kWh | £731 | £162 a year for 6.5 kWh more |
On Flux the step from 10 to 13.5 kWh still pays reasonably, because spare stored energy can be exported at the peak rate. Whether it pays elsewhere depends on how much the home uses and whether it has solar. Here is the extra that 13.5 kWh earns over 10 kWh in the same model for four kinds of home (London, Octopus’s 26 September rates):
| Home | Annual use | 10 kWh | 13.5 kWh | Extra for 3.5 kWh |
|---|---|---|---|---|
| 4 kWp solar, Octopus Flux | 2,000 kWh | £432 | £593 | £161 |
| 4 kWp solar, Octopus Flux | 4,500 kWh | £388 | £530 | £142 |
| 4 kWp solar, Octopus Flux | 6,000 kWh | £374 | £496 | £122 |
| No solar, Octopus Go | 3,000 kWh | £489 | £503 | £14 |
| No solar, Octopus Go | 4,500 kWh | £495 | £692 | £197 |
| No solar, Octopus Go | 6,000 kWh | £425 | £671 | £246 |
| No solar, Agile Octopus | 3,000 kWh | £335 | £342 | £7 |
| No solar, Agile Octopus | 4,500 kWh | £387 | £482 | £95 |
| 4 kWp solar, flat tariff | any from 2,000 to 6,000 | £141 to £212 | the same | £0 |
The pattern is that solar plus Flux lets a 13.5 kWh battery trade in the peak, so it earns its extra capacity at any usage, and without solar it earns it only for a home using about 4,500 kWh a year or more. On a flat tariff the extra capacity earns nothing at all, because the battery only holds the surplus solar the home would have exported at 12p. At the Octopus prices the extra 3.5 kWh costs £2,663 more than the 10 kWh Fox package (£7,999 against £5,336, though they are different products), which at £155 a year is a payback of about 17 years for the increment; at the official medians the larger battery costs less in total, which is the price cliff above. Run your own figures in the calculator.
The Flux figures use a peak export rate of about 27p to 31p; the other export tariffs Octopus offers pay between 4.1p and 16p flat, or follow wholesale prices half-hour by half-hour.
13kWh or 13.5kWh: which number to compare
People search for both, and the Tesla Powerwall 3 is the product sold as 13.5 kWh of usable energy (Tesla’s datasheet gives usable AC energy, as a minimum at the start of life). The figure that matters on a quote is usable kWh, because manufacturers list nominal capacity, usable capacity or both and the difference can be several percent: this site assumes usable energy is 95% of nominal when it compares products that only list nominal capacity, such as the 10.24 kWh Fox ESS EVO 10. Divide the installed price by usable kWh before comparing two batteries.
How long it runs the house, and how much solar fills it
A full 13.5 kWh battery covers about two days of an Ofgem medium home’s use (2,500 kWh a year, 6.8 kWh a day), three days for a low-use home (1,600 kWh) and 1.3 days for a high-use home (3,800 kWh). At a constant 3 kW load it lasts 4.5 hours, and at the Powerwall 3’s 11.04 kW maximum about 1.2 hours. The full table by load is in how long a 13.5kWh battery lasts.
Refilling takes about 15 kWh of generation, because the round trip loses 10% in this site’s model. A 4 kWp south-facing array in London makes about 16.3 kWh a day in June, so the battery can refill in under a day, but only about 4.7 kWh a day in December, which is three days. To fill the battery from solar alone you would need about 3.7 kWp in June (roughly eight 450 W panels) and about 12.8 kWp in December (roughly 28), which is why a 13.5 kWh battery in winter depends on a cheap overnight tariff rather than on the roof. In Glasgow the December figure is about 24 kWp.
Tesla Powerwall 3 at this size
The Powerwall 3 packs 13.5 kWh with a built-in solar inverter (three strings, up to 20 kW of panels) into one 130 kg wall unit. Its output can be configured between 3.68 kW and 11.04 kW at 230 V. The warranty is 10 years with 80% of capacity retained, valid only when bought from Tesla or a Tesla certified installer, and dependent on a working internet connection for the full term. Backup needs the Backup Gateway 2. Full detail on the Powerwall 3 page.
