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Solar battery storage: how it works and when it pays
How home solar batteries work in the UK, what time-of-use tariffs add, worked examples with losses and export given up, and what to check before you buy.
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Tesla Powerwall in the UK: Powerwall 3, backup and warranty
Powerwall 3 is a 13.5 kWh home battery with a solar inverter built in. What it does in your home depends on the power setting, the backup design and the paperwork your installer does.
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Tesla Powerwall 3 is worth comparing if you want a large home battery and a solar inverter in one wall-mounted unit. Each Powerwall 3 stores 13.5 kWh and, on Tesla's UK datasheet, can be set up to deliver anywhere from 3.68 kW to 11.04 kW on a single-phase supply. A Powerwall 3 Expansion adds another 13.5 kWh to it. Tesla's UK warranty covers 10 years and 80% of the battery's rated capacity. Whether it suits your home depends on your evening use, your tariff and whether the quote includes the Gateway needed for backup in a power cut.
In the UK you buy a Powerwall through a Tesla certified installer, not off the shelf: Tesla's warranty only covers units bought from Tesla or one of those installers. Octopus Energy also fits it: on 23 September 2026, its installation packages page listed Powerwall 3, with optional Expansion units, among the batteries it installs. For installed prices and what they include, see our Powerwall 3 price guide.
Which Powerwall fits your situation?
Tesla's UK Powerwall page leads with Powerwall 3, released in 2023 with a built-in solar inverter. Powerwall 2 is the earlier generation, which you may already have: it has the same 13.5 kWh capacity, but no solar inverter, and Tesla's UK page lists it at 5 kW continuous power. Some price comparisons still quote Powerwall 2's power figures for Powerwall 3, so check which model a figure belongs to.
My yearly electricity use is
Your yearly electricity use helps us size your solar panel system.
| Your situation | What to look at | The decision that matters |
|---|---|---|
| New solar panels and a battery, or a battery added to existing panels | Powerwall 3, with its built-in solar inverter | Power setting, backup design and whether your current inverter stays |
| A Powerwall 3 system that needs more storage | Powerwall 3 Expansion, 13.5 kWh per unit | Whether you have the surplus solar or cheap-rate hours to fill it |
| You already own a Powerwall 2 | Service and support for the existing unit | Keep the serial number, installation date and warranty papers |
An Expansion is not a stand-alone battery. Tesla's datasheet says it works only with Powerwall 3. It adds storage and lifts the maximum charging power from 5 kW to 8 kW, but not the output power: for more output you need another Powerwall 3.
Powerwall 3 in numbers
These are the figures that change the decision, from the Tesla UK Powerwall 3 datasheet we checked (2026 edition). The full specification is in Tesla's datasheet; your installed system may be set up differently.
| Specification | Powerwall 3 (UK) | What it means for you |
|---|---|---|
| Usable energy | 13.5 kWh (AC) | Energy you can draw from a full battery at the start of its life |
| Output power | Configurable from 3.68 kW to 11.04 kW | The setting decides how many appliances run at once, and the grid paperwork |
| Charging power | Up to 5 kW (up to 8 kW with Expansion units) | How fast it refills from solar or a cheap overnight rate |
| Solar to battery to home | 89% | The share of solar energy you get back after storing it |
| Solar to home or grid | 97.5% | Solar used directly, without going through the battery |
Two of these numbers are often mixed up. 97.5% is the efficiency for solar power going straight to your home or the grid. The figure for energy that is stored and used later is 89%, which is the one to use when you work out what a battery saves.
kWh and kW answer different questions: kWh is how much energy is stored, kW how much power it delivers at one moment. A full battery running a steady 5 kW load lasts under three hours (13.5 kWh ÷ 5 kW = 2.7 hours), before any backup reserve.
Is 13.5 kWh the right size for you?
Start with two numbers from your smart meter or your installer's survey: how much electricity you use between the end of the solar day and the next morning, and how much your panels export on a typical day. A battery only saves money on energy it can fill and then use.
Example (illustrative, not a forecast): your home uses 8 kWh in the evening and overnight, and your panels leave 5 kWh of surplus on a spring day. Stored and used later at the datasheet's 89%, that surplus gives about 4.5 kWh back (5 × 0.89 = 4.45 kWh). The rest of your evening still comes from the grid, and a 13.5 kWh battery would be partly empty. In winter the surplus is usually smaller still.
On a time-of-use tariff, a Powerwall can also charge from the grid at the cheap rate and cover your peak hours. At its maximum charging power of 5 kW, filling an empty Powerwall 3 takes at least 2.7 hours (13.5 kWh ÷ 5 kW), so it fits comfortably into a five-hour overnight cheap-rate window. With one Expansion unit (27 kWh at up to 8 kW), it takes at least about 3.4 hours. Tariff availability changes often, so check the current terms before you count on one; our guide to solar battery storage covers how tariffs change a battery's value.
If you are adding a heat pump or an electric car, ask for a month-by-month estimate rather than one sunny day multiplied by 365.
Backup reserve reduces daily savings
Energy you keep back for power cuts is not available for everyday use. For example, a 20% reserve on 13.5 kWh holds back 2.7 kWh and leaves 10.8 kWh for daily cycling.