The other 13.5 kWh unit, the GivEnergy All in One, comes from a company in administration; the GivEnergy All in One page sets out what that means for the warranty and the app.
Siting a 13.5kWh battery
PAS 63100 limits a single battery enclosure to 20 kWh and the total in a home to 40 kWh indoors, or 80 kWh outdoors or in a garage or outbuilding, so a 13.5 kWh unit is well within the rules. The location rules are the harder constraint: outdoors is preferred, and bedrooms, escape routes such as hallways and landings, lofts and cellars without outside access are ruled out. A unit this size is also heavy, so ask the installer to confirm the wall or plinth. See PAS 63100 explained.
Homes that would fill more than 13.5 kWh, such as those with a heat pump on an overnight tariff, can also price a 15 kWh battery built from modules, the step between one large unit and a 20 kWh stack. Get home battery storage quotes for both 10 kWh and 13.5 kWh and compare the extra cost against the modelled extra saving before choosing. If a quote arrives at 12 kWh, the 12kWh battery page shows why it often costs about the same as a Powerwall 3.
Questions people ask
How much does a 13.5kWh battery cost?
Four installed Tesla Powerwall 3 prices were published in October 2026: E.ON Next from £7,795, EDF from £7,995, Octopus at £7,999 (April 2026 prices) and Fresh Solar through Costco UK at £8,499.99. Tesla's own UK list price is £5,000 without its Backup Gateway 2 and £5,975 with it, before VAT, delivery and installation. The official median for MCS-certified batteries of 11 kWh or more was £630 per kWh in 2025/26, about £8,500 for 13.5 kWh.
Who needs a 13kWh or 13.5kWh battery?
Homes that use a lot of electricity outside daylight hours: typically 4,000 kWh a year or more, an electric car charged at home, a heat pump, or a household that wants whole-home backup. With solar on Octopus Flux, the model shows the extra capacity earning about £120 to £160 a year more than 10 kWh for homes using 2,000 to 6,000 kWh. Without solar, a home using 3,000 kWh a year gains almost nothing over 10 kWh.
How long will a 13.5 kWh battery power a house?
About two days for a home using Ofgem's medium 2,500 kWh a year (6.8 kWh a day), three days for low use and 1.3 days for high use. In a power cut with essentials only, a few hundred watts, it lasts more than a day; at 3 kW it lasts 4.5 hours. The how-long post has the full table by load.
How many solar panels do I need to charge a 13.5 kWh battery?
To put 15 kWh into the battery from solar alone, which allows for the round-trip loss, you need about 3.7 kWp in a London June, roughly eight 450 W panels, and about 12.8 kWp, roughly 28 panels, in December, if the battery gets all of the output. In practice the home uses some of it and a cheap overnight tariff fills the battery in winter.
Is 13.5 kWh too big for my home?
It can be if you have no solar and use about 3,000 kWh a year or less: in this site's model a London home using 3,000 kWh on Octopus Go saves £489 a year with 10 kWh and £503 with 13.5 kWh, and on Agile £335 and £342. At 4,500 kWh a year, or with solar on Octopus Flux, the extra capacity earns its keep.
Is the official median price for 13.5 kWh really £630 per kWh?
It is the median for the whole 11 kWh and over band, which DESNZ's figures show averaging 15.5 kWh per installation in 2025/26, so it describes bigger batteries than 13.5 kWh. At the band's median, a 13.5 kWh battery is about £8,500, which is less than the £8,900 a 10 kWh battery comes to at the 6 to 11 kWh band's median of £890 per kWh.
Is a Tesla Powerwall 3 the only 13.5kWh option?
No. The GivEnergy All in One is also 13.5 kWh, but its maker has been in administration since April 2026, so it effectively has no manufacturer warranty. Modular systems from Fox ESS, Sunsynk and others reach similar capacities with stacked modules.