New installation or adding to existing panels
On a new installation, the built-in inverter means the panels connect straight to the Powerwall, so there is no separate solar inverter to buy or mount. Compare that quote with a hybrid inverter and battery from another brand for the same panels and the same backup.
If you already have panels, Tesla's UK datasheet shows designs where your existing solar inverter stays in place and a Tesla Remote Meter measures its output. Keeping a working inverter can save money. The trade-off is grid connection. Under the G98 rules, the connect-and-notify route covers up to 16 A per phase in total, which is 3.68 kW on a single-phase supply, and a battery's inverter counts towards that total. A 3.68 kW solar inverter plus a Powerwall 3 is over the limit, so your installer has to apply to your network operator under G99 and get approval before connecting. The same applies to a Powerwall 3 set above 3.68 kW on its own. Ask which power setting the quote assumes and who makes the application.
Backup in a power cut needs the Gateway
Tesla's UK datasheet says Backup Gateway 2 is required for a backup installation. The Gateway detects a power cut, disconnects your home from the grid and switches the chosen circuits to the battery. The datasheet shows whole-home and partial-home backup designs.
Powerwall 3 can also be installed without backup, using a Tesla Remote Meter instead of the Gateway. That system still stores solar power and shifts your grid use to cheaper hours, but it will not power your home during a power cut. A backup reserve set in the app does not change that.
Before you sign, ask for:
- the circuits that will stay on in a power cut, and any that won't;
- the power setting and whether large loads (an electric shower, oven or heat pump) can run in backup;
- the backup reserve the installer recommends and how much daily capacity that leaves;
- a test of the changeover at handover.
Tesla's warranty says Powerwall is not intended as a power source for life-support or other medical equipment.
What the UK warranty covers
Tesla's European warranty region includes the UK. The edition we checked (revision 1.16, effective 3 December 2025) gives:
- 10 years against defects from the date of installation;
- 80% of 13.5 kWh after 10 years for Powerwalls installed in the UK, with unlimited cycles when used for solar self-consumption, time-based control and backup;
- a limit of 37.8 MWh of total output for any other use, or any mix that includes another use.
The same document gives 70% for Italy, Spain, Portugal, Malta and Slovenia, which is why you'll see both numbers quoted online.
Three conditions matter. Tesla needs to update the system remotely: if it is offline for a long period or not registered, Tesla may not honour the full 10 years, though it says it will always honour at least 4. Keep the Powerwall on your home internet; the datasheet says the mobile connection shouldn't be the main one. Claims go through the installer who sold you the Powerwall, or to Tesla if you can't reach them. Tesla's warranty comes on top of your rights under the Consumer Rights Act 2015 against whoever sold and installed the Powerwall. And a later owner can claim with proof of ownership, which helps if you sell your home.
The 80% threshold is a warranty condition, not a forecast of your battery's capacity after 10 years. The installer's workmanship guarantee is separate: ask how long it lasts and whether it is insurance-backed.
Checking a Powerwall quote
A Powerwall quote should itemise:
- the number of Powerwall 3 and Expansion units, and the power setting;
- the Backup Gateway 2, or the Remote Meter for a non-backup system;
- any change to your consumer unit, cabling, isolators and a separate backup board;
- the G98 notification or G99 application, and who handles it;
- whether the planned spot (garage, outside wall or utility room) meets PAS 63100:2024, the BSI fire-safety specification for home batteries, and can take a 130 kg unit on the wall or floor;
- new panels and roof work, if the quote includes them.
Batteries are zero-rated for VAT until 31 March 2027, then 5% from 1 April 2027 unless the relief is extended, when they are supplied and installed in a home, including when you add one to existing panels or have no panels at all. A battery bought without installation is charged at 20%.
MCS's product directory does not list home batteries, so the check is on the installer: ask for their MCS certificate number and look it up. Under MCS's redeveloped installer scheme, consumer-code membership is no longer required, so check RECC or HIES membership separately if you want those deposit and warranty protections. Our guides to comparing solar quotes and choosing an installer go through the paperwork.
Our view: Powerwall 3 makes sense when you can use most of its 13.5 kWh on most days, from solar surplus or a cheap overnight rate, and when backup is designed and included in the price. If your evening use is modest, or you only need a few kWh, compare smaller batteries in our best solar batteries guide before paying for capacity you won't use.
Tesla Powerwall FAQs
Does Powerwall 3 always deliver 11.04 kW?
No. 11.04 kW is the highest setting on Tesla's UK datasheet. Your installer chooses the setting, and anything above 3.68 kW on a single-phase supply needs your network operator's approval under G99 first.
Is the Powerwall 3 warranty 70% or 80% in the UK?
80%. Tesla's European warranty (revision 1.16, December 2025) gives 80% of 13.5 kWh after 10 years for Powerwalls installed in the UK. The 70% figure applies to Italy, Spain, Portugal, Malta and Slovenia.
Will a Powerwall keep my lights on in a power cut?
Only in a backup design with Backup Gateway 2. A non-backup Powerwall 3 with a Tesla Remote Meter does not power your home when the grid goes down. Ask which circuits are backed up and test the changeover at handover.
Can I add a Powerwall 3 Expansion to a Powerwall 2?
No. Tesla's datasheet says the Expansion works only with Powerwall 3. If you own a Powerwall 2 and need more storage, ask an installer what your options are for your specific system.